EFFECTS OF NITROGEN-OXIDES AND DENITRIFICATION BY PSEUDOMONAS-STUTZERI ON ACETOTROPHIC METHANOGENESIS BY METHANOSARCINA-MAZEI

Nitrate and its reduced forms produced during denitrification, nitrite and nitrous oxide, were studied for their influence on methane produc tion from acetate by Methanosarcina mazei. While 0.18 mM nitrite and 0 .8% nitrous oxide in the gas phase completely suppressed methane produ ction, 71.4 mM nitrate resulted in only 83.3% inhibition. Go-culture e xperiments showed that M. mazei growing with 15 mM nitrate produced me thane from acetate until the denitrifying bacterium Pseudomonas stutze ri was inoculated and nitrate denitrification began. The presence of n itrous oxide in the gas phase after cessation of denitrification activ ity by P. stutzeri in co-cultures flasks prevented M. mazei resuming m ethane production. Nitrous oxide, instead of dinitrogen, was the end p roduct of denitrification by P. stutzeri either in pure cultures or in co-cultures with M. mazei, probably because of the highly reduced cul ture conditions that were used. This study strongly suggests that acet ate-dependent methane production by M. mazei was inhibited by reduced nitrogen forms produced during bacterial nitrate denitrification, rath er than by competition for acetate between denitrifying and methanogen ic bacteria. These results are consistent with previous studies with H -2/CO2 methanogens. (C) 1998 Federation of European Microbiological So cieties. Published by Elsevier Science B.V.