Role of natural organic matter and iron(III) for methanogenesis and methane oxidation in thawing permafrost soils

Permafrost soils store about twice as much organic carbon as the atmosphere. In the future, certain permafrost regions will develop anoxic soil conditions due to thaw-induced soil subsidence and waterlogging. Under these conditions, methane (CH₄) emissions due to decomposition of newly thawed organic carbon will likely increase. The net release of CH₄ from soil depends on the availability of more energetically favorable electron acceptors than CO₂, which could on the one hand suppress methanogenesis and on the other hand act as an electron acceptor for anaerobic CH₄ oxidation. Since many common inorganic electron acceptors (sulfate, nitrate) are present only in low concentrations in permafrost soils, we hypothesize that natural organic matter (NOM) and/or ferric iron (Fe(III)) are more abundant and can act as significant electron acceptors. However, to which extent NOM fractions such as dissolved organic matter (DOM) and particulate organic matter (POM) as well as Fe(III) minerals influence methane production and methane oxidation in permafrost soils is unknown. In this project, we therefore aim (i) to characterize the redox-active moieties of DOM and POM fractions from permafrost soils, (ii) to quantify the effect of these NOM fractions on methanogenesis suppression and/or CH₄ oxidation, and (iii) to identify the microorganisms that are able to oxidize CH₄ coupled to NOM or Fe(III) reduction by performing enrichment culture experiments. To achieve this, we collected and isolated NOM from a thawing permafrost peatland in Sweden (Stordalen Mire, Abisko) across multiple thaw stages. We analyzed the changes in electron accepting and donating capacity of NOM fractions across permafrost thaw stages via mediated electrochemical reduction and oxidation, respectively. Enrichments targeting anaerobic CH₄-oxidizers were set up using an inoculum from partly thawed and fully thawed permafrost thaw stages, amended with poorly crystalline Fe(III) minerals, AQDS (a model compound for redox-active moieties in NOM) and POM. In the future, microcosm experiments with isolated NOM fractions and ¹³C-labeled CH₄ or ¹³C-labeled CO₂ will be performed in order to quantify the influence of NOM on methane oxidation or methanogenesis suppression, respectively. Spectroscopic, isotope-tracing and molecular biology techniques will be used to track the reduction of amended electron acceptors, concentration of labeled gases and the change in abundance of targeted microorganisms. Overall, this work will help to assess the role of NOM and Fe(III) in influencing CH₄ cycling in thawing permafrost peatlands.