Microb Ecol (2015) 69:245–253 DOI 10.1007/s00248-014-0505-3
MICROBIOLOGY OF AQUATIC SYSTEMS
Preliminary Evidence for the Organisation of a Bacterial Community by Zooplanktivores at the Top of an Estuarine Planktonic Food Web R. J. Wasserman & G. F. Matcher & T. J. F. Vink & P. W. Froneman
Received: 19 April 2014 / Accepted: 25 September 2014 / Published online: 10 October 2014 # Springer Science+Business Media New York 2014
Abstract As part of a larger investigation, the effect of apex predation on estuarine bacterial community structure, through trophic cascading, was investigated using experimental in situ mesocosms. Through either the removal (filtration) or addition of specific size classes of planktonic groups, four different trophic scenarios were established using estuarine water and its associated plankton. One such treatment represented a “natural” scenario in which stable apex predatory pressure was qualified. Water samples were collected over time from each of the treatments for bacterial community evaluation. These samples were assessed through pyrosequencing of the variable regions 4 and 5 of the bacterial 16S rRNA gene and analysed at the species operational taxonomic unit (OTU) level using a community procedure. The blue-green group dominated the samples, followed by Proteobacteria and Bacteroidetes. Samples were the most similar among treatments at the commencement of the experiment. While the bacterial communities sampled within each treatment changed over time, the deviation from initial appeared to be linked to the treatment trophic scenarios. The least temporal deviationfrom-initial in bacterial community was found within the stable apex predatory pressure treatment. These findings are consistent with trophic cascade theory, whereby predators
R. J. Wasserman (*) : P. W. Froneman Department of Zoology and Entomology, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa e-mail: [email protected] G. F. Matcher Department of Biochemistry, Microbiology and Biotechnology, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa T. J. F. Vink Department of Botany, Coastal and Marine Research Unit, Nelson Mandela Metropolitan University, P O Box 77000, Port Elizabeth 6031, South Africa
mediate interactions at multiple lower trophic levels with consequent repercussions for diversity. Keywords Bacteria . Unicellular eukaryotic algae . Apex predation . Trophic cascading . 16S rRNA . Pyrosequencing
Introduction Eukaryotic phytoplankton and prokaryotic bacteria have been demonstrated to be the two largest contributors to primary production in marine ecosystems [1]. Moreover, these organisms often represent an important carbon source for the heterotrophic components within the microbial loop [2]. As a consequence, these ubiquitous, abundant and diverse organisms are shown to be key regulators of various biogeochemical processes, including the carbon and nitrogen cycles in aquatic environments [3]. While many studies have investigated the role of various physico-chemical drivers on bacterial community dynamics [2–5], few studies have investigated the role of keystone predation in this regard within aquatic ecosystems [6]. Trophic interactions play an essential organisational role in community and ecosystem ecology [7]. Substantial research has been conducted on this field of ecology which has revealed that communities and associated food webs are often controlled by predators through trophic cascades [8–10]. Trophic cascading, defined as the downward transmission of predatory control through multiple trophic levels [11, 12], outlines the importance of predation in structuring food webs and ecological communities. Cascade investigations and their elucidation with regard to empirical and theoretical science have in recent times marked some of the most exciting advances in trophic ecology [7]. In addition to their contribution in controlling prey numbers, predators may also affect the diversity at lower trophic levels through selective feeding, potentially increasing species richness by
246
removing or decreasing superior competitors [3, 13–15]. As such, apex predators can have a stabilizing effect on prey communities, with implications for multiple lower trophic levels, making them key components of aquatic food webs [6, 16–18]. Estuaries are among the most productive of all ecosystems, supporting high levels of primary productivity [19–21] and secondary consumer biomass [22]. While the classic food web has received much attention in these systems [23, 24], there is relatively little information available on microbial dynamics of estuaries and indeed, marine ecosystems worldwide. In southern African estuaries, as in many aquatic environments, zooplanktivorous predation is, at least partly, performed by early life-history fish [25–27]. Zooplanktivorous fish have long been shown to influence invertebrate prey communities [28, 29]. Furthermore, by consuming herbivorous zooplankton, zooplanktivores can indirectly alter phytoplankton communities [23, 24]. However, the indirect effects of zooplanktivores on the microbial planktonic community composition and diversity remains poorly understood [6]. In a recent mesocosm study, Wasserman et al. [30] demonstrated that the presence of an apex zooplanktivorous predator contributed to the control of zooplankton numbers, with a subsequent increase in stability of phytoplankton and bacterial abundances. Their study, however, focused only on absolute number of organisms at the broad taxonomic level, with no information at lower taxonomic resolution. Here, we focus on the impact of these apex predators on bacterial diversity by using pyrosequencing for the identification of sampled bacteria at a fine resolution [31–33]. Using pyrosequenced water samples collected from the various treatments in Wasserman’s study [30], we relate the bacteria at the species operational taxonomic unit (OTU) level, to the bacterial abundances found in their study. We postulated that the presence of the stable apex predatory pressure, which was shown to stabilize the entire food web—including bacterial abundances [30], would have repercussions for the bacterial community as specified by OTU (hereafter referred to as species level) analyses.
