Effects of alternative electron acceptors on the activity and community structure of methane-producing and consuming microbes in the sediments of two shallow boreal lakes

Abstract

The role of anaerobic CH₄ oxidation in controlling lake sediment CH₄ emissions remains unclear. Therefore, we tested how relevant EAs (SO₄²⁻, NO₃⁻, Fe³⁺, Mn⁴⁺, O₂) affect CH₄ production and oxidation in the sediments of two shallow boreal lakes. The changes induced to microbial communities by the addition of Fe³⁺ and Mn⁴⁺ were studied using next-generation sequencing targeting the 16S rRNA and methyl-coenzyme M reductase (mcrA) genes and mcrA transcripts. Putative anaerobic CH₄-oxidizing archaea (ANME-2D) and bacteria (NC 10) were scarce (up to 3.4% and 0.5% of archaeal and bacterial 16S rRNA genes, respectively), likely due to the low environmental stability associated with shallow depths. Consequently, the potential anaerobic CH₄ oxidation (0–2.1 nmol g⁻¹_{dry weight (DW)}d⁻¹) was not enhanced by the addition of EAs, nor important in consuming the produced CH₄ (0.6–82.5 nmol g⁻¹_DWd⁻¹). Instead, the increased EA availability suppressed CH₄ production via the outcompetition of methanogens by anaerobically respiring bacteria and via the increased protection of organic matter from microbial degradation induced by Fe³⁺ and Mn⁴⁺. Future studies could particularly assess whether anaerobic CH₄ oxidation has any ecological relevance in reducing CH₄ emissions from the numerous CH₄-emitting shallow lakes in boreal and tundra landscapes.

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