INVESTIGATING FE(III)-REDUCING MICROBIAL ACTIVITY UNDER 100% CARBON DIOXIDE HEADSPACE

CO₂ sequestration aims to use CO₂ for enhanced oil recovery or to inject into confined subsurface reservoirs for geologic storage. The effect of increased CO₂ on subsurface microbial communities is unknown. Microorganisms can influence groundwater chemistry and potentially impact the efficacy of C­O₂ sequestration. This work is part of an ongoing investigation aimed at understanding the potential for microorganisms to affect biogeochemical cycling in naturally high CO₂ systems. Previously, Fe(III)-reducing bacteria (FeRB) from aquifers with naturally high CO₂ were enriched under a 100% CO₂ headspace. We evaluated the ability of these FeRB enrichments to impact solid and aqueous phase Fe transformations.

FeRB cultures and controls (in triplicate) were cultured in anoxic seawater media with synthetic ferrihydrite and organic acids as electron acceptor and donors, respectively. Incubations were performed under 100% CO₂, at 37°C for 82 days, and in the presence of resazurin as a redox indicator. Over time, aqueous Fe(II), aqueous total Fe [Fe(II+III)], and HCl-extractable Fe(II) [Fe(II)_HCl] were measured using a ferrozine assay. Color changes and mineralogy were also monitored.

Cultures showed visible evidence of Fe(III) reduction in 7 days, indicated by a dark mineral substrate. Chemical measurements showed that Fe(III) reduction peaked at 16 days. No Fe(III) reduction was observed in control bottles. In culture bottles, aqueous Fe(II) initially increased along with increasing Fe(II)_HCl concentrations. A decrease in aqueous Fe(II) soon followed, decreasing to below detection. Mineralogical data indicated the initial presence of ferrihydrite (Fe₂O₃•H₂O), followed by siderite (FeCO₃) precipitation (observed at day 16), which indicates sequestration of CO₂ into a solid phase. These results indicate Fe(III)-reducing bacteria native to high CO₂ accumulation sites can impact the geochemistry of their environment and enhance sequestration through mineral formation.