ISOTOPIC AND CHEMICAL INVESTIGATIONS OF FE (III) REDUCTION BY SHEWANELLA PUTREFACIENS STRAIN 200R

Stable isotope fractionation based studies on metabolic pathways during metal reduction are recent and limited. Equilibrium fractionation between bacterial biomass and synthesized fatty acids were studied previously to identify the predominant carbon fixation pathways for Shewanella (~14.3‰). In this study, carbon isotope disproportionation among organic carbon substrate used (lactate), biomass and respired carbon dioxide at the lag to stationary phase of the growth curve of Fe⁺³ -reducing bacterium (FeRB) Shewanella Putrefaciens strain 200R was investigated during dissimilatory Fe (III) reduction at circum-neutral pH condition. Ferric citrate (δ¹³C, -24.7‰) and sodium lactate (δ¹³C, -24.8‰) were used as electron acceptor and donor respectively. Sodium bicarbonate (δ¹³C, +10‰) or potassium phosphate was used as buffering agent. Time series aqueous iron (II)/(III) were determined using the ferrozine method. The resulting carbon dioxide was extracted cryogenically for subsequent DIC (dissolved inorganic carbon) and carbon isotope ratios measurement. Our results showed that bicarbonate system has an enhancing effect in the reduction process compared to the phosphate system. Both systems resulted in carbon isotope fractionations that could be modeled as a Rayleigh process. The biomass in both buffer conditions was depleted by ~2‰ and enriched by ~5‰ compared to the substrate and the DIC respectively. This translates to an overall isotopic fractionation of 10 - 12‰ between the biomass and respired CO₂ in both systems.