Deleterious microbial activity is commonly controlled with biocides at significant cost to the driller. However, despite biocide use, microbial activity is prevalent in produced water. Previous studies have shown that biocide effectiveness may be limited by high salt concentrations, organic compounds, and long residence times in the subsurface. Other studies have shown that microbial communities in produced water are distinct from those in the injected fracturing fluid, and correlate well with changes in geochemical and environmental conditions. This implies that the common practice of recycling produced water for subsequent hydraulic fracturing may introduce adapted populations into the formation. Over the past decade molecular ecology surveys based on the 16S rRNA gene have increased our knowledge about the taxonomic Silmitasertib citations composition of microbial communities in reservoir environments,,,. However, these studies offer limited insights on the metabolic capabilities of the microbial community, as they rely on taxonomic inference based on 16S rRNA gene similarity to previously isolated microorganisms. As an example of the limitations of using previously isolated microorganisms to infer metabolic capability, the ‘core genome’ of the well-studied Escherichia coli is typically less than 50% of the genes in the genome, and,30% of the E. coli pan-genome. On the other hand, shotgun metagenomic surveys enable access to complete genetic information within microbial genomes from uncultured, mixed consortia. These surveys have provided significant insights on the functional potential of microorganisms in diverse environments such as marine samples, corals, activated sludge, permafrost, hydrocarbon and sandstone reservoirs, and swine gut. Despite the importance of microbial activity in produced water brines from hydraulic fracturing operations, the functional potential of associated microbial communities has not yet been studied. In this study, the metagenome of fracturing source water and produced water at two different time points from a Marcellus Shale natural gas well in Westmoreland County, PA was generated using Illumina MiSeq technology. The microbial ecology from 16S rRNA surveys and chemical composition of these samples has been described in a previous publication. Sequences from each sample were assembled into contiguous sequences and analyzed for taxonomic affiliations and functional potential of the microbial communities. These functional categories were similarly identified as dominant in previous studies of soil, marine samples, activated sludge, freshwater and hypersaline environments. Normalization of gene abundance data shows a relative increase in each of the above functional categories in the produced water samples as compared to the fracturing source water implying that core systems necessary for survival are enriched in the produced water community.