Linking stream microbial community function to dissolved organic matter and inorganic nutrients

There is now increasing evidence for the importance of microbial regulation of biogeochemical cycling in streams. Resource availability shapes microbial community structure, but less is known about how landscape-mediated availability of nutrients and carbon can control microbial functions in streams. Using comparative metagenomics, we examined the relationship between microbial function and composition of dissolved organic matter (DOM), nutrients, and suspended microbial communities in 11 streams that were divided into 3 groups based on whether their catchment was predominantly agriculture, forested or wetlands. Using weighted gene co-occurrence network analysis, we identified clusters of functions related to DOM composition, agriculture land use and/or wetland and forest land cover. Wetland-dominated streams were characterized by functions related to nitrogen metabolism and processing of aromatic carbon compounds, with strong positive correlations with DOC concentration and DOM aromaticity. Forested streams were characterized by metabolic functions related to monomer uptake and carbohydrates, such as mannose and fructose metabolism. In agricultural streams, microbial functions were correlated to more labile, protein-like DOM, PO4 and NO3, likely reflecting functional adaptation to labile DOM and higher nutrient concentrations. Distinct changes in the functional composition and loss of functional diversity of microorganisms became evident when comparing natural to agricultural catchments. At the same time, we found signs of functional redundancy across all study streams an indication that microbial communities can maintain core functions regardless of catchment land use. Our results provide new insight into microbial community functions involved in nutrient and carbon biogeochemical cycles and their dependence on specific environmental settings.