Ecotoxicology DOI 10.1007/s10646-014-1221-y
Complex interactions between climate change and toxicants: evidence that temperature variability increases sensitivity to cadmium David A. Kimberly • Christopher J. Salice
Accepted: 24 February 2014 Ó Springer Science+Business Media New York 2014
Abstract The Intergovernmental Panel on Climate Change projects that global climate change will have significant impacts on environmental conditions including potential effects on sensitivity of organisms to environmental contaminants. The objective of this study was to test the climate-induced toxicant sensitivity (CITS) hypothesis in which acclimation to altered climate parameters increases toxicant sensitivity. Adult Physa pomilia snails were acclimated to a near optimal 22 °C or a high-normal 28 °C for 28 days. After 28 days, snails from each temperature group were challenged with either low (150 lg/L) or high (300 lg/L) cadmium at each temperature (28 or 22 °C). In contrast to the CITS hypothesis, we found that acclimation temperature did not have a strong influence on cadmium sensitivity except at the high cadmium test concentration where snails acclimated to 28 °C were more cadmium tolerant. However, snails that experienced a switch in temperature for the cadmium challenge, regardless of the switch direction, were the most sensitive to cadmium. Within the snails that were switched between temperatures, snails acclimated at 28 °C and then exposed to high cadmium at 22 °C exhibited significantly greater mortality than those snails acclimated to 22 °C and then exposed to cadmium at 28 °C. Our results point to the importance of temperature variability in increasing toxicant
D. A. Kimberly C. J. Salice Department of Environmental Toxicology, The Institute of Environmental and Human Health, Texas Tech University, 1207 Gilbert Drive, Lubbock, TX 79416, USA D. A. Kimberly (&) Department of Biology, Westminster College, 215 Meldrum Science Center, 1840 S 1300 East, Salt Lake City, UT 84105, USA e-mail: [email protected]; [email protected]
sensitivity but also suggest a potentially complex cost of temperature acclimation. Broadly, the type of temporal stressor exposures we simulated may reduce overall plasticity in responses to stress ultimately rendering populations more vulnerable to adverse effects. Keywords Climate change Temperature Cadmium Physa pomilia Stressor variability CITS
Introduction The Intergovernmental Panel on Climate Change (IPCC) projects that global climate change (GCC) will have significant impacts on environmental conditions such as higher ocean acidity, changes in precipitation patterns, and higher mean temperature (IPCC 2007). While the projections include significant uncertainties and regional variation, in general, increased frequency in extreme weather events such as heat waves, droughts, and storms is also expected. Substantial uncertainty in understanding the impacts of GCC extends to how climate changes will interact with other anthropogenic stress factors such as chemical contaminants. Concerns include the role of GCC in determining environmental concentration of contaminants (Gouin et al. 2013) as GCC is expected to have a substantial effect on release, fate, behavior, and exposure of toxicants (Noyes et al. 2009). As well, the sensitivity of organisms to environmental toxicants could be drastically modified by GCC-induced changes in environmental conditions (Hooper et al. 2013). Hence there is the potential for GCC to modify both exposure and responses to environmental contaminants. The combination of chemical and non-chemical stressors like those related to GCC may act in a greater-than-
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additive manor (Coors and de Meester 2008; Kimberly and Salice 2013; Holmstrup et al. 2010), which may be difficult to predict when extrapolating from individual stressor exposures. For example, when exposed to temperatures at the upper end of the normal range many developmental processes, such as time-to-hatching and growth, occur more rapidly (Van der Schalie and Berry 1971; Heugens et al. 2001; Kimberly and Salice 2013). However, upon exposure to another stressor, these stimulatory effects may be reversed or ameliorated (Kimberly and Salice 2013). Interactions that are nonadditve are particularly important for ecological risk assessments (ERA) because they complicate extrapolation and predictive efforts. Because effects of toxicants under future climatic conditions may be more severe, challenges remain in producing adequate environmental quality standards and conservation plans (Moe et al. 2013). While the interactions between GCC and contaminants in the environment has received more attention recently (Schiedek et al. 2007; Noyes et al. 2009; Seeland et al. 2013), the implications of these interactions in the assessment of chemicals still need to be evaluated. One potential implication for climate change is its contribution towards temporal variability in stressor exposure history and how this may alter subsequent responses to stress (Fischer et al. 2013). Exposure to stressful conditions at one age or life cycle stage may restrict the ability to produce adaptive responses that would allow organisms to successfully respond do future, novel stress (De Block and Stoks 2005; Kishida et al. 2010; Salice et al. 2010). As an example, adult Physa pomilia exposed to cadmium during development displayed significant deficits in growth and reproduction as adults. Further, those snails that were exposed to cadmium developmentally and experienced novel cadmium and temperature stress as adults exhibited the most adverse effects in reproductive assays, including time to reproduction, clutch size, and even hatching success of the F2 generation (Kimberly and Salice, 2014). This type of temporal disconnect in the occurrence and effect of stressors is not commonly considered in multiple stressor studies but may, in fact, be a more common exposure scenario under global climate change. Understanding the effects of temporal variability in exposures to toxicants and climatic stressors can be interpreted along two different perspectives: toxicant-induced climate susceptibility (TICS), where toxicant exposure increases vulnerability to subsequent changes in climatic conditions, or climate-induced toxicant sensitivity (CITS), where exposure to a climate-related stressors renders organisms more sensitive to toxicant exposure (Hooper et al. 2013). Toxicant-induced climate sensitivity involves alterations caused by toxicant exposure that impact the ability of an organism to acclimate to a GCC related
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stressor such as temperature. Alternatively, climateinduced toxicant sensitivity scenarios produce altered or increased toxicity of chemicals as a result of exposure to changes in climate related conditions. Neither toxicantinduced nor climate-induced effects may be predicted from controlled laboratory studies in which these interactions are not explicitly evaluated (Hooper et al. 2013). Furthermore, environmental changes and pollution events may place populations at the edge of their physiological tolerance range, where the greatest risk exists (Parmesan 2006; Heugens et al. 2001, 2003; Tagatz 1969; Becker and Genoway 1979). Populations that can acclimate to fluctuations in environmental conditions increase their likelihood for persistence; however, even if populations are successful in acclimating to a specific stressor, the cost of that acclimation may leave animals more vulnerable to novel stressors or environmental conditions (Parmesan 2006). Aquatic systems may be particularly susceptible to GCC impacts because many aquatic species are range-restricted and lack long-range dispersal mechanisms that would allow them to tolerate changing conditions. Even if an adult stage is particularly mobile, other life stages, especially developmental stages, may have limited mobility. Freshwater gastropods, for example, have very limited dispersal capabilities and therefore, may be exposed to multiple (and temporally varying) stressors without relief by means other than physiological tolerance. They are also important components of freshwater ecosystems, contributing heavily to both community function and structure (Anderson and Smith 2000). Further, almost a quarter of known gastropod species are considered imperiled (Ricciardi and Rasmussen 1990). Thus, freshwater gastropods may be particularly susceptible to GCC related stressors and are useful models for understanding stressor interactions and the potential impacts at higher levels of biological organization. In the present study, we focused on the perspective that climate acclimation can impact responses to toxicant exposure and specifically tested the climate-induced toxicant sensitivity hypothesis. Our objective was to increase understanding of how previous acclimation to one stressor, in this case higher temperature, influences response to subsequent exposure to the model environmental toxicant, cadmium. Temperature has been the focus on an impressive breadth of study, especially in the context of temperature-toxicant interactions (Heugens et al. 2003). Here we build on previous research and focus our efforts on understanding how temperature and temperature fluctuation impact responses to cadmium. This is a potentially important consideration as organisms in the wild can frequently experience multiple, fluctuating stressors. Further, rapid temperature changes represent a scenario that may be more common with continued climate change. We
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hypothesized that (1) acclimation to a higher temperature would increase sensitivity to cadmium and (2) that snails acclimated to a low, optimal temperature and exposed to cadmium at a higher, more stressful temperature will experience the most adverse effect. While a considerable number of studies have evaluated impacts of co-exposures to elevated temperature and toxicants, few have explicitly considered temporal variability in the occurrence of different stressors.
