REGULATION OF METHANE OXIDATION IN A FRESH-WATER WETLAND BY WATER-TABLE CHANGES AND ANOXIA

The effects of water table fluctuations and anoxia on methane emission and methane oxidation were studied in a freshwater marsh. Seasonal ae robic methane oxidation rates varied between 15% and 76% of the potent ial diffusive methane flux (diffusive flux in the absence of aerobic o xidation). On an annual basis, approximately 43% of the methane diffus ing into the oxic zone was oxidized before reaching the atmosphere. Th e highest methane oxidation was observed when the water table was belo w the peat surface. This was confirmed in laboratory experiments where short-term decreases in water table levels increased methane oxidatio n but also net methane emission. Although methane emission was general ly not observed during the winter, stems of soft rush (Juncus effusus) emitted methane when the marsh was ice covered, Indigenous methanotro phic bacteria from the wetland studied were relatively anoxia tolerant . Surface peat incubated under anoxic conditions maintained 30% of the initial methane oxidation capacity after 32 days of anoxia. Methanotr ophs from anoxic peat initiated aerobic methane oxidation relatively q uickly after oxygen addition (1-7 hours). These results were supported by culture experiments with the methanotroph Methylosinus trichospori um OB3b. This organism maintained a greater capacity for aerobic metha ne oxidation when starved under anoxic compared to oxic conditions. An oxic incubation of M. trichosporium OB3b in the presence of sulfide (2 mM) and a low redox potential(-110 mV) did not decrease the capacity for methane oxidation relative to anoxic cultures incubated without su lfide. The results suggest that aerobic methane oxidation was a major regulator of seasonal methane emission from the investigated wetland. The observed water table fluctuations affected net methane oxidation p resumably due to associated changes in oxygen gradients. However, chan ges from oxic to anoxic conditions in situ had relatively little effec t on survival of the methanotrophic bacteria and thus on methane oxida tion potential per se.