Methane Oxidation by Nitrosococcus oceanus and Nitrosomonas europaea

Chemolithotrophic ammonium-oxidizing and nitrite-oxidizing bacteria including Nitrosomonas europaea, Nitrosococcus oceanus, Nitrobacter sp., Nitiospina gracilis, and Nitrococcus mobilis were examined as to their ability to oxidize methane in the absence of ammonium or nitrite. All ammonium oxidizers tested had the ability to oxidize significant amounts of methane to CO(2) and incorporate various amounts into cellular components. None of the nitrite-oxidizing bacteria were capable of methane oxidation. The methane-oxidizing capabilities of Nitrosococcus oceanus and Nitrosomonas europaea were examined with respect to ammonium and methane concentrations, nitrogen source, and pH. The addition of ammonium stimulated both CO(2) production and cellular incorporation of methane-carbon by both organisms. Less than 0.1 mM CH(4) in solution inhibited the oxidation of ammonium by Nitrosococcus oceanus by 87%. Methane concentrations up to 1.0 mM had no inhibitory effects on ammonium oxidation by Nitrosomonas europaea. In the absence of NH(4)-N, Nitrosococcus oceanus achieved a maximum methane oxidation rate of 2.20 x 10 mumol of CH(4) h mg (dry weight) of cells, which remained constant as the methane concentration was increased. In the presence of NH(4)-N (10 ppm 10 mug/ml), its maximum rate was 26.4 x 10 mumol of CH(4) h mg (dry weight) of cells at a methane concentration of 1.19 x 10 mM. Increasing the methane concentration above this level decreased CO(2) production, whereas cellular incorporation of methane-carbon continued to increase. Nitrosomonas europaea showed a linear response throughout the test range, with an activity of 196.0 x 10 mumol of CH(4) h mg (dry weight) of cells at a methane concentration of 1.38 x 10 mM. Both nitrite and nitrate stimulated the oxidation of methane. The pH range was similar to that for ammonium oxidation, but the points of maximum activity were at lower values for the oxidation of methane.