Methods Study species The family Physidae is a globally distributed group of aquatic, pulmonate gastropod mollusks, which includes the genus Physa (Wethington and Lydeard 2007). These snails are characterized by a relatively large aperture, sinistral coiling, pointed spire, and no operculum. Physa sp. consume periphyton and algae but also graze detritus. In West Texas and other parts of the Southwest U.S, Physa pomilia (Say) is among the more abundant freshwater gastropod species. The primary habitat includes eutrophic ephemeral playa wetlands and slow moving streams. Adults from the field typically range from 3 to 11 mm in length and have light brown coloration, with speckling or striping. Laboratory acclimation conditions Approximately 180 adult P. pomilia were collected from a playa wetland near Abernathy, Texas, in October 2012, and allowed to acclimate to laboratory conditions for three weeks prior to being introduced into experimental conditions. All snails were housed in an incubator (BOD/low temperature, VMR; 12:12 light:dark cycle) at 22° C (± 1° C) in reconstituted, moderately hard water (60 lg/L CaSO4, 60 lg/L MgSO4, 4 lg/L KCl, 98 lg/L NaHCO3; henceforth lab water) in 20-L glass aquaria and fed cooked romaine lettuce, ad libitum (Kimberly and Salice 2012). Complete water changes of acclimation containers were conducted once a week. The water quality of lab water used to acclimate and house snails was comparable to that of the collection site water with total conductivity of approximately 350 ls/cm, pH in the range of 7.5. Additionally, the temperature recorded from bodies of water near the collection sites was 23.5° C (± 2° C; n = 9). Experimental conditions For the current experiment, all cadmium (Cd2?) test solutions (150 and 300 lg/L) were created by serial dilution from a 100 mg/L of dissolved Cd (cadmium chloride,
99.00 ? %, Sigma-Aldrich, St. Louis, MO) stock solution with 3 % nitric acid (for stabilization) and lab water. At the above concentrations, cadmium was chosen as a model stressor, as these represent concentrations greater than what is commonly found in the environment. Cadmium concentrations in natural waters are typically \1 lg/L, but can range up to 10 lg/L (World Bank Group 1999) and can reach as high as [40 lg/L in heavily contaminated sites (Environmental Integrity Project and Earthjustice 2010). Exposures followed a static-renewal design with 100 % renewal of solutions every 4 days. After water changes, water samples were collected from newly created solutions for cadmium concentration verification by atomic absorption spectrophotometry (Solaar-AA, Thermo Scientific) using flame ionization. Measured concentrations were 139 (± 6.23 lg/L; mean ± std. error, n = 5) and 283 (± 13.8 lg/L; mean ± std. error, n = 5). Since measured cadmium concentrations were within 10 % of nominal, we used nominal concentrations throughout. After the 3 week laboratory acclimation period, 180 adult snails were divided into 36–500 mL BPA-free Glad LockwareTM plastic containers with five snails each and filled with 450 mL lab water. Snails were fed cooked romaine lettuce, ad libitum. Those 36 containers were then divided between two temperature treatments, 18 containers each at temperatures of 22 and 28 °C. Experiments in our lab have shown that P. pomilia reared at 22° C hatch, grow, and reproduce better than snails reared at 30° C or higher, with mortality increasing at temperatures above 30 °C (e.g., Kimberly and Salice 2013). We used a ‘‘control’’ temperature of 22° C and a high temperature of 28° C which encompass the range of relatively optimal to a somewhat stressful condition. P. pomilia collected for this study are often found in shallow playa wetlands of West Texas where water temperatures closely track air temperatures and, thus, experience elevated water temperatures. For example, Sublette and Sublette (1967) described playa lakes near Eastern New Mexico and West Texas as having water temperatures near 33° C. As a result, the high temperature used in this study represents a reasonable high-end scenario but one that is also increasingly likely given climate change projections. In separate incubators (12:12 light:dark cycle), snails were acclimated at their respective temperature for 4 weeks. We recorded length at the beginning of the study by measuring the spire to the bottom tip of the aperture with digital calipers. Starting adult shell lengths were 4.16 ± 0.33 mm. We also recorded survival and number of egg masses produced during the temperature acclimation period. Both endpoints were checked daily. After 4 weeks, a cadmium challenge was conducted in a full factorial design where, for example, snails initially reared at 22 °C were either continued at 22 °C (control) or exposed to low or high cadmium (150 or 300 lg/L,
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D. A. Kimberly, C. J. Salice Fig. 1 Schematic showing the experimental design meant to evaluate the effects acclimation temperature and temperature switch in cadmium sensitivity. Adult snails were first acclimated to one of two temperatures, 22 or 28 °C for 28 days. At the end of the acclimation period, there were additional exposures to low (150 lg/L) and high (300 lg/L) cadmium at acclimation and switched temperatures in a full factorial design. Switched temperature exposure included, for example, acclimating to 28 °C then exposed to low or high cadmium at 22 °C. Mortality and egg mass production was observed during the acclimation phase while mortality was observed twice daily during the 14 day exposure phase
respectively) at 22 °C or switched from 22 to 28 °C and then exposed to either control (no added Cd), low, or high cadmium (Fig. 1). This full factorial including a temperature switch produced 12 final treatments with three replicates of five adult snails each. During the cadmium exposure phase, snails were fed cooked romaine lettuce every third day with a full water change and cadmium renewal occurring on the fourth day. Observations of mortality were taken twice daily over a 14 day period. Statistical analysis Egg mass production per replicate during the acclimation phase was analyzed using a mixed effects model to take into account that observations were not independent through time (Zuur et al. 2009). Temperature treatments were treated as fixed factors while time was included as a random factor. The model was constructed using the nlme package in R (R-development core team 2010) and the resulting model was then analyzed with analysis of variance. Survival analyses were performed on all mortality data, including the acclimation phase, using the SURVIVAL and KMsurv (Venables and Ripley 2002) packages for R. Survival analyses are time-to event regression methods that are well suited to mortality data (Newman and Aplin 1992). The logrank test (aka Mantel-Cox test) was used to compare survival curves between treatments, which computes observed and expected number of events at each time point for each group. The expected values are
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subtracted from observed values for each time point and then summed across all time points. In the analysis, individuals that survived the duration of the experiment were statistically accounted for through censoring. Through the 14-day period, individual observations (snails) were censored from all treatments except the 28-22H treatment, where complete mortality was reached. Survival analyses were conducted among each cadmium treatment (0, low, and high Cd) but were also done as pairwise comparisons by temperature within each concentration. Results were considered significant at the p B0.05 level and R version 2.12.2 was used for all analyses (R-development core team 2010).
