Previous studies show that subglacial lakes can host endemic communities of microorganisms adapted to cold, dark, and nutrient-poor waters, but mechanism by which microbes disseminate under the ice and colonize these lakes is unknown. We present new molecular data on the subglacial microbial communities of Vatnajökull generated from samples of two subglacial lakes, a subglacial flood, and a lake that was formerly subglacial but is now partly exposed.  These data include paired pyrosequence libraries generated with novel primers that span the v3-v5 and v4-v6 hypervariable regions of the bacterial 16S rRNA gene.  In both subglacial lakes the community is dominated by five bacterial taxa, and lacks a detectable archaeal population.  The dominant taxa are closely related to cultured anaerobic or micro-aerobic species in the genera Acetobacterium, Geobacter, Sulfuricurvum, Sulfurospirillum, and Desulfosporosinus, and the metabolisms of these relatives suggests a chemolithotrophic ecosystem based on homoacetogenesis, ferric iron reduction, and sulfur cycling.  The populations of the major taxa in the subglacial lakes are indistinguishable (>99% 16S rRNA sequence identity), despite separation by 6 kilometers and an ice divide; the Sulfuricurvum-like taxon is ubiquitous in our samples.  We propose that much of the glacial bed of Vatnajökull is connected by an aquifer in the underlying permeable basalt, and that these subglacial lakes are colonized by members of a subterranean microbiome that are adapted to ambient conditions and energy sources.  Our hypothesis of a "deep" source is supported by growth of thermophiles in enrichments from lake samples.  Some preliminary results of metagenomic sequencing will be presented.