Microb Ecol (1983) 9:15-26
MICROBI#L ECOLOGV @ 1983 Springer-Verlag
Habitat Selection in Two Species of Aquatic Hyphomycetes G 6 r a n Bengtsson Laboratory of EcologicalChemistry, Ecology Building, Helgonaviigen 5, S-223 62 Lund, Sweden
Abstract. T w o species o f aquatic hyphomycetes, Tetracladium marchalianum and Tricladium splendens, were isolated from decaying leaves in a stream. T. marchalianum was a b u n d a n t on alder leaves but absent on beech leaves, which were d o m i n a t e d by T. splendens. It was hypothesized that differences in some chemical key factors in the leaves would account for differences in the distribution o f the 2 species. In the e x p e r i m e n t designed to test the hypothesis, combinations o f sterilized leaves and isolated fungi were used. Differences in growth o f F D A active m y c e l i u m were related to differences in leaf weight loss; T. splendens d e c o m p o s e d beech leaves and T. marchalianurn d e c o m p o s e d alder leaves. Extracellular protease activity corresponded with these trends but there was no detectable protein loss in the leaves. Both fungi showed a nitrogen d e m a n d , and hydrolysis o f leaf proteins was c o m p l e m e n t e d with absorption o f free a m i n o acids and a m m o n i u m . High concentrations o f free a m i n o acids modified the pattern for habitat selection so that T. splendens grew substantially on alder leaves and T. marchalianum colonized beech leaves. N o protease activity was, however, found f r o m T. marchalianum on beech leaves, and it is concluded that a m o r e general metabolic inhibition prevents extensive growth o f this species on beech leaves. T h e low natural abundance o f T. splendens on alder leaves, where it m a y grow well, m a y be a consequence o f a specific protease inhibition and c o m p e t i t i o n from other species.
Introduction Decay o f a u t u m n - s h e d leaves in a stream is p r o m o t e d by associated microorganisms, which are numerous on most leaves [ 1, 2, 8, 1 1]. It is generally assumed that these microorganisms process leaves by enzymatic hydrolysis, but there is evidence that energy and nutrients m a y also be derived by absorption o f dissolved organic m a t t e r [2, 5]. Preferential uptake o f dissolved organic m a t t e r at high concentrations m a y cause a t e m p o r a r y decline in leaf decay, but the low decay rate o f certain leaves, e.g., beech leaves, m o r e likely depends on the chemical nature o f the leaves. Chemical c o m p o s i t i o n o f leaves m a y select for guilds o f fungi with special physiological adaptations on different leaves. Whereas m a n y fungi can be found on a variety o f leaves, some species are m o r e restricted in their distribution. 0095-3628/83/0009-0015502.40
16
G. Bengtsson
Swift [13] suggested the use o f the terms resource specific and nonspecific to distinguish between habitat specialist and habitat generalist fungi and bacteria in terrestrial habitats. The a u t h o r attributes the basis o f the resource specificity to the presence o f specific chemicals, such as terpenes and polyphenols, that m a y be inhibitory at relatively low concentrations, or nutrient molecules, such as those containing N, that m a y be essential to the growth o f microbes. The depletion o f the energy and nutrient resources o f the substrate during d e c o m position m a y lead to increasing niche overlap and c o m p e t i t i o n for the remaining substrate. The fungal species that m o s t successfully utilize the m o s t persistent substrate probably have a wide range o f enzymatic diversity and capacity for p r o d u c t i o n o f antibiotics [17]. Beech leaves are more persistent that alder leaves in a running water system, and support fewer species o f aquatic h y p h o m y c e t e s and a lower mycelial growth rate than alder leaves [5]. Fungi on both kinds o f leaves use extracellular proteases to hydrolyze proteins in leaf tissue, and fungi on alder leaves efficiently absorb free a m i n o acids (FAA) in the water. The fungi show strong preferential uptake o f F A A when its concentration in relation to the protein concentration in leaves exceeds a certain level. These findings, and other evidence reported in an earlier paper [5], suggest that the chemical c o m p o s i t i o n o f alder and beech leaves m a y have selected for different guilds o f fungal communities. Judging from species n u m b e r and dec o m p o s i t i o n rate, alder leaves appeared to be the m o r e favorable habitat, and c o m p e t i t i o n m a y occur between fungi for space and other resources. Consequently, species with high dispersal rates, rapid mycelial growth, and efficient absorption o f nutrients released from leaves would be m o s t competitive on alder leaves; selection on the m o r e persistent beech leaves m a y have favored species with m o r e emphasis on enzymes that are efficient in the presence o f strong complexing agents. T o examine the extent o f habitat selection, 2 species o f fungi were isolated a n d incubated with the leaves. Tricladium splendens (Ingold) was p r e d o m i n a n t on beech leaves, and Tetracladium rnarchalianum (deWild) was a b u n d a n t on alder leaves and absent on beech leaves. It was hypothesized that the difference in performance between the 2 species would be small on alder leaves, with T. marchalianum slightly m o r e successful, and only T. splendens would successfully macerate a n d colonize beech leaves. Different aspects o f habitat selection were studied by determination o f mycelial growth, weight and protein loss o f leaves, absorption o f free a m i n o acids and a m m o n i u m ions, and extracellular protease activity.