Results During the acclimation phase, there was no effect of temperature on mortality of snails (P = 0.328), with snails from both temperatures having greater than 92 % survival. Additionally, there was no difference in egg mass production per day between snails at 22 or 28 °C (2.40 ± 0.56 and 2.19 ± 0.95 mean egg mass/day ± std. error, respectively; F1,36 = 0.637, P = 0.128). During the cadmium challenge, mortality increased through time in snails from both cadmium concentrations but there was overall greater mortality at the higher cadmium concentration (Fig. 2). Within each cadmium challenge group, snails in the temperature switch treatments
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snails was not different, except between 28 °C acclimation treatments and 22–28 °C treatments, where mortality was greater for snails acclimated at 28 °C (P = 0.040). Among temperature treatments in the low cadmium challenge, snails acclimated at 22 °C and then exposed to low cadmium at 22 °C did not have different mortality compared to snails acclimated at 28 °C and then exposed to the low cadmium challenge at 28 °C (P = 0.100). However, there was significantly lower mortality in snails from the low cadmium challenge at 22 °C compared to snails from the low cadmium challenge with a temperature switch from 22 to 28 °C (P \ 0.001). Mortality was nearly different between low cadmium challenge at constant 22 °C and low cadmium challenge at 28–22 °C (P = 0.061). Snails exposed to low cadmium and acclimated at a constant 28 °C did not have different mortality than either temperature switch treatment (22–28 °C, P = 0.326; 28–22 °C, P = 0.890). Again, snails from the two temperature switch treatments exposed to the low cadmium challenge did not differ in sensitivity to cadmium (P = 0.358). In the high cadmium challenge, mortality of snails acclimated to 22 °C then exposed to high cadmium at 22 °C was not different than the snails acclimated to 28 °C and exposed to high cadmium at 28 °C (P = 0.092). However, mortality was significantly less for snails acclimated to 22 °C and exposed to high cadmium at 22 °C than snails from both temperature switch treatments (22–28 °C, P \ 0.001; 28–22 °C, P \ 0.001). Additionally, snail mortality was significantly lower in snails acclimated to 28 °C and exposed to high cadmium at 28 °C compared to snails from both of the temperature switch treatments that were exposed to the high cadmium challenge (22–28 °C, P \ 0.001; 28–22 °C, P \ 0.001). Within the treatments that experienced a temperature switch only, snails from the 28–22 °C switch control treatment had significantly higher mortality than the 22–28 °C switch control treatment (P = 0.019).
1 A - Control
Percent of mortality
22C 28C 22-28C 28-22C
0
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B - Low Cd
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C - High Cd
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Fig. 2 Proportion mortality of snails exposed to cadmium and temperature after an acclimation period at specific temperatures. Mortality was observed twice daily over a 14-day period. Panel A includes responses from control treatments, Panel B includes responses from low cadmium treatments, and Panel C shows responses from high cadmium treatments. Panel B represents responses from treatments acclimated to 28 °C. Error bars represent on standard error
had the greatest mortality, with those in the 28–22 °C-high cadmium exposure reaching complete mortality. Snails in the constant 22 °C treatment had the next highest mortality, except at the low cadmium exposure. In both constant and switched temperature only treatments, mortality of
Discussion Global climate change is an increasingly important issue in applied ecology with direct, profound effects documented for a variety of taxa. While there are clear concerns of GCC related stressors, wildlife also experience other substantial anthropogenic stress, including environmental contamination. Little is known, however, as to how the exposure to either GCC stressors or contaminants will modify responses to the other. Two recent hypotheses that can be used to help explore the potential interactions include toxicantinduced climate sensitivity (TICS) and climate-induced toxicant sensitivity (CITS) (Hooper et al. 2013). We were
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interested in how acclimating to climate change related stress influenced sensitivity to toxicant exposure in an adult freshwater snail; a specific test of the CITS hypothesis. Using cadmium as a model environmental toxicant, we found that while cadmium sensitivity was highest when acclimation temperatures were switched before cadmium exposure, sensitivity was not predictable based solely on pre-exposure to different temperatures. Overall, we found that more than increased temperature alone, when snails experienced a switch from acclimation temperature to a different temperature combined with toxicant challenge they were most sensitive. This points to the importance of temporal patterns in the occurrence of different stressors and the fact that fluctuations in environmental conditions, especially in short timeframes, may be particularly stressful to natural populations. In general, mortality increased with cadmium exposure duration and concentration but animals acclimated to the higher temperature were not more sensitive to the cadmium challenge than animals at the lower temperature except at the high cadmium challenge concentration. Because survival and egg mass production were not different during the acclimation phase for snails at 28 °C and 22 °C, and because mortality did not differ between snails that were reared and exposed to the cadmium challenges at the same temperature, it seems likely that the higher temperature was not overtly stressful to snails and that they acclimated successfully. Abrupt shifts to the higher temperature, however, resulted in significant stress as indicated by greater mortality in snails exposed to 28 °C after 22 °C (without cadmium). Further evidence that the temperature switch was highly stressful was that snails acclimated at 28 °C and then switched to 22 °C and exposed to high cadmium, exhibited the highest mortality, even higher than the 22–28 °C-high cadmium treatment. This result is interesting, as it indicates a potential cost of acclimation to the high temperature treatment. Thus, snails acclimated at a higher temperature and then exposed to cadmium at the lower temperature (28–22 °C) apparently were more vulnerable than snails that were acclimated at a lower temperature and then switched to the higher temperature and exposed to cadmium (22–28 °C). Because many stressors vary temporally (Fischer et al. 2013), it is likely that organisms will experience multiple exposures that vary over the course of their life span. Any experience during one stage may impart a tolerance or sensitivity to that same stressor if experienced again later in the life cycle. This scenario is likely true in cases where species have short generation times, or where chemical concentrations vary (e.g., pesticide applications, runoff events), where climate parameters change abruptly, or organism inhabit areas near thermal effluent discharge
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(Brett 1952, Kita et al. 1996). More specifically, projections of climate change suggest there will likely be increases in extreme events and overall climate variability (IPCC 2007). Therefore, the environmental conditions that animals acclimate to could quickly and sharply change (Feldmeth et al. 1974) and our data suggest that these changes in environmental conditions can increase the vulnerability of populations. Importantly, the magnitude and duration of effects on natural populations will be a function of the specific temporal pattern of stressor occurrence (e.g., how quickly temperatures can change) as well as adaptive responses and dispersal among habitats both of which can ameliorate negative stressor effects. In our study, it is important to note that the difference in temperatures was only 6 degrees centigrade and that P. pomilia is a passive disperser. Hence, although our cadmium exposure concentrations were relatively high, it seems likely that freshwater gastropods may have increased susceptibility to novel stressors when temperatures change over even a fairly narrow range. Acclimating to one set of environmental stressors may influence responses to novel stressors, suggesting a potential cost of acclimating to the initial stressor (Adeyeme and Klerks 2012). This change in stressor tolerance resulting from previous exposures is not a new concept (Salice et al. 2010; Ward and Robinson 2005; Bossuyt and Janssen 2003; Van et al. 2013) and may be expressed as altered life history traits (Postma et al. 1995; Xie and Klerks 2004) or impaired tolerance to additional stressors (Meyer and Di Giulio 2003; Ward and Robinson 2005; Salice et al. 2010). While many of the cited examples are described in the context of adaptive costs, which can accrue over many generations, costs of acclimation within an individual organism’s lifespan can also occur. In acclimating to a new environment, energy may be shifted among different processes such as maintenance, growth and reproduction (e.g., Sokolova 2013). If maintenance costs are increased to counter toxicity, there is then less energy allocated to growth, reproduction, etc. There also may be less energy available to contest a new stressor. In a recent example, sheepshead minnows exposed to different salinities had different tolerance to acute copper challenges with those fish acclimated to low salinity showing the most copper sensitivity (Adeyeme and Klerks 2012). Conversely, the effects of acclimation to copper, zinc and cadmium by D. magna have been reported by several authors (LeBlanc 1982; Bodar et al. 1990; Stuhlbacher et al. 1992; Muyssen and Janssen 2001). For all these examples, a decrease in metal sensitivity was observed with increasing concentration when these concentrations were below lethal levels. In our study, the immediate effects of the higher temperature were not strong as snails reared at the higher temperature reproduced similarly to
