Effects of nitrogen oxides and denitrification by Pseudomonas stutzeri on acetotrophic methanogenesis by Methanosarcina mazei

Abstract

Nitrate and its reduced forms produced during denitrification, nitrite and nitrous oxide, were studied for their influence on methane production from acetate by <it>Methanosarcina mazei</it>. While 0.18 mM nitrite and 0.8% nitrous oxide in the gas phase completely suppressed methane production, 71.4 mM nitrate resulted in only 83.3% inhibition. Co-culture experiments showed that <it>M. mazei</it> growing with 15 mM nitrate produced methane from acetate until the denitrifying bacterium <it>Pseudomonas stutzeri</it> was inoculated and nitrate denitrification began. The presence of nitrous oxide in the gas phase after cessation of denitrification activity by <it>P. stutzeri</it> in co-cultures flasks prevented <it>M. mazei</it> resuming methane production. Nitrous oxide, instead of dinitrogen, was the end product of denitrification by <it>P. stutzeri</it> either in pure cultures or in co-cultures with <it>M. mazei</it>, probably because of the highly reduced culture conditions that were used. This study strongly suggests that acetate-dependent methane production by <it>M. mazei</it> was inhibited by reduced nitrogen forms produced during bacterial nitrate denitrification, rather than by competition for acetate between denitrifying and methanogenic bacteria. These results are consistent with previous studies with H<inf>2</inf>/CO<inf>2</inf> methanogens.