Ammonium addition inhibits 13C-methane incorporation into methanotroph membrane lipids in a freshwater sediment

Abstract

To investigate the effect of ammonium addition on the species composition and activity of freshwater methane oxidizing bacteria, intact sediment cores were labeled with (CH4)-C-13 and incubated under ambient and elevated ammonium concentrations. After 7 days, methanotroph activity was assessed by quantifying the isotopic composition of the carbon in membrane lipids. The 16-carbon rather than the 18-carbon methanotroph-specific biomarkers showed a clear enrichment in C-13, suggesting the importance of group I methanotrophs in these sediments. Ammonium addition resulted in a depleted isotopic signal compared to ambient controls, suggesting that high ammonium concentrations inhibit methane incorporation into cellular components. These results compare favorably with studies that showed ammonium inhibition of methane oxidation, and extend these findings by demonstrating the effect of nitrogen fertilization on methanotroph lipid synthesis. [KEYWORDS: methane; freshwater; sediment; ammonium; lipid; stable carbon isotope Methane oxidation; fatty-acid; atmospheric methane; community structure; loosdrecht lakes; peaty sediments; eutrophic lake; bacteria; carbon; nitrification]