Carbon isotope fractionation by circumneutral iron-oxidizing bacteria

Bacteriogenic iron oxides in natural environments are characterized by an abundance of ferrihydrite precipitates intermixed with bacterial structures that commonly resemble those produced by the lithoautotrophic microorganisms Gallionella ferruginea and Leptohtrix ochracea. These species have been inferred to play a causal role in the formation of bacteriogenic iron oxides, providing a pathway for the reduction of CO (sub 2) and the depletion of (super 13) C in the organic constituents of bacteriogenic iron oxides. In this study, stable carbon isotope fractionation was determined for bacteriogenic iron oxide samples collected from submarine hydrothermal vents (Axial Volcano, Juan de Fuca Ridge), subterranean (Aspo Hard Rock Laboratory, Sweden) and surficial (Chalk River, Canada) groundwater seeps, and cultures of G. ferruginea. Data were also collected from ferrihydrite samples lacking evidence of bacteria from Bounty Seamount in the vicinity of Pitcairn Island. The mean delta (super 13) C ( per mil) of ferrihydrite was determined to be -15.87 per mil + or -4.96 per mil for the samples from Axial Volcano, -24.97 per mil + or -0.43 per mil for Aspo, -27.80 per mil + or -0.85 per mil for Chalk River, -29.3 per mil + or -0.2 per mil for the microbial culture, and -8.43 per mil + or -1.89 per mil for the samples from Pitcairn. Samples with the highest concentration of organic carbon also had the lightest delta (super 13) C in a logarithmic relationship. The consistency of carbon isotope values in relation to the presence of iron-oxidizing bacteria from natural and laboratory samples is interpreted as the ability of these microorganisms to fractionate carbon. The potential of this fractionation to serve as a biosignature holds promise when the resistance of carbon and bacteriogenic ferrihydrite to diagenesis is taken into consideration.