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

Abstract

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 14CO 2 from 14CH 4 containing 14CO. 14C-labelled carbon monoxide was removed from 14CH 4 by oxidation over hopcalite to carbon dioxide and fixation in sodium hydroxide solution. Measurable formation of 14CO 2 from purified 14C-labelled methane by sulfate-reducing bacteria was not observed. Therefore, reported anaerobic methane oxidation rates in marine habitats measured with 14CH 4 from M. thermoautotrophicum are inclined to include carbon monoxide oxidation rates. Anaerobic oxidation of 14CH 4 by sulfate-reducing and acetogenic bacteria and methanogenic archaebacteria was tested. Only methanogenic species produced up to 900 ppm 14CO 2 from 14CH 4 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.