Identification of D esulfobacterales as primary hydrogenotrophs in a complex microbial mat community

Hypersaline microbial mats have been shown to produce significant quantities of H 2 under dark, anoxic conditions via cyanobacterial fermentation. This flux of a widely accessible microbial substrate has potential to significantly influence the ecology of the mat, and any consumption will affect the net efflux of H 2 that might otherwise be captured as a resource. Here, we focus on H 2 consumption in a microbial mat from E lkhorn S lough, C alifornia, USA , for which H 2 production has been previously characterized. Active biologic H 2 consumption in this mat is indicated by a significant time‐dependent decrease in added H 2 compared with a killed control. Inhibition of sulfate reduction, as indicated by a decrease in hydrogen sulfide production relative to controls, resulted in a significant increase in H 2 efflux, suggesting that sulfate‐reducing bacteria ( SRB ) are important hydrogenotrophs. Low methane efflux under these same conditions indicated that methanogens are likely not important hydrogenotrophs. Analyses of genes and transcripts that encode for r RNA or dissimilatory sulfite reductase, using both PCR ‐dependent and PCR ‐independent metatranscriptomic sequencing methods, demonstrated that D esulfobacterales are the dominant, active SRB in the upper, H 2 ‐producing layer of the mat (0–2 mm). This hypothesis was further supported by the identification of transcripts encoding hydrogenases derived from D esulfobacterales capable of H 2 oxidation. Analysis of molecular data provided no evidence for the activity of hydrogenotrophic methanogens. The combined biogeochemical and molecular data strongly indicate that SRB belonging to the D esulfobacterales are the quantitatively important hydrogenotrophs in the Elkhorn Slough mat.