Fe-oxide grain coatings support bacterial Fe-reducing metabolisms in 1.7−2.0 km-deep subsurface quartz arenite sandstone reservoirs of the Illinois Basin (USA)

Abstract

The Cambrian-age Mt. Simon Sandstone, deeply buried within the Illinois Basin of the midcontinent of North America, contains quartz sand grains ubiquitously encrusted with iron-oxide cements and dissolved ferrous iron in pore-water. Although microbial iron reduction has previously been documented in the deep terrestrial subsurface, the potential for diagenetic mineral cementation to drive microbial activity has not been well studied. In this study, two subsurface formation water samples were collected at 1.72 and 2.02 km, respectively, from the Mt. Simon Sandstone in Decatur, Illinois. Low-diversity microbial communities were detected from both horizons and were dominated by Halanaerobiales of Phylum Firmicutes. Iron-reducing enrichment cultures fed with ferric citrate were successfully established using the formation water. Phylogenetic classification identified the enriched species to be related to Vulcanibacillus from the 1.72 km depth sample, while Orenia dominated the communities at 2.02 km of burial depth. Species-specific quantitative analyses of the enriched organisms in the microbial communities suggest that they are indigenous to the Mt. Simon Sandstone. Optimal iron reduction by the 1.72 km enrichment culture occurred at a temperature of 40°C (range 20–60°C) and a salinity of 25 parts per thousand (range 25–75 ppt). This culture also mediated fermentation and nitrate reduction. In contrast, the 2.02 km enrichment culture exclusively utilized hydrogen and pyruvate as the electron donors for iron reduction, tolerated a wider range of salinities (25–200 ppt), and exhibited only minimal nitrate- and sulfate-reduction. In addition, the 2.02 km depth community actively reduces the more crystalline ferric iron minerals goethite and hematite. The results suggest evolutionary adaptation of the autochthonous microbial communities to the Mt. Simon Sandstone and carries potentially important implications for future utilization of this reservoir for CO2 injection.

Orenia metallireducens sp. nov. Strain Z6, a Novel Metal-Reducing Member of the Phylum Firmicutes from the Deep Subsurface

“…Strain Z6 was isolated from an iron-reducing enrichment culture created using the groundwater taken at a depth of 2.02 km in the Illinois Basin as an inoculum (27). Unless otherwise specified, this isolate was cultivated anaerobically in 25-ml anaerobic culture tubes or 120-ml serum bottles.…”

“…Anoxic culture tubes were loaded with 6 ml of basal medium (18) and amended with low-melting-point agarose (Thermo Fisher Scientific, Inc., MA). After autoclaving, 4 ml of N 2 -bubbled and degassed filter-sterilized formation water (see Table S1 in the supplemental material) (27) was added to each tube, giving a final concentration of 1.5% agarose. Each anoxic culture tube was then amended with 5 mM ferric citrate, H 2 (202 mol/tube), and 5 mM (each) formate, acetate, pyruvate, and lactate that had been used in the enrichment culture (27).…”

“…Our previous work identified indigenous microbial communities within different stratigraphic horizons in the Mt. Simon formation, and using this water, we successfully developed enrichment cultures capable of reducing ferric iron compounds (27,28). In this paper, we describe a bacterium isolated from these enrichments that is capable of reducing a broad suite of iron minerals.…”

“…Simon formation within the Illinois Basin, IL, USA, is composed primarily of highly porous and permeable quartz sandstones coated with crystalline ferric minerals, including hematite and goethite (25). The presence of these iron minerals along with millimolar concentrations of dissolved ferrous iron in the groundwater suggests that microbially mediated iron reduction is active in this environment (27). This groundwater is thermal (50°C), briny (19.1% total dissolved salts [TDS]), anoxic, and under considerable pressure (21 ϫ 10 6 Pa) (27).…”

“…The presence of these iron minerals along with millimolar concentrations of dissolved ferrous iron in the groundwater suggests that microbially mediated iron reduction is active in this environment (27). This groundwater is thermal (50°C), briny (19.1% total dissolved salts [TDS]), anoxic, and under considerable pressure (21 ϫ 10 6 Pa) (27). Our previous work identified indigenous microbial communities within different stratigraphic horizons in the Mt.…”