Products of trace methane oxidation during nonmethyltrophic growth by Methanosarcina

Our understanding of the metabolic pathways involved in the anaerobic oxidation of methane (AOM) is challenged by not having an isolated anaerobic methanotroph. Fortunately, trace methane oxidation (TMO) is carried out by isolated organisms closely related to anaerobic methanotrophs. As TMO likely shares metabolic similarities with the AOM metabolism, studies of TMO in culture experiments can serve to enhance our understanding of AOM. Here we explored TMO in cultures of Methanosarcina acetivorans grown separately on acetate and carbon monoxide. We observed no methane oxidation to carbon dioxide in the cultures grown on acetate, but did observe methane oxidation to acetate (1.1 × 10^-6 ± 3.5 × 10^-8 moles CH₄ oxidized, 0.10% of the CH₄ produced). Methane was exclusively converted to the methyl position. Because AOM lipids exhibit strong ¹³C depletion in nature, our results imply acetate does not play a major role as an AOM intermediate. The carbon monoxide cultures showed little or no methane conversion to carbon dioxide (1.9 × 10^-7 × 2.0 × 10^-7 moles CH₄ oxidized), moderate methane oxidation to acetate (1.8 × 10^-7 ± 7.3 × 10^-8 moles CH₄ oxidized, 0.14% of biogenic CH₄ production), and high methane conversion to the methyl groups of product methyl sulfides (4.0 × 10^-6 ± 7.7 × 10^-7 moles CH₄ oxidized, 3.1% of biogenic CH₄ production). Taken together, our results identify methyl sulfides as more likely intermediates in natural AOM than acetate.