Description:
<jats:title>Summary</jats:title><jats:p>A multilevel sampler (MLS) was emplaced in a borehole straddling anaerobic, sulfate‐rich Cretaceous‐era shale and sandstone rock formations ∼200 m below ground surface at Cerro Negro, New Mexico. Sterile quartzite sand contained in chambers in the sampler allowed <jats:italic>in situ</jats:italic> colonization and recovery of nucleic acids for molecular analyses. Denaturing gradient gel electrophoresis and 16S rRNA gene cloning results indicated a homogeneously distributed bacterial community across the shale–sandstone interface. δ‐Proteobacteria sequences were common at all depths, and were dominated by members of the <jats:italic>Geobacteraceae</jats:italic> family (<jats:italic>Pelobacter</jats:italic>, <jats:italic>Desulphuromonas</jats:italic> and <jats:italic>Geobacter</jats:italic>). Other members of this group are capable of dissimilatory Fe(III) and/or S° reduction, but not sulfate reduction. RNA hybridization data also suggested that Fe(III)‐/S°‐reducing bacteria were predominant. These findings are striking considering the lack of significant concentrations of these electron acceptors in this environment. The next most abundant bacterial group indicated was the sulfate reducers, including <jats:italic>Desulfobacterium</jats:italic>, <jats:italic>Desulfocapsa</jats:italic> and <jats:italic>Desulfobulbus</jats:italic>. Sequences related to fermenters, denitrifiers and acetogens were also recovered. The presence of a phylogenetically and functionally diverse microbial community in this deep subsurface environment likely reflects the complex nature of the primary energy and carbon sources, kerogen associated with the shale.</jats:p>