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

14C-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 14CO2 from 14CH4 containing 14CO. 14C-labelled carbon monoxide was removed from 14CH4 by oxidation over hopcalite to carbon dioxide and fixation in sodium hydroxide solution. Measurable formation of 14CO2 from purified 14C-labelled methane by sulfate-reducing bacteria was not observed. Therefore, reported anaerobic methane oxidation rates in marine habitats measured with 14CH4 from M. thermoautotrophicum are inclined to include carbon monoxide oxidation rates. Anaerobic oxidation of 14CH4 by sulfate-reducing and acetogenic bacteria and methanogenic archaebacteria was tested. Only methanogenic species produced up to 900 ppm 14CO2 from 14CH4 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.

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