The Powder River Basin (PRB) in southeastern Montana and northeastern Wyoming contains massive coal deposits with biologically generated coal bed methane (CBM). The microbial ecology of an area within a coal bed influenced by recent groundwater recharge was sampled with a diffusive microbial sampler (DMS). The DMS contained native coal material and was incubated in situ (57 m depth) to allow colonization of the coal particles. Pyrotag sequence analyses of SSU rRNA gene sequences from the coal contained within the post-incubation DMS detected methylotrophic and hydrogenotrophic methanogenic archaea along with diverse bacterial communities. Microbial enrichments (coal or acetate/H-2) were established from the DMS, and the enriched bacterial and archaeal communities were characterized via clone library analysis. The in situ bacterial communities were more diverse than the archaeal communities, and the archaeal populations differed between coal incubated in situ and in laboratory enrichments. In addition, bacterial diversity was higher for laboratory enrichments with coal compared to enrichments without coal. The elucidation of relationships between microorganisms involved in coal degradation and metabolite (acetate, H-2) utilization within coal-dependent microbial communities is crucial to understanding and improving in situ coal bed methane production. (C) 2013 Elsevier B.V. All rights reserved.