Spatial Variability in Streambed Microbial Community Structure Across Two Watersheds

The spatial patterns of microbial communities are largely unknown compared to those of macro-fauna and flora. We investigated patterns of microbial community structure on streambed sediments from two watersheds across spatial scales spanning < 1m within a single stream to several hundred km between watersheds. Analyses of phospholipid fatty acids (PLFA) profiles indicated that the variations in microbial community structure were driven by increases in the relative abundance of microeukaryotic photoautotrophs and their contribution to total microbial biomass. Furthermore, streams within watersheds had similar microbial community structure, underscoring within-watershed controls of microbial communities. Moreover, bacterial community structure assayed as either polymerase chain reaction-denaturing gradient gelelectrophoresis (PCR-DGGE) fingerprints or PLFA profiles edited to remove microeukaryotes indicated a distinct watershed-level biogeography. No distinct stream order-level distributions were identified although DGGE analysis clearly indicated that there was greater variability in community structure among 1st-order streams compared to 2nd- and 3rd-order streams into which they flowed. Longitudinal gradients in microbial biomass and structure showed that the greatest variations were associated with 1st order streams within a watershed and 68% of the variation in total microbial biomass was explained by sediment C:N mass ratio, percent Carbon, sediment surface area, and percent water content. This study confirms a distinct microbial biogeography for headwater stream communities driven by environmental heterogeneity across distant watersheds and suggests that eukaryotic photoautotrophs play a key role in structuring sediment microbial communities. IMPORTANCE Microorganisms in streams drive many biogeochemical reactions of global significance, including nutrient cycling and energy flow, yet the mechanisms responsible for the distribution and composition of streambed microbial communities are not well known. We sampled sediments from multiple streams in two watersheds; Neversink River (New York) and White Clay Creek (Pennsylvania) watersheds and measured microbial biomass, total microbial and bacterial community structures using phospholipid and molecular methods. Microbial and bacterial community structures displayed a distinct watershed-level biogeography. The smallest headwater streams within a watershed showed the greatest variation in microbial biomass, and C:N ratio, percent carbon, sediment surface area and percent water content explained 68% of the variations in microbial biomass. This study indicates a non-random distribution of microbial communities in streambeds, and that microeukaryotic photoautotrophs, environmental heterogeneity and geographical distance influence microbial composition and spatial distribution.