FIELD AND LABORATORY STUDIES OF METHANE OXIDATION IN AN ANOXIC MARINESEDIMENT - EVIDENCE FOR A METHANOGEN-SULFATE REDUCER CONSORTIUM

Field and laboratory studies of anoxic sediments from Cape Lookout Big ht, North Carolina, suggest that anaerobic methane oxidation is mediat ed by a consortium of methanogenic and sulfate-reducing bacteria. A se asonal survey of methane oxidation and CO2 reduction rates indicates t hat methane production was confined to sulfate-depleted sediments at a ll times of year, while methane oxidation occurred in two modes. In th e summer, methane oxidation was confined to sulfate-depleted sediments and occurred at rates lower than those of CO2 reduction. In the winte r, net methane oxidation occurred in an interval at the base of the su lfate-containing zone. Sediment incubation experiments suggest both me thanogens and sulfate reducers were responsible for the observed metha ne oxidation. In one incubation experiment both modes of oxidation wer e partially inhibited by 2-bromoethanesulfonic acid (a specific inhibi tor of methanogens). This evidence, along with the apparent confinemen t of methane oxidation to sulfate-depleted sediments in the summer, in dicates that methanogenic bacteria are involved in methane oxidation. In a second incubation experiment, net methane oxidation was induced b y adding sulfate to homogenized methanogenic sediments, suggesting tha t sulfate reducers also play a role in the process. We hypothesize tha t methanogens oxidize methane and produce hydrogen via a reversal of C O2 reduction. The hydrogen is efficiently removed and maintained at lo w concentrations by sulfate reducers. Pore water Hz concentrations in the sediment incubation experiments (while net methane oxidation was o ccurring) were low enough that methanogenic bacteria could derive suff icient energy for growth from the oxidation of methane. The methanogen -sulfate reducer consortium is consistent not only with the results of this study, but may also be a feasible mechanism for previously docum ented anaerobic methane oxidation in both freshwater and marine enviro nments.