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Abstract

The contribution of volatile fatty acids (VFA) as e–-donors for anaerobic terminal oxidation of organic carbon through iron and sulfate reduction was studied in Arctic fjord sediment. Dissolved inorganic carbon, Fe2+, VFA concentrations, and sulfate reduction were monitored in slurries from the oxidized (0–2 cm) and the reduced (5–9 cm) zone. In the 0–2 cm layer, 2/3 of the mineralization could be attributed to sulfate reduction and 1/3 to iron reduction. In the 5–9 cm layer, sulfate reduction was the sole mineralization process. Acetate and lactate turnover rates were measured by radiotracer. Inhibition of sulfate reduction with selenate resulted in the accumulation of acetate, propionate, and isobutyrate. The acetate turnover rates determined by radiotracer and accumulation after inhibition were similar. VFA turnover accounted for 21% and 52% of the mineralization through sulfate reduction in the 0–2 and 5–9 cm layer, respectively. Acetate and lactate turnover in the inhibited 0–2 cm slurry was attributed to iron reduction and accounted for 10% and 2% of the iron reduction. Therefore, 88% and 79% of the iron and sulfate reduction in the 0–2 cm layer, respectively, must be fueled by alternative e−-donors. The accumulation of VFA in the selenate–inhibited 0–2 cm slurry did not enhance iron reduction, indicating that iron reducers were not limited by VFA availability.