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Hypersaline microbial mats have been shown to produce significant quantities of H2 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 H2 that might otherwise be captured as a resource. Here, we focus on H2 consumption in a microbial mat from Elkhorn Slough, California, USA, for which H2 production has been previously characterized. Active biologic H2 consumption in this mat is indicated by a significant time-dependent decrease in added H2 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 H2 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 Desulfobacterales are the dominant, active SRB in the upper, H2-producing layer of the mat (0-2 mm). This hypothesis was further supported by the identification of transcripts encoding hydrogenases derived from Desulfobacterales capable of H2 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. [ABSTRACT FROM AUTHOR]

l.c. burow, k.s. gobius, b.a. vanselow and a.v. klieve. 2004.To investigate interactions between rumen protozoa and Shiga toxin-producingEscherichia coli(STEC) and to ascertain whether it is likely that rumen protozoa act as ruminant hosts for STEC.The presence ofstxgenes in different microbial fractions recovered from cattle and sheep rumen contents and faeces was examined using PCR. In animals shedding faecal STEC,stxgenes were not detected in the rumen bacterial or rumen protozoal fractions. Direct interactions between ruminal protozoa and STEC were investigated byin vitroco-incubation. Rumen protozoa did not appear to ingest STEC, a STEC lysogen or non-STECE. colipopulations when co-incubated.The ruminal environment is unlikely to be a preferred habitat for STEC. Bacterial grazing by rumen protozoa appears to have little, if any, effect on STEC populations.This study indicates that ruminal protozoa are unlikely to be a major factor in the survival of STEC in ruminants. They appear as neither a host that protects STEC from the ruminal environment nor a predator that might reduce STEC numbers. [ABSTRACT FROM AUTHOR]