Anaerobic methane oxidation in sulfate depleted sediments: effects of sulfate and molybdate additions

Anaerobic methane oxidation and sulfate reduction were investigated in intact marine sediment cores and in headspace-free, undiluted, homogenized, incubation bags. In intact cores, the typical upward concave methane concentration profile indicated methane oxidation in the anoxic part of the sediment. Generally, sulfate reduction rates exceeded methane oxidation rates many-fold, except in one case, where methane oxidation exceeded sulfate reduction 2 to 8 times. In the sulfate-methane transition zone, sulfate reduction was stimulated compared to rates measured above and below. Methane oxidation rates determined in incubation bags were equivalent to rates determined in intact sediment cores. Methane oxidation rates were proportional to the concentrations of methane and also increased with increasing methane concentrations in the absence of sulfate or the presence of molybdate. When sulfate was added to sulfate-depleted incubation bags, methane oxidation rates decreased immediately to less than half the rate measured prior to the addition, while sulfate reduction was stimulated. When molybdate (a specific inhibitor of sulfate-reducing bacteria) was added to a sulfate-free incubation bag, methane oxidation responded after a lag period of approximately 3 d, by uncoupling methane oxidation rates from methane concentrations. Methane production was not affected. From the outcome of our incubation bag experiments we conclude that methane is not, as previously proposed, oxidized by sulfate reducers alone. Our results support the hypothesis of Hoehler et al. (1994; Global Biogeochem Cycles 8:451-463), who proposed that a consortium of methanogenic bacteria and sulfate reducers is responsible for net oxidation of methane under anoxic conditions, a process called 'reverse methanogenesis'.