Role of redox-active dissolved organic matter for methane cycling in thawing permafrost peatlands

Permafrost peatlands represent a large organic carbon stock and are currently a net carbon sink. However, some permafrost regions will develop anoxic conditions due to soil subsidence and waterlogging in the future. Under these conditions, it is estimated that methane (CH₄) emissions will increase due to the higher availability of newly mobilized dissolved organic matter (DOM) for microorganisms. However, little attention has been given to redox-active functional groups within DOM, which could also play a role in lowering CH₄ emissions. On the one hand, oxidized redox-active DOM could suppress methanogenesis thermodynamically, while on the other hand it could act as an electron acceptor for anaerobic CH₄ oxidation (AOM). Both processes would decrease net CH₄ release. However, the change in redox-active moieties in DOM across thaw in permafrost peatlands and their role in AOM have not been determined yet. In this project, we therefore aim (i) to quantify the changes in abundance and oxidation state of redox-active DOM along a thaw gradient and (ii) to determine the effect of oxidized redox-active DOM on AOM. To achieve this, we collected porewater samples over four depths (10-40 cm) across multiple thaw stages during July and September 2025 in a thawing permafrost peatland in Sweden (Stordalen Mire, Abisko). We analyzed the electron accepting and donating capacity of the anoxic porewater via mediated electrochemical reduction and oxidation, respectively. We found that the electron accepting capacity attributable to DOM significantly decreases from recently thawed to fully thawed sites (from 1.68±0.65 to 0.77±0.58 mmol e^- g^-1 C^-1, p<0.01). Further, mean electron donating capacity attributable to DOM was positively correlated to the average CH₄ flux per site (R=0.53, p<0.05), suggesting that more reduced redox-active DOM co-occurs with a higher CH₄ release. Additionally, microcosm experiments with water-extracted DOM from the peat and ¹³C-labeled CH₄ were performed in order to quantify the rates of methane oxidation in the presence and absence of redox-active DOM. We used a combination of electrochemical, isotope-tracing and molecular biology techniques to track the reduction of amended DOM, production of ¹³C-CO₂ and the change in abundance of methane-oxidizing microorganisms. Overall, this work will help to assess the importance of redox-active DOM for CH₄ cycling in thawing permafrost peatlands.