Nutrient availability and organic matter quality shape bacterial community structure in a lake biofilm

Heterotrophic bacteria play a key role in ecosystem processes, but little is known about the factors that shape bacterial community structure in aquatic biofilms, especially in lakes. We used molecular techniques (16S rRNA) to evaluate resource controls on biofilm bacterial community structure in an oligotrophic subalpine lake. We manipulated nutrients (nitrogen and phosphorus; NP) and glucose (G) on inorganic (rock) and organic (wood) substrates under light and dark conditions (i.e. with and without autotrophy, respectively) in a full factorial design using nutrient diffusing substrates in situ for 20 d. Distinct patterns of separation in community structure between treatments with nutrients (NP, NP+G) and without nutrients (control, G-only) indicated that community structure was more strongly influenced by nutrients than organic matter irrespective of substrate type or light availability. Further separation in community structure between treatments with nutrients only (NP) and nutrients with glucose (NP+G) on both organic and inorganic substrates indicated that once nutrient limitation was alleviated, organic matter quality played an important role in shaping community structure. Differences in the relative abundance of 6 phyla, 3 classes, and 19 genera among treatments revealed (1) contrasting taxa-specific re - source requirements, (2) the influence of interspecific interactions on composition, and (3) the potential for individual taxa to participate in the decomposition of recalcitrant organic matter. Our findings provide insight into the role that nutrients and organic matter quality play in shaping bacterial community structure, which is a critical step in bridging the knowledge gap between microbial composition and ecosystem function within aquatic environments

Resource constraints highlight complex microbial interactions during lake biofilm development

This study evaluated how the availability of nutrients and organic carbon interact to influence the associations between autotrophic and heterotrophic micro-organisms during lake biofilm development. Considering that decomposers are often better competitors for nutrients than producers in aquatic environments, we hypothesized that heterotrophs would outcompete autotrophs for available nutrients unless heterotrophs were limited by organic carbon provided by autotrophs. To test our hypothesis, we evaluated autotrophic (algae) and heterotrophic (fungi, bacteria) biomass in response to a factorial enrichment of nutrients (nitrogen and phosphorus in combination) and glucose using nutrient-diffusing substrates with either inorganic or organic discs in a subalpine lake. In the field, nutrient-diffusing substrates were exposed to either natural sunlight or placed under a darkened experimental canopy to evaluate the response of heterotrophs to nutrients and carbon subsidies in the presence or absence of algae. We expected that heterotrophs would be limited by organic carbon on inorganic substrates in the absence of autotrophic production (i.e., dark treatments), and that organic substrates would provide a carbon subsidy for heterotrophic metabolism. Fungi were stimulated by nutrient enrichment on inorganic substrates in the presence of algae (light treatment), but not in the dark (without algae). The response of fungi to algal presence on inorganic substrates was similar in magnitude to the response of fungi to nutrients and glucose substrates incubated in the dark. In contrast to our expectations, elevated algal biomass did not stimulate heterotrophic bacteria in the presence of elevated nutrient levels on inorganic substrates, possibly owing to antagonistic interactions between bacteria and fungi. The positive effect of nutrients on algal biomass was significantly reduced in favour of heterotrophs when nutrients were combined with glucose, suggesting that heterotrophs were able to outcompete algae for available nutrients in the absence of carbon limitation. Synthesis. These results expand our understanding of how the availability of limiting resources governs the outcomes of complex interactions among micro-organisms in aquatic biofilms, and suggests that background levels of organic carbon regulate producer and decomposer responses to nutrient availability during biofilm development

Wyatt et al_2019_dataset

These data were collected as part of a biofilm experiment conducted in Castle Lake, California. The data file is in csv format

Resource Constraints Highlight Complex Microbial Interactions During Lake Biofilm Development

This study evaluated how the availability of nutrients and organic carbon interact to influence the associations between autotrophic and heterotrophic micro-organisms during lake biofilm development. Considering that decomposers are often better competitors for nutrients than producers in aquatic environments, we hypothesized that heterotrophs would outcompete autotrophs for available nutrients unless heterotrophs were limited by organic carbon provided by autotrophs.To test our hypothesis, we evaluated autotrophic (algae) and hetertrophic (fungi, bacteria) biomass in response to a factorial enrichment of nutrients (nitrogen and phosphorus in combination) and blucose using nutrient-diffusing substrates with either inorganic or organic discs in a subalpine lake. In the field, nutrient-diffusing substrates were exposed to either natural sunlight or placed under a darkened experimental canopy to evaluate the resonse of heterotrophs to nutrients and carbon subsidies in the presence or absence of algae. We expected that heterotrophs would be limited by organic carbon on inorganic substrates in the absence of autotrophic production (i.e., dark treatments), and that organic substrates would provide a carbon subsidy for heterotrophic metabolism.Fungi were stimulated by nutrient enrichment on inorganic substrates in the presence of algae (light treatment), but not in the dark (without algae). The response of fungi to algal presence on inorganic substrates was similar in magnitude to the response of fungi to nutrients and glucose substrates incubated in the dark. In contrast to our expectations, elevated algal biomass did not stimulate heterotrophic bacteria in the presence of elevated nutrient levels on inorganic substrates, possibly owning to antagonistic interactions between bacteria and fungi.The positive effect of nutrients on algal biomass was significantly reduced in favour of heterotrophs when nutrients were combined with glucose, suggesting that heterotrophs were able to outcompete algae for available nutrients in the absence of carbon limitation.Synthesis. These results expand our understanding of how the availablility of limiting resources governs the outcomes of complex interactions among micro-organisms in aquatic biofilms, and suggests that background levels of organic carbon regulate producer and decomposer responses to nutrient availability during biofilm development