Anaerobic oxidation of methane mediated by iron and graphene oxides in coastal sediments

Methane (CH₄) is a potent greenhouse gas. Coastal systems account for a large fraction of marine CH₄ emissions. This emphasizes the need to understand coastal sources and sinks of CH₄. Most of the CH₄ in coastal systems is produced in sediments during organic matter degradation and consumed through anaerobic oxidation of CH₄ (AOM), a process predominantly mediated by anaerobic methanotrophic archaea (ANME). Typically, AOM coupled to sulfate reduction is considered the dominant CH₄ sink. However, in iron (Fe)-rich sediments, CH₄ can also be oxidized either directly via coupling to Fe(III) reduction or indirectly via an Fe-driven cryptic sulfur cycle that sustains sulfate-dependent AOM. Additionally, natural organic matter (NOM) may also act as an electron acceptor in AOM.

While Fe-dependent AOM has been demonstrated in surface sediments, experimental evidence for such processes in deeper sediment layers (>1 m) remains limited and is largely inferred from model studies. Furthermore, experimental evidence for NOM-dependent AOM in coastal sediments remains scarce. The Bothnian Sea is a brackish basin in the northern Baltic Sea that receives high inputs of reactive Fe oxides and organic matter, creating conditions that may favor Fe- and NOM-coupled AOM in its deep sediments.

In this study we assess whether there is potential for AOM coupled to Fe and NOM reduction in deep sediments (>1 m) of the Bothnian Sea. We present results from long-term incubation experiments using sediments retrieved from the Bothnian Sea, site US5B which we amend with Fe oxide and graphene oxide, a NOM analogue, to evaluate their effect on CH₄ oxidation. Our incubations, using ¹³CH₄, show that Fe oxide and graphene oxide both stimulate AOM. In the case of Fe oxide, this could potentially involve a cryptic sulfur cycle. Based on metagenomic sequencing, ANME-2a/b archaea and potential metal-oxide reducing bacteria were enriched over time in both treatments. These findings provide new experimental constraints on the occurrence and relevance of Fe oxide and natural organic matter as electron acceptors in AOM in Fe-oxide and organic rich coastal sediments.