Abstract:
Rapid warming in the Arctic is driving permafrost thaw, which in turn alters hydrology, vegetation, carbon accumulation, and microbial communities. All of these changes significantly influence greenhouse gas (GHG) emissions, including carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). A key factor controlling these emissions is the association between organic carbon (OC) and iron (Fe) minerals, as up to 50% of OC in permafrost soils is stabilized by Fe(III) minerals. As the climate warms, these Fe(III)-OC associations become destabilized through the reductive dissolution of Fe(III) minerals, a process driven by Fe(III)-reducing bacteria in waterlogged and oxygen-limited conditions. This process releases dissolved organic carbon (DOC), which can then be metabolized, leading to greater GHG emissions (primarily CO2 and CH4).
As permafrost continues to thaw, aqueous iron (Fe2+) and associated OC are released in both dissolved and particulate forms and are transported into small thaw ponds where redox conditions fluctuate. Seasonal runoff and extended waterlogging can shift redox conditions from oxic to anoxic, affecting the balance between Fe(II) oxidation and Fe(III) reduction. However, the impact of these redox shifts on Fe-OC interactions, and consequently on carbon mobilization and GHG production, remains unclear.
To address this knowledge gap, I first characterized Fe and OC phases in partially thawed ‘bog’ and fully thawed ‘fen’ ponds at Stordalen Mire near Abisko, Sweden. I collected samples and analyzed them for Fe and OC concentrations, Fe redox states, mineralogy, OC functional groups, and the spatial correlation between Fe and OC using both bulk and nanoscale techniques. I then incubated bog pond samples in the laboratory under different redox conditions (anoxic, oxic, and fluctuating anoxic-oxic), and monitored Fe(II) and total Fe concentrations, OC levels, CO2 and CH4 fluxes, Fe mineral speciation, and particle charge interactions.
The results showed that bog thaw ponds had higher OC concentrations (50 to 352 mg/L), whereas fen thaw ponds contained more iron (1.5 to 212 mg/L). Iron occurred in both Fe(II) and Fe(III) redox states, either associated with OC (8 to 80%) or precipitated as poorly crystalline Fe(III) oxyhydroxides. Fe-OC associations were present in dissolved, small particulate, and large particulate size fractions. While Fe(II) dominated the large particulate fraction, Fe(III) was detected in the dissolved fraction, possibly as Fe(III)-OC associations or ferrihydrite, and remained stable under the anoxic conditions typical of thaw ponds.
Subjecting pond water to anoxic incubation indicated that microbial Fe(III) reduction released OC into the dissolved phase of thaw ponds. Under oxic conditions, however, OC re-adsorbed onto Fe(III) minerals. It can be concluded that microbial Fe(III) reduction not only drove iron cycling but also controlled OC availability and transformation, particularly when redox conditions changed. With continuous thawing, intensified redox fluctuations may speed up OC breakdown, increasing GHG emissions and accelerating climate change. Ultimately, mineral-associated OC in thaw ponds was susceptible to microbial decomposition and shifting redox conditions promoted its mobilization and the release of GHGs.
In addition to environmental samples, I examined an iron-organic complex called pulcherrimin which is produced by the aerobic bacterium Staphylococcus epidermidis. I observed that iron concentration in the growth medium influenced pulcherrimin production. This concentration affected the distribution of iron between the supernatant and pellet, with approximately 70% of the iron remaining in the supernatant alongside extracellular pulcherrimin. This served as another example of an iron-organic complex with potential applications, such as managing biofilm-related infections through iron sequestration and complexation.
Iron‐carbon interactions