Ammonium conversion and its feedback effect on methane oxidation of Methylosinus sporium

Ammonium (NH4+) is not only nitrogen source that can support methanotrophic growth, but also it can inhibit methane (CH4) oxidation by competing with CH4 for the active site of methane monooxygenase. NH4+ conversion and its feedback effect on the growth and activity of methanotrophs were evaluated with Methylosinus sporium used as a model methanotroph. Nitrogen sources could affect the CH4-derived carbon distribution, which varied with incubation time and nitrogen concentrations. More CH4-derived carbon was incorporated into biomass in the media with NH4+-N, compared to nitrate-nitrogen (NO3−-N), as sole nitrogen source at the nitrogen concentrations of 10-18 mmol L−1. Although ammonia (NH3) oxidation activity of methanotrophs was considerably lower, only accounting for 0.01-0.06% of CH4 oxidation activity in the experimental cultures, NH4+ conversion could lead to the pH decrease and toxic intermediates accumulation in the their habits. Compared with NH4+, nitrite (NO2−) accumulation in the NH4+ conversion of methanotroph had stronger inhibition on its activity, especially the joint inhibition of NO2− accumulation and the pH decrease during the NH4+-N conversion. These results suggested that more attention should be paid to the feedback effects of NH4+ conversion by methanotrophs to understand effects of NH4+ on CH4 oxidation in the environments.