Complex interactions between climate change
snails at the lower temperatures and there was no apparent impact on subsequent cadmium tolerance. Costs were only evident when there was a switch in temperatures concomitant with acute cadmium challenges. More specifically, those snails acclimated at 28 °C and exposed to cadmium at 22 °C experienced significantly greater mortality than snails acclimated to 22 °C and exposed to cadmium at 28 °C. Thus, as a result of acclimating to a high but sublethal stress (28 °C), snails were more sensitive to a novel stress (cadmium) when exposed outside of the acclimation conditions (22 °C). The importance of temperature and toxicant interaction has been the subject of numerous research efforts (Schiedek et al. 2007; Sokolova and Lannig 2008; Noyes et al. 2009), and in general, adverse effects of one stress increase as temperature or toxicant concentration increase (Heugens et al. 2003). It is well established that organisms living near or beyond their thermal tolerance exhibit greater sensitivity to other stressors, like toxicants, than those living closer to optimal conditions (Heugens et al. 2003; Gordon 2003). Our research also shows that even in temperatures that do not cause an overt stress response, if conditions change rapidly, an increased sensitivity to additional stressors can manifest. Importantly, the outcomes of the interactions between temperature and toxicant are condition specific which creates a challenge for developing a predictive model for adverse effects. An organized framework for understanding the interactive effects of climate change and toxicants has only recently been recognized (Stahl et al. 2013). One approach is to place interactive effects in the adverse outcome pathway framework (AOP, Hooper et al. 2013), which is a stepwise progression from exposure of any relevant stressor to adverse responses at all levels of organization. The CITS hypothesis is useful in this context for exploring the influence of climate change factors on toxicant exposures and may inform the eventual development of AOPs. Just as many toxicants share similar modes of action, many organisms may respond to stressor interactions similarly. For example, interactions between environmental temperature and metal toxicants strongly affect physiological tolerance to both stressors and can limit the survival and distribution of organisms. Interference with aerobic metabolism (Calow and Forbes 1998; Roff 2002), including energy demand (Po¨rtner 2002), oxygen supply (Lannig et al. 2006; 2008) and mitochondrial function (Cherkasova et al. 2006), forms a physiological basis for these interactions. Stressors that vary temporally and fluctuate rapidly may also compound these physiological effects. Another consideration is the importance of behavior when discussing an organisms’ ability to tolerate suites of stressors. Many aquatic organisms can behaviorally exploit stressor heterogeneity to increase survivability (Hutchison and Maness, 1979). For example, when first exposed to the insecticide malathion, Physa pomilia behaviorally escaped
the chemical by moving above the water line, similarly to a predator response (Salice and Kimberly, 2013). Regardless of the tolerance traits, contributing empirical data on specific interactions, like the ones in this study, help to perhaps reduce the uncertainties involved with understanding a changing world and can help guide future studies. Although the efforts described here are likely just the first step, they bring the reality of predictive risk assessments and efficient ecological management one step closer. Other ideas that need exploring, and that would be another step to increased realism, include the relative changes in population genetics as those dynamics strongly influence acclimation and adaptive processes to changing environments. In conclusion, we have demonstrated that exposure history plays an important role in how organisms respond to future stress. Specifically, snails that experienced a change in temperature responded to cadmium stress with increased sensitivity compared to snails that were exposed to cadmium with no change in temperature. Hence, the CITS hypothesis may need modification to encompass temperature change as an important source of vulnerability to contaminants. This result is particularly important in the context of climate change as many systems will experience significant temperature fluctuations with potential for more extreme conditions. Thus, in order to adequately predict adverse effects, a better understanding of how stressor exposures mediate sensitivity to other stressors appears critical. Acknowledgments DAK would like to thank Texas Tech University, The Institute of Environmental and Human Health (TIEHH) and the Conchologists of America for funding. This manuscript was improved with helpful comments from two anonymous reviewers. Conflict of interest of interest.
The authors declare that they have no conflict
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Lannig G, Cherkasov A, Portner H, Bock C, Sokolova I (2008) Cadmium-dependent oxygen limitation affects temperature tolerance in eastern oysters (Crassostrea virginica Gmelin). Am J Physiol Regul Integr Comp Physiol 294:1338–1346 LeBlanc GA (1982) Laboratory investigations into the development of resistance of Daphnia magna (Straus) to environmental pollutants. Environ. Pollut. (Series A) 27:309–322 Meyer JN, Di Giulio RT (2003) Heritable adaptation and fitness costs in killifish (Fundulus heteroclitus) inhabiting a polluted estuary. Ecologicla Applica 13:490–503 Moe S, De Schamphelaere K, Clements W, Sorensen M, Van den Brink P, Liess M (2013) Combined and interactive effects of global climate change and toxicants on populations and communities. Environ Toxicol Chem 32:49–61 Muyssen BA, Janssen CR (2001) Multi-generation zinc acclimation and tolerance in Daphnia magna: implications for water quality guidelines and ecological risk assessment. Environ Toxicol Chem 20:2053–2060 Newman MC, Aplin MS (1992) Enhancing toxicity data interpretation and prediction of ecological risk with survival time modeling: an illustration using sodium chloride toxicity to mosquitofish (Gambusia holbrooki). Aquat Toxicol 23:85–96 Noyes PD, McElwee MK, Miller HD, Clark BW, Van Tiem LA, Walcott KC, Erwin KN, Levin ED (2009) The toxicology of climate change: environmental contaminants in a warming world. Environ Int 35:971–986 Parmesan C (2006) Ecological and evolutionary responses to recent climate change. Annu Rev Ecol Evol Syst 37:637–669 Po¨rtner HO (2002) Climate variations and the physiological basis of temperature dependent biogeography: systemic to molecular hierarchy of thermal tolerance in animals. Comp Biochem Physiol A: Mol Integr Physiol 132:739–761 Postma J, Vankleunen A, Admiraal W (1995) Alterations in life-history traits of chironomus-riparius (diptera) obtained from metalcontaminated rivers. Arch Environ Contam Toxicol 29:469–475 Ricciardi A, Rasmussen J (1990) Extinction rates of North American freshwater fauna. Conserv Biol 13:1220–1222 Roff DA (2002) Life history evolution; Sinauer, Sunderland Salice CJ, Kimberly DA (2013) Envionrmentally relevant concentrations of a common insecticide increase predation risk in a freshwater gastropod 22:42–49 Salice CJ, Anderson T, Roesijadi G (2010) Adaptive responses and latent costs of multigeneration cadmium exposure in parasite resistant and susceptible strains of a freshwater snail. Ecotoxicology 19:1466–1475 Seeland A, Albrand J, Oehlmann J, Mu¨ller R (2013) Life stagespecific effects of the fungicide pyrimethanil and temperature on the snail Physella acuta (Draparnaud, 1805) disclose the pitfalls for the aquatic risk assessment under global climate change. Environ Pollut 174:1–9 Schiedek D, Sundelin B, Readman JW, Macdonald RW (2007) Interactions between climate change and contaminants. Mar Pollut Bull 54:1845–1856 Sokolova IM (2013) Energy-limited tolerance to stress as a conceptual framework to integrate the effects of mulitple stressors. Integr Comp Biol 53:597–608 Sokolova IM, Lannig G (2008) Interactive effects of metal pollution and temperature on metabolism in aquatic ectotherms: implications of global climate change. Clim Res 37:181–201 Stahl RG, Hooper MJ, Balbus JM, Clements W, Fritz A, Gouin T, Helm R, Hickey C, Landis W, Moe SJ (2013) The influence of global climate change on the scientific foundations and applications of environmental toxicology and chemistry: introduction to a SETAC international workshop. Environ Toxicol Chem 32:13–19
