In situ light and phosphorus manipulations reveal potential role of biofilm algae in enhancing enzyme-mediated decomposition of organic matter in streams

Biofilms play crucial roles in the carbon and nutrient dynamics of stream ecosystems. Measuring extracellular enzyme activities can reveal universal patterns in C, N and P dynamics at the microbial level and enhance our understanding of how microorganisms influence ecosystem-level processes. Because algae may potentially enhance the activities of enzymes produced by heterotrophic microorganisms for degrading allochthonous organic matter, more work is needed to understand how factors that influence biofilm algae could in turn alter activities of these enzymes and how these influences alter the demand for inorganic nutrients. The purpose of this study was to use extracellular enzymes (alkaline or acid phosphatase, beta-glucosidase, beta-xylosidase, leucine aminopeptidase and phenol oxidase) to examine the response of biofilms to in situ shade and phosphorus manipulations in two streams with contrasting DOC quantity and quality. Despite the water column nutrient ratios indicating P limitation in both streams, shade had the biggest effect on algal and bacterial biomass. Enzyme results clearly demonstrated the positive effects of algae on the activities of both hydrolytic and oxidative enzymes in stream biofilms. In most cases, we observed at least a doubling in activities between the shaded and ambient treatments, with P additions indicating enhanced P demand under ambient light. Furthermore, similar phenol oxidase activities between the two streams suggests pH or enzyme inhibitors might have played a role or that a proportion of the measured enzyme activities was directed at the degradation of fine particulate organic matter embedded in the biofilm matrix in addition to water column DOC. This study provides further evidence that algae might contribute to the degradation of terrestrial organic matter through indirect influences on extracellular enzyme activities and possibly priming.