SummaryA low-diversity microbial community, dominated by the γ-proteobacterium Halomonas sulfidaeris, was detected in samples of warm saline formation porewater collected from the Cambrian Mt. Simon Sandstone in the Illinois Basin of the North American Midcontinent (1.8 km/5872 ft burial depth, 50°C, pH 8, 181 bars pressure). These highly porous and permeable quartz arenite sandstones are directly analogous to reservoirs around the world targeted for largescale hydrocarbon extraction, as well as subsurface gas and carbon storage. A new downhole lowcontamination subsurface sampling probe was used to collect in situ formation water samples for microbial environmental metagenomic analyses. Multiple lines of evidence suggest that this H. sulfidaerisdominated subsurface microbial community is indigenous and not derived from drilling mud microbial contamination. Data to support this includes V1-V3 pyrosequencing of formation water and drilling mud, as well as comparison with previously published microbial analyses of drilling muds in other sites. Metabolic pathway reconstruction, constrained by the

A novel halophilic and metal-reducing bacterium, Orenia metallireducens strain Z6, was isolated from briny groundwater extracted from a 2.02 km-deep borehole in the Illinois Basin, IL. This organism shared 96% 16S rRNA gene similarity with Orenia marismortui but demonstrated physiological properties previously unknown for this genus. In addition to exhibiting a fermentative metabolism typical of the genus Orenia, strain Z6 reduces various metal oxides [Fe(III), Mn(IV), Co(III), and Cr(VI)], using H 2 as the electron donor. Strain Z6 actively reduced ferrihydrite over broad ranges of pH (6 to 9.6), salinity (0.4 to 3.5 M NaCl), and temperature (20 to 60°C). At pH 6.5, strain Z6 also reduced more crystalline iron oxides, such as lepidocrocite (␥- IMPORTANCEA novel iron-reducing species, Orenia metallireducens sp. nov., strain Z6, was isolated from groundwater collected from a geological formation located 2.02 km below land surface in the Illinois Basin, USA. Phylogenetic, physiologic, and genomic analyses of strain Z6 found it to have unique properties for iron reducers, including (i) active microbial iron-reducing capacity under broad ranges of temperatures (20 to 60°C), pHs (6 to 9.6), and salinities (0.4 to 3.5 M NaCl), (ii) lack of c-type cytochromes typically affiliated with iron reduction in Geobacter and Shewanella species, and (iii) being the only member of the Halanaerobiales capable of reducing crystalline goethite and hematite. This study expands the scope of phylogenetic affiliations, metabolic capacities, and catalytic mechanisms for iron-reducing microbes.T he reduction of ferric [Fe(III)-bearing] minerals by microorganisms is widespread in both terrestrial and marine environments (1, 2) and is potentially one of the earliest forms of metabolism (3, 4). Due to the abundance of ferric minerals in the Earth's crust and the ubiquity of dissimilatory metal-reducing bacteria (DMRB), Fe(III) reduction is of global environmental significance, particularly in the subsurface. A phylogenetic variety of organisms has been reported for the capacity to reduce iron in a dissimilatory manner (1, 5). Dissimilatory iron reduction can be classified into two major groups. Many of the iron-reducing organisms (e.g., fermentative iron reducers) use Fe(III) as a minor side reaction in their metabolism but do not appear to conserve energy to support growth from this electron transfer. In comparison, the respiratory iron reducers conserve energy to support growth from Fe(III) via an electron transfer chain (1,5). DMRB strongly influence the biogeochemical cycling of metals and mineralization of organic matter in aquatic and sedimentary environments (1,(6)(7)(8). In addition, DMRB play a key role in the bioremediation of hazardous contaminants, such as heavy metals, radionuclides, and hydrocarbons (9). DMRB capable of reducing ferric minerals have previously been isolated from gold mines, petroleum reservoirs, and other