Prokaryotic diversity and biogeochemistry in hyporheic zone fluids from the McCarran Ranch channel bar, Truckee River, Nevada

The hyporheic zone is defined as the subsurface zone beneath and alongside a streambed where shallow groundwater and surface water interact. Nutrient and energy exchange within this zone have been shown to stimulate biological activity, including aerobic respiration, denitrification, and elemental cycling. However, the microbial ecology of the hyporheic zone, especially that of fluvial channel bars, remains poorly understood. To obtain a greater understanding of hyporheic zone prokaryotic biogeochemistry, we took advantage of a piezometer array to obtain pore fluids from multiple sites throughout the McCarran Ranch Channel Bar (MRCB); a fluvial geomorphic feature in the Truckee River, NV. Utilizing fluids pumped from this array, we characterized the prokaryotic community structure, metabolic potential, and aqueous chemistries of flowing river surface water and porewater. Physical measurements, with the exception of pH and dissolved oxygen concentration, were relatively consistent among all samples. The concentrations of potential respiratory electron acceptors were highest in the surface water and riverbed porewater samples and were sequentially depleted (O2, then NO3, then SO42-) in porewater from the MRCB. Similarly, the concentrations of cultivable nitrate reducers and denitrifiers were highest in river surface water and riverbed porewater despite overall oxic conditions. Cultivable sulfate reducers were most abundant in surface water and porewater from a single location in the MRCB but were detected in all samples. Prokaryotic community profile reconstruction from 16S rRNA gene sequences indicated that the river surface water community was less diverse than that of all piezometer samples despite higher cell concentrations and high concentrations of various cultivable microbial physiotypes. Furthermore, the putative metabolic capacity of abundant OTUs from all samples was indicative of a shift in metabolic strategy, from aerobic heterotrophy in river surface waters (e.g.: OTUs in the Comamonadaceae and Sporichthyaceae families) to chemolithotrophy and anaerobic metabolisms (e.g.: Hydrogenophaga spp., Ferribacterium spp., Methanobacterium spp.), along the hyporheic zone flow path. Collectively, these data indicate that prokaryotic communities within the MRCB are phylogenetically and metabolically diverse and contribute to biogeochemical cycling in this common yet relatively understudied habitat.