EFFECTS OF METHANE METABOLISM ON NITRIFICATION AND NITROUS-OXIDE PRODUCTION IN POLLUTED FRESH-WATER SEDIMENT

We report the effect of CH4 and of CH4 oxidation on nitrification in freshwater sediment from Hamilton Harbour, Ontario, Canada, a highly polluted ecosystem. Aerobic slurry experiments showed a high potential for aerobic N4O production in some sites. It was suppressed by C2H2, correlated to NO3- production, and stimulated by NH4+ concentration, supporting the hypothesis of a nitrification-dependent source for this N2O production. Diluted sediment slurries supplemented with CH4 (1 to 24 mu M) showed earlier and enhanced nitrification and N2O production compared with unsupplemented slurries (less than or equal to 1 mu M CH4). This suggests that nitrification by methanotrophs may be significant in freshwater sediment under certain conditions. Suppression of nitrification was observed at CH4 concentrations of 84 mu M and greater, possibly through competition for O-2 between methanotrophs and NH4+-oxidizing bacteria and/or competition for mineral N between these two groups of organisms. In Hamilton Harbour sediment, the very high CH4 concentrations (1.02 to 6.83 mM) which exist would probably suppress nitrification and favor NH4+ accumulation in the pore water. Indeed, NH4+ concentrations in Hamilton Harbour sediment are higher than those found in other lakes. We conclude that the impact of CH4 metabolism on N cycling processes in freshwater ecosystems should be given more attention.