Bacterial Halanaerobium strains become the dominant persisting microbial community member in produced fluids across geographically distinct hydraulically fractured shales. Halanaerobium is believed to be inadvertently introduced into this environment during the drilling and fracturing process and must therefore tolerate large changes in pressure, temperature, and salinity. Here, we used a Halanaerobium strain isolated from a natural gas well in the Utica Point Pleasant formation to investigate metabolic and physiological responses to growth under high-pressure subsurface conditions. Laboratory incubations confirmed the ability of H. congolense strain WG8 to grow under pressures representative of deep shale formations (21-48 MPa). Under these conditions, broad metabolic and physiological shifts were identified, including higher abundances of proteins associated with the production of extracellular polymeric substances. Confocal laser scanning microscopy indicated that EPS production was associated with greater cell aggregation when biomass was cultured at high-pressure. Changes in Halanaerobium central carbon metabolism under the same conditions were inferred from NMR and gas chromatography measurements, revealing large per-cell increases in production of ethanol, acetate and propanol and cessation of hydrogen production. These metabolic shifts were associated with carbon flux through 1,2 propanediol in response to slower fluxes of carbon through stage 3 of glycolysis. Together, these results reveal the potential for bio-clogging and corrosion (via organic acid fermentation products) associated with persistent Halanaerobium growth in deep, hydraulically fractured shale ecosystems, and offer new insights into cellular mechanisms that enable these strains to dominate deep shale microbiomes. [ABSTRACT FROM AUTHOR]