Hydrogen, acetate, and lactate as electron donors for microbial manganese reduction in a manganese-rich coastal marine sediment

The role of hydrogen, acetate, and lactate as electron donors for microbial manganese reduction was investigated in manganese-rich marine sediment from Gullmar Fjord (Sweden). Here, manganese reduction accounted for 50% of the anaerobic carbon oxidation at 0-15cm sediment depth. In anoxic incubations from 0 to 5cm depth, where manganese reduction dominated completely as terminal electron-accepting process, the combined contribution of acetate and lactate as electron donors for manganese reducers corresponded to <1/4 of the electron flow. The concentrations, C-14-radiotracer turnover rates, and contributions to carbon oxidation of acetate and lactate associated with manganese reduction were similar to those found in deeper horizons dominated by concomitant iron and sulfate reduction and sulfate reduction alone, respectively. By contrast, hydrogen concentrations increased considerably with sediment depth, indicating thermodynamic control of the competition between the electron-accepting processes, and hydrogen may have contributed substantially to the >75% of the electron flow that did not involve acetate and lactate. Alternatively, the oxidation of more complex organic substrates could be involved. Our study provides the first direct evidence of substrate utilization by a natural manganese-reducing community and indicates similar mechanisms of thermodynamic control and competition for electron donors as known from sediments dominated by iron reduction, sulfate reduction, or methanogenesis.