Material and M e t h o d s Alder (Alnus glutinosa L.) and beech (Fagus sylvatica L.) leaves were collected from two 4 m 2 nylon nets that were stretched above a stream bordered by trees. Leaves were enclosed in 2 mm mesh nylon bags and left in the stream for 2 days to ensure proper conditioning, such as softening of the tissue and removal of initial excess leachate. The bags were brought to the laboratory and the leaves rinsed in cold tap water to remove extraneous sediment and invertebrates. The leaves were moved to plastic jars, leached in running tap water for 2 days, and dried in an oven at 60~ for 2 days. Leaves were weighed and then placed in 250 ml Erlenmeyer flasks, 1 leaf in each flask,
Habitat Selection of Hyphomycetes
17
sealed with a cotton plug. The leaves were autoclaved at 120~ for 15 min, and 100 ml distilled autoclaved water was added to each flask. Storage of autoclaved and gamma-irradiated substrate for a couple of months may change the content of soluble tannins and carbohydrates, respectively [7], but a short sterilization period (I 5 min), short incubation time (
MICROBI#L ECOLOGV @ 1983 Springer-Verlag
Habitat Selection in Two Species of Aquatic Hyphomycetes G 6 r a n Bengtsson Laboratory of EcologicalChemistry, Ecology Building, Helgonaviigen 5, S-223 62 Lund, Sweden
Abstract. T w o species o f aquatic hyphomycetes, Tetracladium marchalianum and Tricladium splendens, were isolated from decaying leaves in a stream. T. marchalianum was a b u n d a n t on alder leaves but absent on beech leaves, which were d o m i n a t e d by T. splendens. It was hypothesized that differences in some chemical key factors in the leaves would account for differences in the distribution o f the 2 species. In the e x p e r i m e n t designed to test the hypothesis, combinations o f sterilized leaves and isolated fungi were used. Differences in growth o f F D A active m y c e l i u m were related to differences in leaf weight loss; T. splendens d e c o m p o s e d beech leaves and T. marchalianurn d e c o m p o s e d alder leaves. Extracellular protease activity corresponded with these trends but there was no detectable protein loss in the leaves. Both fungi showed a nitrogen d e m a n d , and hydrolysis o f leaf proteins was c o m p l e m e n t e d with absorption o f free a m i n o acids and a m m o n i u m . High concentrations o f free a m i n o acids modified the pattern for habitat selection so that T. splendens grew substantially on alder leaves and T. marchalianum colonized beech leaves. N o protease activity was, however, found f r o m T. marchalianum on beech leaves, and it is concluded that a m o r e general metabolic inhibition prevents extensive growth o f this species on beech leaves. T h e low natural abundance o f T. splendens on alder leaves, where it m a y grow well, m a y be a consequence o f a specific protease inhibition and c o m p e t i t i o n from other species.
