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.