Complex interactions between climate change Stuhlbacher A, Bradley MC, Naylor C, Calow P (1992) Induction of cadmium tolerance in two clones of Daphnia magna Straus. Comp Biochem Physiol C 101:571–577 Sublette JE, Sublette MS (1967) The limnology of playa lakes on the Llano Estacado, New Mexico and Texas. Southwest Nat 12:369–406 Tagatz ME (1969) Some relations of temperature acclimation and salinity to thermal tolerance of the blue crab, Callinectus sapidus. Trans Amer Fish Soc 98:713–716 R development core team (2010) R: a language and environment for statistical computing. R foundation for statistical computig; Vienna, Austria Van der Schalie H, Berry HE (1971) Effects of temperature on groth and reproduction in aquatic snails. US Environemntal Protectoin Agency; Washington, DC Van KD, Janssens L, Debecker S, De Jonge M, Lambert P, Nilsson¨ rtman V, Bervoets L, Stoks R (2013) Susceptibility to a metal O
under global change warming is shaped by thermal adaptation along a latitudinal gradient. Glob Change Biol. doi:10.1111/gcb. 12243 Venables WN, Ripley BD (2002) Modern applied statistics with S; Springer, New York Ward T, Robinson W (2005) Evolution of cadmium resistance in Daphnia magna. Environ Toxicol Chem 24:2341–2349 Wethington AR, Lydeard C (2007) A molecular phylogeny of Physidae (Gastropoda: basommatophora) based on mitochondrial DNA sequences. J Molluscan Stud 73:241–257 World Bank Group (1999) Polution prevetnion and abatement handbook; Washington, DC Xie L, Klerks P (2004) Fitness cost of resistance to cadmium in the least killifish (Heterandria formosa). Environ Toxicol Chem 23:1499–1503 Zuur A, Leno EN, Walker N, Saveliev AA, Smith GM (2009) Mixed effects models and extensions in ecology with R. Springer
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Complex interactions between climate change and toxicants: evidence that temperature variability increases sensitivity to cadmium David A. Kimberly • Christopher J. Salice
Accepted: 24 February 2014 Ó Springer Science+Business Media New York 2014
Abstract The Intergovernmental Panel on Climate Change projects that global climate change will have significant impacts on environmental conditions including potential effects on sensitivity of organisms to environmental contaminants. The objective of this study was to test the climate-induced toxicant sensitivity (CITS) hypothesis in which acclimation to altered climate parameters increases toxicant sensitivity. Adult Physa pomilia snails were acclimated to a near optimal 22 °C or a high-normal 28 °C for 28 days. After 28 days, snails from each temperature group were challenged with either low (150 lg/L) or high (300 lg/L) cadmium at each temperature (28 or 22 °C). In contrast to the CITS hypothesis, we found that acclimation temperature did not have a strong influence on cadmium sensitivity except at the high cadmium test concentration where snails acclimated to 28 °C were more cadmium tolerant. However, snails that experienced a switch in temperature for the cadmium challenge, regardless of the switch direction, were the most sensitive to cadmium. Within the snails that were switched between temperatures, snails acclimated at 28 °C and then exposed to high cadmium at 22 °C exhibited significantly greater mortality than those snails acclimated to 22 °C and then exposed to cadmium at 28 °C. Our results point to the importance of temperature variability in increasing toxicant
D. A. Kimberly C. J. Salice Department of Environmental Toxicology, The Institute of Environmental and Human Health, Texas Tech University, 1207 Gilbert Drive, Lubbock, TX 79416, USA D. A. Kimberly (&) Department of Biology, Westminster College, 215 Meldrum Science Center, 1840 S 1300 East, Salt Lake City, UT 84105, USA e-mail: [email protected]; [email protected]
sensitivity but also suggest a potentially complex cost of temperature acclimation. Broadly, the type of temporal stressor exposures we simulated may reduce overall plasticity in responses to stress ultimately rendering populations more vulnerable to adverse effects. Keywords Climate change Temperature Cadmium Physa pomilia Stressor variability CITS
Introduction The Intergovernmental Panel on Climate Change (IPCC) projects that global climate change (GCC) will have significant impacts on environmental conditions such as higher ocean acidity, changes in precipitation patterns, and higher mean temperature (IPCC 2007). While the projections include significant uncertainties and regional variation, in general, increased frequency in extreme weather events such as heat waves, droughts, and storms is also expected. Substantial uncertainty in understanding the impacts of GCC extends to how climate changes will interact with other anthropogenic stress factors such as chemical contaminants. Concerns include the role of GCC in determining environmental concentration of contaminants (Gouin et al. 2013) as GCC is expected to have a substantial effect on release, fate, behavior, and exposure of toxicants (Noyes et al. 2009). As well, the sensitivity of organisms to environmental toxicants could be drastically modified by GCC-induced changes in environmental conditions (Hooper et al. 2013). Hence there is the potential for GCC to modify both exposure and responses to environmental contaminants. The combination of chemical and non-chemical stressors like those related to GCC may act in a greater-than-
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additive manor (Coors and de Meester 2008; Kimberly and Salice 2013; Holmstrup et al. 2010), which may be difficult to predict when extrapolating from individual stressor exposures. For example, when exposed to temperatures at the upper end of the normal range many developmental processes, such as time-to-hatching and growth, occur more rapidly (Van der Schalie and Berry 1971; Heugens et al. 2001; Kimberly and Salice 2013). However, upon exposure to another stressor, these stimulatory effects may be reversed or ameliorated (Kimberly and Salice 2013). Interactions that are nonadditve are particularly important for ecological risk assessments (ERA) because they complicate extrapolation and predictive efforts. Because effects of toxicants under future climatic conditions may be more severe, challenges remain in producing adequate environmental quality standards and conservation plans (Moe et al. 2013). While the interactions between GCC and contaminants in the environment has received more attention recently (Schiedek et al. 2007; Noyes et al. 2009; Seeland et al. 2013), the implications of these interactions in the assessment of chemicals still need to be evaluated. One potential implication for climate change is its contribution towards temporal variability in stressor exposure history and how this may alter subsequent responses to stress (Fischer et al. 2013). Exposure to stressful conditions at one age or life cycle stage may restrict the ability to produce adaptive responses that would allow organisms to successfully respond do future, novel stress (De Block and Stoks 2005; Kishida et al. 2010; Salice et al. 2010). As an example, adult Physa pomilia exposed to cadmium during development displayed significant deficits in growth and reproduction as adults. Further, those snails that were exposed to cadmium developmentally and experienced novel cadmium and temperature stress as adults exhibited the most adverse effects in reproductive assays, including time to reproduction, clutch size, and even hatching success of the F2 generation (Kimberly and Salice, 2014). This type of temporal disconnect in the occurrence and effect of stressors is not commonly considered in multiple stressor studies but may, in fact, be a more common exposure scenario under global climate change. Understanding the effects of temporal variability in exposures to toxicants and climatic stressors can be interpreted along two different perspectives: toxicant-induced climate susceptibility (TICS), where toxicant exposure increases vulnerability to subsequent changes in climatic conditions, or climate-induced toxicant sensitivity (CITS), where exposure to a climate-related stressors renders organisms more sensitive to toxicant exposure (Hooper et al. 2013). Toxicant-induced climate sensitivity involves alterations caused by toxicant exposure that impact the ability of an organism to acclimate to a GCC related
