Identification of Desulfobacterales 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 Elkhorn Slough, California, 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 rRNA or dissimilatory sulfite reductase, using both PCR-dependent and PCR-independent metatranscriptomic sequencing methods, demonstrated that Desulfobacterales are the dominant, active SRB in the upper, H-2-producing layer of the mat (0-2mm). This hypothesis was further supported by the identification of transcripts encoding hydrogenases derived from Desulfobacterales 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 Desulfobacterales are the quantitatively important hydrogenotrophs in the Elkhorn Slough mat.