Stability of microbial communities through seasonal hydrological transitions and the resulting biogeochemical redox response

Alluvial floodplains act as an intermediate between terrestrial and riverine systems, with subsurface exchanges of water, nutrients, and contaminants occurring across saturated-unsaturated interfaces. Coupling between hydrology and biogeochemistry in these areas are particularly complex due to the many hydrologic processes at play (percolation, capillary rise, advection) and biogeochemical reactions that can be triggered (redox reactions, mineral dissolution and precipitation, sorption and desorption, etc.). Intense, intermittent hydrologic transitions and their biogeochemical consequences are governed in large part by the subsurface microbial community response, which is then responsible for ensuing impacts on groundwater quality, including contaminant release and transport. Detailed knowledge of how various microbial and geochemical processes are connected to hydrological processes in such spatially-heterogeneous systems is lacking. We aimed to address this knowledge gap by collecting both microbial and geochemical samples to capture key redox transitions at the DOE legacy uranium ore processing site in Riverton, WY, from April to September 2017. Our goal was to characterize the microbial community throughout the soil column and observe its response to wet-dry transitions across a complete season. Next-generation sequencing (Illumina) was employed to identify biogeochemically-relevant microbial taxa based on the 16S rRNA gene; we found a broad diversity of microbial clades including bacteria involved in sulfur cycling,dissimilatorymetal-reducing bacteria, and ammonia-oxidizing Archaea. These data were paired with measurements of soil temperature and moisture, major biogeochemical elements (C, N, Fe, S), and metal contaminants (U, Mo). Overall microbial community composition was dependent on soil depth or type, with little seasonal effects except those in the subsurface aquifer. This indicates that major microbial groups are present during both wet (reducing) and dry (oxidizing) periods but may have varied metabolic activity throughout the year, depending on optimal conditions; this will be clarified with metagenomic and metatranscriptomic sequencing.