Microbial Methane Oxidation Rates in Guandu Wetland of northern Taiwan

Microbially-mediated methane oxidation plays an important role in global methane budgets. Aerobic methanotroph uses O2 to serve as an electron acceptor, and is widespread in ground surface. Anaerobic oxidation of methane (AOM) can couple to sulfate reduction, denitrification, iron reduction and manganese reduction, which means that SO42-, NO3-, iron oxide and manganese oxide are able to be the electron acceptors of AOM. Wetlands exist anaerobic conditions and are the largest source of methane emissions, but the AOM mechanisms and their abilities of methane removal have not been fully examined. In this study, a sub-tropical wetland in northern Taiwan, Guandu, was chosen to examine the tidal effects and effects of electron acceptors on microbial methane regulation. We conducted laboratory experiments with sediments collected during high tide and low tide periods from the Guandu wetland and both aerobic and anaerobic potential methane oxidation rates were estimated. Results showed that aerobic oxidation rates during low tide periods were higher than those during high tide periods. Because the methane concentrations in pore water were higher in low tide sediments than those in high tide sediments, the microbial activities might also be higher during low tides. On the contrary, anaerobic oxidation rates were higher during high tide periods than those during low tide periods. The difference of AOM rates between high tide and low tide periods may be due to microbial competition. Sulfate reduction coupled to organic matter oxidation may outcompete sulfate reduction coupled to methane oxidation, resulting in slow AOM rates during low tide periods. In electron acceptors addition experiments, the highest potential rates were observed in the sulfate addition and no addition treatments. The AQDS treatment showed lower rates than that in previous two treatments. The AOM rates in the iron oxide, ferric citrate and fumarate treatments were even slower. The AOM was only detected in part of the incubation periods in nitrate and manganese treatments. These results indicated that the major electron acceptor for AOM in the Guandu wetland is sulfate. The addition of two humic acids, ferric citrate and fumarate were not helpful for methane removal, because microbial methane generation was stimulated in much higher rates than those of AOM.