Trace methane oxidation and the methane dependency of sulfate reduction in anaerobic granular sludge

This study investigates the oxidation of labeled methane (CH₄) and the CH₄ dependence of sulfate reduction in three types of anaerobic granular sludge. In all samples, ¹³C-labeled CH₄ was anaerobically oxidized to ¹³C-labeled CO₂, while net endogenous CH₄ production was observed. Labeled-CH₄ oxidation rates followed CH₄ production rates, and the presence of sulfate hampered both labeled-CH₄ oxidation and methanogenesis. Labeled-CH₄ oxidation was therefore linked to methanogenesis. This process is referred to as trace CH₄ oxidation and has been demonstrated in methanogenic pure cultures. This study shows that the ratio between labeled-CH₄ oxidation and methanogenesis is positively affected by the CH₄ partial pressure and that this ratio is in methanogenic granular sludge more than 40 times higher than that in pure cultures of methanogens. The CH₄ partial pressure also positively affected sulfate reduction and negatively affected methanogenesis: a repression of methanogenesis at elevated CH₄ partial pressures confers an advantage to sulfate reducers that compete with methanogens for common substrates, formed from endogenous material. The oxidation of labeled CH₄ and the CH₄ dependence of sulfate reduction are thus not necessarily evidence of anaerobic oxidation of CH₄ coupled to sulfate reduction.