Introduction Decay o f a u t u m n - s h e d leaves in a stream is p r o m o t e d by associated microorganisms, which are numerous on most leaves [ 1, 2, 8, 1 1]. It is generally assumed that these microorganisms process leaves by enzymatic hydrolysis, but there is evidence that energy and nutrients m a y also be derived by absorption o f dissolved organic m a t t e r [2, 5]. Preferential uptake o f dissolved organic m a t t e r at high concentrations m a y cause a t e m p o r a r y decline in leaf decay, but the low decay rate o f certain leaves, e.g., beech leaves, m o r e likely depends on the chemical nature o f the leaves. Chemical c o m p o s i t i o n o f leaves m a y select for guilds o f fungi with special physiological adaptations on different leaves. Whereas m a n y fungi can be found on a variety o f leaves, some species are m o r e restricted in their distribution. 0095-3628/83/0009-0015502.40
16
G. Bengtsson
Swift [13] suggested the use o f the terms resource specific and nonspecific to distinguish between habitat specialist and habitat generalist fungi and bacteria in terrestrial habitats. The a u t h o r attributes the basis o f the resource specificity to the presence o f specific chemicals, such as terpenes and polyphenols, that m a y be inhibitory at relatively low concentrations, or nutrient molecules, such as those containing N, that m a y be essential to the growth o f microbes. The depletion o f the energy and nutrient resources o f the substrate during d e c o m position m a y lead to increasing niche overlap and c o m p e t i t i o n for the remaining substrate. The fungal species that m o s t successfully utilize the m o s t persistent substrate probably have a wide range o f enzymatic diversity and capacity for p r o d u c t i o n o f antibiotics [17]. Beech leaves are more persistent that alder leaves in a running water system, and support fewer species o f aquatic h y p h o m y c e t e s and a lower mycelial growth rate than alder leaves [5]. Fungi on both kinds o f leaves use extracellular proteases to hydrolyze proteins in leaf tissue, and fungi on alder leaves efficiently absorb free a m i n o acids (FAA) in the water. The fungi show strong preferential uptake o f F A A when its concentration in relation to the protein concentration in leaves exceeds a certain level. These findings, and other evidence reported in an earlier paper [5], suggest that the chemical c o m p o s i t i o n o f alder and beech leaves m a y have selected for different guilds o f fungal communities. Judging from species n u m b e r and dec o m p o s i t i o n rate, alder leaves appeared to be the m o r e favorable habitat, and c o m p e t i t i o n m a y occur between fungi for space and other resources. Consequently, species with high dispersal rates, rapid mycelial growth, and efficient absorption o f nutrients released from leaves would be m o s t competitive on alder leaves; selection on the m o r e persistent beech leaves m a y have favored species with m o r e emphasis on enzymes that are efficient in the presence o f strong complexing agents. T o examine the extent o f habitat selection, 2 species o f fungi were isolated a n d incubated with the leaves. Tricladium splendens (Ingold) was p r e d o m i n a n t on beech leaves, and Tetracladium rnarchalianum (deWild) was a b u n d a n t on alder leaves and absent on beech leaves. It was hypothesized that the difference in performance between the 2 species would be small on alder leaves, with T. marchalianum slightly m o r e successful, and only T. splendens would successfully macerate a n d colonize beech leaves. Different aspects o f habitat selection were studied by determination o f mycelial growth, weight and protein loss o f leaves, absorption o f free a m i n o acids and a m m o n i u m ions, and extracellular protease activity.
Material and M e t h o d s Alder (Alnus glutinosa L.) and beech (Fagus sylvatica L.) leaves were collected from two 4 m 2 nylon nets that were stretched above a stream bordered by trees. Leaves were enclosed in 2 mm mesh nylon bags and left in the stream for 2 days to ensure proper conditioning, such as softening of the tissue and removal of initial excess leachate. The bags were brought to the laboratory and the leaves rinsed in cold tap water to remove extraneous sediment and invertebrates. The leaves were moved to plastic jars, leached in running tap water for 2 days, and dried in an oven at 60~ for 2 days. Leaves were weighed and then placed in 250 ml Erlenmeyer flasks, 1 leaf in each flask,
Habitat Selection of Hyphomycetes
17
sealed with a cotton plug. The leaves were autoclaved at 120~ for 15 min, and 100 ml distilled autoclaved water was added to each flask. Storage of autoclaved and gamma-irradiated substrate for a couple of months may change the content of soluble tannins and carbohydrates, respectively [7], but a short sterilization period (I 5 min), short incubation time (
