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

To investigate the effect of ammonium addition on the species composition a nd activity of freshwater methane oxidizing bacteria, intact sediment cores were labeled with (CH4)-C-13 and incubated under ambient and elevated ammo nium concentrations. After 7 days, methanotroph activity was assessed by qu antifying 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 methanotrop hs in these sediments. Ammonium addition resulted in a depleted isotopic si gnal compared to ambient controls, suggesting that high ammonium concentrat ions inhibit methane incorporation into cellular components. These results compare favorably with studies that showed ammonium inhibition of methane o xidation, and extend these findings by demonstrating the effect of nitrogen fertilization on methanotroph lipid synthesis. (C) 1999 Federation of Euro pean Microbiological Societies. Published by Elsevier Science B.V. All righ ts reserved.