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stressor such as temperature. Alternatively, climateinduced toxicant sensitivity scenarios produce altered or increased toxicity of chemicals as a result of exposure to changes in climate related conditions. Neither toxicantinduced nor climate-induced effects may be predicted from controlled laboratory studies in which these interactions are not explicitly evaluated (Hooper et al. 2013). Furthermore, environmental changes and pollution events may place populations at the edge of their physiological tolerance range, where the greatest risk exists (Parmesan 2006; Heugens et al. 2001, 2003; Tagatz 1969; Becker and Genoway 1979). Populations that can acclimate to fluctuations in environmental conditions increase their likelihood for persistence; however, even if populations are successful in acclimating to a specific stressor, the cost of that acclimation may leave animals more vulnerable to novel stressors or environmental conditions (Parmesan 2006). Aquatic systems may be particularly susceptible to GCC impacts because many aquatic species are range-restricted and lack long-range dispersal mechanisms that would allow them to tolerate changing conditions. Even if an adult stage is particularly mobile, other life stages, especially developmental stages, may have limited mobility. Freshwater gastropods, for example, have very limited dispersal capabilities and therefore, may be exposed to multiple (and temporally varying) stressors without relief by means other than physiological tolerance. They are also important components of freshwater ecosystems, contributing heavily to both community function and structure (Anderson and Smith 2000). Further, almost a quarter of known gastropod species are considered imperiled (Ricciardi and Rasmussen 1990). Thus, freshwater gastropods may be particularly susceptible to GCC related stressors and are useful models for understanding stressor interactions and the potential impacts at higher levels of biological organization. In the present study, we focused on the perspective that climate acclimation can impact responses to toxicant exposure and specifically tested the climate-induced toxicant sensitivity hypothesis. Our objective was to increase understanding of how previous acclimation to one stressor, in this case higher temperature, influences response to subsequent exposure to the model environmental toxicant, cadmium. Temperature has been the focus on an impressive breadth of study, especially in the context of temperature-toxicant interactions (Heugens et al. 2003). Here we build on previous research and focus our efforts on understanding how temperature and temperature fluctuation impact responses to cadmium. This is a potentially important consideration as organisms in the wild can frequently experience multiple, fluctuating stressors. Further, rapid temperature changes represent a scenario that may be more common with continued climate change. We
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hypothesized that (1) acclimation to a higher temperature would increase sensitivity to cadmium and (2) that snails acclimated to a low, optimal temperature and exposed to cadmium at a higher, more stressful temperature will experience the most adverse effect. While a considerable number of studies have evaluated impacts of co-exposures to elevated temperature and toxicants, few have explicitly considered temporal variability in the occurrence of different stressors.
Methods Study species The family Physidae is a globally distributed group of aquatic, pulmonate gastropod mollusks, which includes the genus Physa (Wethington and Lydeard 2007). These snails are characterized by a relatively large aperture, sinistral coiling, pointed spire, and no operculum. Physa sp. consume periphyton and algae but also graze detritus. In West Texas and other parts of the Southwest U.S, Physa pomilia (Say) is among the more abundant freshwater gastropod species. The primary habitat includes eutrophic ephemeral playa wetlands and slow moving streams. Adults from the field typically range from 3 to 11 mm in length and have light brown coloration, with speckling or striping. Laboratory acclimation conditions Approximately 180 adult P. pomilia were collected from a playa wetland near Abernathy, Texas, in October 2012, and allowed to acclimate to laboratory conditions for three weeks prior to being introduced into experimental conditions. All snails were housed in an incubator (BOD/low temperature, VMR; 12:12 light:dark cycle) at 22° C (± 1° C) in reconstituted, moderately hard water (60 lg/L CaSO4, 60 lg/L MgSO4, 4 lg/L KCl, 98 lg/L NaHCO3; henceforth lab water) in 20-L glass aquaria and fed cooked romaine lettuce, ad libitum (Kimberly and Salice 2012). Complete water changes of acclimation containers were conducted once a week. The water quality of lab water used to acclimate and house snails was comparable to that of the collection site water with total conductivity of approximately 350 ls/cm, pH in the range of 7.5. Additionally, the temperature recorded from bodies of water near the collection sites was 23.5° C (± 2° C; n = 9). Experimental conditions For the current experiment, all cadmium (Cd2?) test solutions (150 and 300 lg/L) were created by serial dilution from a 100 mg/L of dissolved Cd (cadmium chloride,
99.00 ? %, Sigma-Aldrich, St. Louis, MO) stock solution with 3 % nitric acid (for stabilization) and lab water. At the above concentrations, cadmium was chosen as a model stressor, as these represent concentrations greater than what is commonly found in the environment. Cadmium concentrations in natural waters are typically \1 lg/L, but can range up to 10 lg/L (World Bank Group 1999) and can reach as high as [40 lg/L in heavily contaminated sites (Environmental Integrity Project and Earthjustice 2010). Exposures followed a static-renewal design with 100 % renewal of solutions every 4 days. After water changes, water samples were collected from newly created solutions for cadmium concentration verification by atomic absorption spectrophotometry (Solaar-AA, Thermo Scientific) using flame ionization. Measured concentrations were 139 (± 6.23 lg/L; mean ± std. error, n = 5) and 283 (± 13.8 lg/L; mean ± std. error, n = 5). Since measured cadmium concentrations were within 10 % of nominal, we used nominal concentrations throughout. After the 3 week laboratory acclimation period, 180 adult snails were divided into 36–500 mL BPA-free Glad LockwareTM plastic containers with five snails each and filled with 450 mL lab water. Snails were fed cooked romaine lettuce, ad libitum. Those 36 containers were then divided between two temperature treatments, 18 containers each at temperatures of 22 and 28 °C. Experiments in our lab have shown that P. pomilia reared at 22° C hatch, grow, and reproduce better than snails reared at 30° C or higher, with mortality increasing at temperatures above 30 °C (e.g., Kimberly and Salice 2013). We used a ‘‘control’’ temperature of 22° C and a high temperature of 28° C which encompass the range of relatively optimal to a somewhat stressful condition. P. pomilia collected for this study are often found in shallow playa wetlands of West Texas where water temperatures closely track air temperatures and, thus, experience elevated water temperatures. For example, Sublette and Sublette (1967) described playa lakes near Eastern New Mexico and West Texas as having water temperatures near 33° C. As a result, the high temperature used in this study represents a reasonable high-end scenario but one that is also increasingly likely given climate change projections. In separate incubators (12:12 light:dark cycle), snails were acclimated at their respective temperature for 4 weeks. We recorded length at the beginning of the study by measuring the spire to the bottom tip of the aperture with digital calipers. Starting adult shell lengths were 4.16 ± 0.33 mm. We also recorded survival and number of egg masses produced during the temperature acclimation period. Both endpoints were checked daily. After 4 weeks, a cadmium challenge was conducted in a full factorial design where, for example, snails initially reared at 22 °C were either continued at 22 °C (control) or exposed to low or high cadmium (150 or 300 lg/L,
