Anaerobic methane oxidation by bacteria employing C-14-methane uncontaminated with C-14-carbon monoxide

C-14-labelled methane, biologically prepared by Methanobacterium thermoautotrophicum, is widely used to determine methane oxidation rates. However, M. thermoautotrophicum synthesizes carbon monoxide as a by-product during methanogenesis. In this study, sulfate-reducing bacteria utilizing the acetyl-CoA/carbon monoxide-dehydrogenase pathway were able to form (CO2)-C-14 from (CH4)-C-14 containing (CO)-C-14. C-14-labelled carbon monoxide was removed from (CH4)-C-14 by oxidation over hopcalite to carbon dioxide and fixation in sodium hydroxide solution. Measurable formation of (CO2)-C-14 from purified C-14-labelled methane by sulfate-reducing bacteria was not observed. Therefore, reported anaerobic methane oxidation rates in marine habitats measured with (CH4)-C-14 from M. thermoautotrophicum are inclined to include carbon monoxide oxidation rates. Anaerobic oxidation of (CH4)-C-14 by sulfate-reducing and acetogenic bacteria and methanogenic archaebacteria was tested. Only methanogenic species produced up to 900 ppm (CO2)-C-14 from (CH4)-C-14 applied. This observation and the absence of methane oxidation by sulfate-reducing bacteria sustain the hypothesis that methanogenic archaebacteria in a syntrophic community might be responsible for the oxidation of methane in anaerobic habitats.