Materials and Methods Study Site The mesocosm experiment was conducted during September (austral spring) in the middle reaches of the temporary open/ closed Kasouga Estuary in the Eastern Cape province of South Africa. Emptying into the Indian Ocean, this estuary is located on the south-eastern coastline of South Africa and has a catchment area of ≈39 km2 [34]. Given the generally shallow depth (
MICROBIOLOGY OF AQUATIC SYSTEMS
Preliminary Evidence for the Organisation of a Bacterial Community by Zooplanktivores at the Top of an Estuarine Planktonic Food Web R. J. Wasserman & G. F. Matcher & T. J. F. Vink & P. W. Froneman
Received: 19 April 2014 / Accepted: 25 September 2014 / Published online: 10 October 2014 # Springer Science+Business Media New York 2014
Abstract As part of a larger investigation, the effect of apex predation on estuarine bacterial community structure, through trophic cascading, was investigated using experimental in situ mesocosms. Through either the removal (filtration) or addition of specific size classes of planktonic groups, four different trophic scenarios were established using estuarine water and its associated plankton. One such treatment represented a “natural” scenario in which stable apex predatory pressure was qualified. Water samples were collected over time from each of the treatments for bacterial community evaluation. These samples were assessed through pyrosequencing of the variable regions 4 and 5 of the bacterial 16S rRNA gene and analysed at the species operational taxonomic unit (OTU) level using a community procedure. The blue-green group dominated the samples, followed by Proteobacteria and Bacteroidetes. Samples were the most similar among treatments at the commencement of the experiment. While the bacterial communities sampled within each treatment changed over time, the deviation from initial appeared to be linked to the treatment trophic scenarios. The least temporal deviationfrom-initial in bacterial community was found within the stable apex predatory pressure treatment. These findings are consistent with trophic cascade theory, whereby predators
R. J. Wasserman (*) : P. W. Froneman Department of Zoology and Entomology, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa e-mail: [email protected] G. F. Matcher Department of Biochemistry, Microbiology and Biotechnology, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa T. J. F. Vink Department of Botany, Coastal and Marine Research Unit, Nelson Mandela Metropolitan University, P O Box 77000, Port Elizabeth 6031, South Africa
mediate interactions at multiple lower trophic levels with consequent repercussions for diversity. Keywords Bacteria . Unicellular eukaryotic algae . Apex predation . Trophic cascading . 16S rRNA . Pyrosequencing
Introduction Eukaryotic phytoplankton and prokaryotic bacteria have been demonstrated to be the two largest contributors to primary production in marine ecosystems [1]. Moreover, these organisms often represent an important carbon source for the heterotrophic components within the microbial loop [2]. As a consequence, these ubiquitous, abundant and diverse organisms are shown to be key regulators of various biogeochemical processes, including the carbon and nitrogen cycles in aquatic environments [3]. While many studies have investigated the role of various physico-chemical drivers on bacterial community dynamics [2–5], few studies have investigated the role of keystone predation in this regard within aquatic ecosystems [6]. Trophic interactions play an essential organisational role in community and ecosystem ecology [7]. Substantial research has been conducted on this field of ecology which has revealed that communities and associated food webs are often controlled by predators through trophic cascades [8–10]. Trophic cascading, defined as the downward transmission of predatory control through multiple trophic levels [11, 12], outlines the importance of predation in structuring food webs and ecological communities. Cascade investigations and their elucidation with regard to empirical and theoretical science have in recent times marked some of the most exciting advances in trophic ecology [7]. In addition to their contribution in controlling prey numbers, predators may also affect the diversity at lower trophic levels through selective feeding, potentially increasing species richness by
246
removing or decreasing superior competitors [3, 13–15]. As such, apex predators can have a stabilizing effect on prey communities, with implications for multiple lower trophic levels, making them key components of aquatic food webs [6, 16–18]. Estuaries are among the most productive of all ecosystems, supporting high levels of primary productivity [19–21] and secondary consumer biomass [22]. While the classic food web has received much attention in these systems [23, 24], there is relatively little information available on microbial dynamics of estuaries and indeed, marine ecosystems worldwide. In southern African estuaries, as in many aquatic environments, zooplanktivorous predation is, at least partly, performed by early life-history fish [25–27]. Zooplanktivorous fish have long been shown to influence invertebrate prey communities [28, 29]. Furthermore, by consuming herbivorous zooplankton, zooplanktivores can indirectly alter phytoplankton communities [23, 24]. However, the indirect effects of zooplanktivores on the microbial planktonic community composition and diversity remains poorly understood [6]. In a recent mesocosm study, Wasserman et al. [30] demonstrated that the presence of an apex zooplanktivorous predator contributed to the control of zooplankton numbers, with a subsequent increase in stability of phytoplankton and bacterial abundances. Their study, however, focused only on absolute number of organisms at the broad taxonomic level, with no information at lower taxonomic resolution. Here, we focus on the impact of these apex predators on bacterial diversity by using pyrosequencing for the identification of sampled bacteria at a fine resolution [31–33]. Using pyrosequenced water samples collected from the various treatments in Wasserman’s study [30], we relate the bacteria at the species operational taxonomic unit (OTU) level, to the bacterial abundances found in their study. We postulated that the presence of the stable apex predatory pressure, which was shown to stabilize the entire food web—including bacterial abundances [30], would have repercussions for the bacterial community as specified by OTU (hereafter referred to as species level) analyses.
Materials and Methods Study Site The mesocosm experiment was conducted during September (austral spring) in the middle reaches of the temporary open/ closed Kasouga Estuary in the Eastern Cape province of South Africa. Emptying into the Indian Ocean, this estuary is located on the south-eastern coastline of South Africa and has a catchment area of ≈39 km2 [34]. Given the generally shallow depth (
