Rapid and stable microbial community assembly in the headwaters of third-order stream

Abstract

Small streams and their headwaters are a key source of microbial diversity in fluvial systems and serve as an entry point for bacteria from the surrounding landscape. Community assembly processes occurring in these streams shape downstream population structure and nutrient cycles. To elucidate the development and stability of microbial communities along the length of a first through third order stream, fine-scale temporal and spatial sampling regimes were employed along McNutt Creek in Athens, Georgia, USA. 16S rRNA gene libraries were constructed from samples collected on a single day from 19 sites spanning the first 16.76 km of the stream. Selected sites at the upper, mid, and lower reaches of the stream were sampled daily for 11 days to evaluate community variability over time. In a second study, sites at and near the creeks headwaters were sampled daily for 11 days to understand the initial stages of bacterioplankton community assembly. In all studies, we observed decreasing alpha and beta diversity with increasing downstream distance. These trends were accompanied by the enrichment of a small fraction of taxa found at low abundance in the furthest-upstream environments. Similar sets of taxa consistently increased significantly in relative abundance in downstream samples over time scales ranging from 1 day to 1 year, many of which belong to microbial clades known to be abundant in freshwater environments. These results underpin the importance of headwaters as the site of rapid in-stream selection that results in the reproducible establishment of a highly stable community of freshwater riverine bacteria.\n\nImportanceHeadwater streams are critical introduction points of microbial diversity for larger connecting rivers and play key roles in the establishment of taxa that partake in in-stream nutrient cycling. We examined microbial community composition of a first- through third-order stream using fine-scale temporal and spatial regimes. Our results show that the bacterioplankton community develops rapidly and predictably from the headwater population with increasing total stream length. Along the length of the stream, the microbial community exhibits substantial diversity loss and enriches repeatedly for select taxa across days and years, although the relative abundances of individual taxa vary over time and space. This repeated enrichment of a stable stream community likely contributes to the stability and flexibility of downstream communities.