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D. A. Kimberly, C. J. Salice Fig. 1 Schematic showing the experimental design meant to evaluate the effects acclimation temperature and temperature switch in cadmium sensitivity. Adult snails were first acclimated to one of two temperatures, 22 or 28 °C for 28 days. At the end of the acclimation period, there were additional exposures to low (150 lg/L) and high (300 lg/L) cadmium at acclimation and switched temperatures in a full factorial design. Switched temperature exposure included, for example, acclimating to 28 °C then exposed to low or high cadmium at 22 °C. Mortality and egg mass production was observed during the acclimation phase while mortality was observed twice daily during the 14 day exposure phase
respectively) at 22 °C or switched from 22 to 28 °C and then exposed to either control (no added Cd), low, or high cadmium (Fig. 1). This full factorial including a temperature switch produced 12 final treatments with three replicates of five adult snails each. During the cadmium exposure phase, snails were fed cooked romaine lettuce every third day with a full water change and cadmium renewal occurring on the fourth day. Observations of mortality were taken twice daily over a 14 day period. Statistical analysis Egg mass production per replicate during the acclimation phase was analyzed using a mixed effects model to take into account that observations were not independent through time (Zuur et al. 2009). Temperature treatments were treated as fixed factors while time was included as a random factor. The model was constructed using the nlme package in R (R-development core team 2010) and the resulting model was then analyzed with analysis of variance. Survival analyses were performed on all mortality data, including the acclimation phase, using the SURVIVAL and KMsurv (Venables and Ripley 2002) packages for R. Survival analyses are time-to event regression methods that are well suited to mortality data (Newman and Aplin 1992). The logrank test (aka Mantel-Cox test) was used to compare survival curves between treatments, which computes observed and expected number of events at each time point for each group. The expected values are
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subtracted from observed values for each time point and then summed across all time points. In the analysis, individuals that survived the duration of the experiment were statistically accounted for through censoring. Through the 14-day period, individual observations (snails) were censored from all treatments except the 28-22H treatment, where complete mortality was reached. Survival analyses were conducted among each cadmium treatment (0, low, and high Cd) but were also done as pairwise comparisons by temperature within each concentration. Results were considered significant at the p B0.05 level and R version 2.12.2 was used for all analyses (R-development core team 2010).
Results During the acclimation phase, there was no effect of temperature on mortality of snails (P = 0.328), with snails from both temperatures having greater than 92 % survival. Additionally, there was no difference in egg mass production per day between snails at 22 or 28 °C (2.40 ± 0.56 and 2.19 ± 0.95 mean egg mass/day ± std. error, respectively; F1,36 = 0.637, P = 0.128). During the cadmium challenge, mortality increased through time in snails from both cadmium concentrations but there was overall greater mortality at the higher cadmium concentration (Fig. 2). Within each cadmium challenge group, snails in the temperature switch treatments
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snails was not different, except between 28 °C acclimation treatments and 22–28 °C treatments, where mortality was greater for snails acclimated at 28 °C (P = 0.040). Among temperature treatments in the low cadmium challenge, snails acclimated at 22 °C and then exposed to low cadmium at 22 °C did not have different mortality compared to snails acclimated at 28 °C and then exposed to the low cadmium challenge at 28 °C (P = 0.100). However, there was significantly lower mortality in snails from the low cadmium challenge at 22 °C compared to snails from the low cadmium challenge with a temperature switch from 22 to 28 °C (P \ 0.001). Mortality was nearly different between low cadmium challenge at constant 22 °C and low cadmium challenge at 28–22 °C (P = 0.061). Snails exposed to low cadmium and acclimated at a constant 28 °C did not have different mortality than either temperature switch treatment (22–28 °C, P = 0.326; 28–22 °C, P = 0.890). Again, snails from the two temperature switch treatments exposed to the low cadmium challenge did not differ in sensitivity to cadmium (P = 0.358). In the high cadmium challenge, mortality of snails acclimated to 22 °C then exposed to high cadmium at 22 °C was not different than the snails acclimated to 28 °C and exposed to high cadmium at 28 °C (P = 0.092). However, mortality was significantly less for snails acclimated to 22 °C and exposed to high cadmium at 22 °C than snails from both temperature switch treatments (22–28 °C, P \ 0.001; 28–22 °C, P \ 0.001). Additionally, snail mortality was significantly lower in snails acclimated to 28 °C and exposed to high cadmium at 28 °C compared to snails from both of the temperature switch treatments that were exposed to the high cadmium challenge (22–28 °C, P \ 0.001; 28–22 °C, P \ 0.001). Within the treatments that experienced a temperature switch only, snails from the 28–22 °C switch control treatment had significantly higher mortality than the 22–28 °C switch control treatment (P = 0.019).
1 A - Control
Percent of mortality
22C 28C 22-28C 28-22C
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B - Low Cd
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C - High Cd
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Fig. 2 Proportion mortality of snails exposed to cadmium and temperature after an acclimation period at specific temperatures. Mortality was observed twice daily over a 14-day period. Panel A includes responses from control treatments, Panel B includes responses from low cadmium treatments, and Panel C shows responses from high cadmium treatments. Panel B represents responses from treatments acclimated to 28 °C. Error bars represent on standard error
had the greatest mortality, with those in the 28–22 °C-high cadmium exposure reaching complete mortality. Snails in the constant 22 °C treatment had the next highest mortality, except at the low cadmium exposure. In both constant and switched temperature only treatments, mortality of
Discussion Global climate change is an increasingly important issue in applied ecology with direct, profound effects documented for a variety of taxa. While there are clear concerns of GCC related stressors, wildlife also experience other substantial anthropogenic stress, including environmental contamination. Little is known, however, as to how the exposure to either GCC stressors or contaminants will modify responses to the other. Two recent hypotheses that can be used to help explore the potential interactions include toxicantinduced climate sensitivity (TICS) and climate-induced toxicant sensitivity (CITS) (Hooper et al. 2013). We were
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interested in how acclimating to climate change related stress influenced sensitivity to toxicant exposure in an adult freshwater snail; a specific test of the CITS hypothesis. Using cadmium as a model environmental toxicant, we found that while cadmium sensitivity was highest when acclimation temperatures were switched before cadmium exposure, sensitivity was not predictable based solely on pre-exposure to different temperatures. Overall, we found that more than increased temperature alone, when snails experienced a switch from acclimation temperature to a different temperature combined with toxicant challenge they were most sensitive. This points to the importance of temporal patterns in the occurrence of different stressors and the fact that fluctuations in environmental conditions, especially in short timeframes, may be particularly stressful to natural populations. In general, mortality increased with cadmium exposure duration and concentration but animals acclimated to the higher temperature were not more sensitive to the cadmium challenge than animals at the lower temperature except at the high cadmium challenge concentration. Because survival and egg mass production were not different during the acclimation phase for snails at 28 °C and 22 °C, and because mortality did not differ between snails that were reared and exposed to the cadmium challenges at the same temperature, it seems likely that the higher temperature was not overtly stressful to snails and that they acclimated successfully. Abrupt shifts to the higher temperature, however, resulted in significant stress as indicated by greater mortality in snails exposed to 28 °C after 22 °C (without cadmium). Further evidence that the temperature switch was highly stressful was that snails acclimated at 28 °C and then switched to 22 °C and exposed to high cadmium, exhibited the highest mortality, even higher than the 22–28 °C-high cadmium treatment. This result is interesting, as it indicates a potential cost of acclimation to the high temperature treatment. Thus, snails acclimated at a higher temperature and then exposed to cadmium at the lower temperature (28–22 °C) apparently were more vulnerable than snails that were acclimated at a lower temperature and then switched to the higher temperature and exposed to cadmium (22–28 °C). Because many stressors vary temporally (Fischer et al. 2013), it is likely that organisms will experience multiple exposures that vary over the course of their life span. Any experience during one stage may impart a tolerance or sensitivity to that same stressor if experienced again later in the life cycle. This scenario is likely true in cases where species have short generation times, or where chemical concentrations vary (e.g., pesticide applications, runoff events), where climate parameters change abruptly, or organism inhabit areas near thermal effluent discharge
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(Brett 1952, Kita et al. 1996). More specifically, projections of climate change suggest there will likely be increases in extreme events and overall climate variability (IPCC 2007). Therefore, the environmental conditions that animals acclimate to could quickly and sharply change (Feldmeth et al. 1974) and our data suggest that these changes in environmental conditions can increase the vulnerability of populations. Importantly, the magnitude and duration of effects on natural populations will be a function of the specific temporal pattern of stressor occurrence (e.g., how quickly temperatures can change) as well as adaptive responses and dispersal among habitats both of which can ameliorate negative stressor effects. In our study, it is important to note that the difference in temperatures was only 6 degrees centigrade and that P. pomilia is a passive disperser. Hence, although our cadmium exposure concentrations were relatively high, it seems likely that freshwater gastropods may have increased susceptibility to novel stressors when temperatures change over even a fairly narrow range. Acclimating to one set of environmental stressors may influence responses to novel stressors, suggesting a potential cost of acclimating to the initial stressor (Adeyeme and Klerks 2012). This change in stressor tolerance resulting from previous exposures is not a new concept (Salice et al. 2010; Ward and Robinson 2005; Bossuyt and Janssen 2003; Van et al. 2013) and may be expressed as altered life history traits (Postma et al. 1995; Xie and Klerks 2004) or impaired tolerance to additional stressors (Meyer and Di Giulio 2003; Ward and Robinson 2005; Salice et al. 2010). While many of the cited examples are described in the context of adaptive costs, which can accrue over many generations, costs of acclimation within an individual organism’s lifespan can also occur. In acclimating to a new environment, energy may be shifted among different processes such as maintenance, growth and reproduction (e.g., Sokolova 2013). If maintenance costs are increased to counter toxicity, there is then less energy allocated to growth, reproduction, etc. There also may be less energy available to contest a new stressor. In a recent example, sheepshead minnows exposed to different salinities had different tolerance to acute copper challenges with those fish acclimated to low salinity showing the most copper sensitivity (Adeyeme and Klerks 2012). Conversely, the effects of acclimation to copper, zinc and cadmium by D. magna have been reported by several authors (LeBlanc 1982; Bodar et al. 1990; Stuhlbacher et al. 1992; Muyssen and Janssen 2001). For all these examples, a decrease in metal sensitivity was observed with increasing concentration when these concentrations were below lethal levels. In our study, the immediate effects of the higher temperature were not strong as snails reared at the higher temperature reproduced similarly to
Complex interactions between climate change
snails at the lower temperatures and there was no apparent impact on subsequent cadmium tolerance. Costs were only evident when there was a switch in temperatures concomitant with acute cadmium challenges. More specifically, those snails acclimated at 28 °C and exposed to cadmium at 22 °C experienced significantly greater mortality than snails acclimated to 22 °C and exposed to cadmium at 28 °C. Thus, as a result of acclimating to a high but sublethal stress (28 °C), snails were more sensitive to a novel stress (cadmium) when exposed outside of the acclimation conditions (22 °C). The importance of temperature and toxicant interaction has been the subject of numerous research efforts (Schiedek et al. 2007; Sokolova and Lannig 2008; Noyes et al. 2009), and in general, adverse effects of one stress increase as temperature or toxicant concentration increase (Heugens et al. 2003). It is well established that organisms living near or beyond their thermal tolerance exhibit greater sensitivity to other stressors, like toxicants, than those living closer to optimal conditions (Heugens et al. 2003; Gordon 2003). Our research also shows that even in temperatures that do not cause an overt stress response, if conditions change rapidly, an increased sensitivity to additional stressors can manifest. Importantly, the outcomes of the interactions between temperature and toxicant are condition specific which creates a challenge for developing a predictive model for adverse effects. An organized framework for understanding the interactive effects of climate change and toxicants has only recently been recognized (Stahl et al. 2013). One approach is to place interactive effects in the adverse outcome pathway framework (AOP, Hooper et al. 2013), which is a stepwise progression from exposure of any relevant stressor to adverse responses at all levels of organization. The CITS hypothesis is useful in this context for exploring the influence of climate change factors on toxicant exposures and may inform the eventual development of AOPs. Just as many toxicants share similar modes of action, many organisms may respond to stressor interactions similarly. For example, interactions between environmental temperature and metal toxicants strongly affect physiological tolerance to both stressors and can limit the survival and distribution of organisms. Interference with aerobic metabolism (Calow and Forbes 1998; Roff 2002), including energy demand (Po¨rtner 2002), oxygen supply (Lannig et al. 2006; 2008) and mitochondrial function (Cherkasova et al. 2006), forms a physiological basis for these interactions. Stressors that vary temporally and fluctuate rapidly may also compound these physiological effects. Another consideration is the importance of behavior when discussing an organisms’ ability to tolerate suites of stressors. Many aquatic organisms can behaviorally exploit stressor heterogeneity to increase survivability (Hutchison and Maness, 1979). For example, when first exposed to the insecticide malathion, Physa pomilia behaviorally escaped
the chemical by moving above the water line, similarly to a predator response (Salice and Kimberly, 2013). Regardless of the tolerance traits, contributing empirical data on specific interactions, like the ones in this study, help to perhaps reduce the uncertainties involved with understanding a changing world and can help guide future studies. Although the efforts described here are likely just the first step, they bring the reality of predictive risk assessments and efficient ecological management one step closer. Other ideas that need exploring, and that would be another step to increased realism, include the relative changes in population genetics as those dynamics strongly influence acclimation and adaptive processes to changing environments. In conclusion, we have demonstrated that exposure history plays an important role in how organisms respond to future stress. Specifically, snails that experienced a change in temperature responded to cadmium stress with increased sensitivity compared to snails that were exposed to cadmium with no change in temperature. Hence, the CITS hypothesis may need modification to encompass temperature change as an important source of vulnerability to contaminants. This result is particularly important in the context of climate change as many systems will experience significant temperature fluctuations with potential for more extreme conditions. Thus, in order to adequately predict adverse effects, a better understanding of how stressor exposures mediate sensitivity to other stressors appears critical. Acknowledgments DAK would like to thank Texas Tech University, The Institute of Environmental and Human Health (TIEHH) and the Conchologists of America for funding. This manuscript was improved with helpful comments from two anonymous reviewers. Conflict of interest of interest.
The authors declare that they have no conflict
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