Christine Riis-Pedersen

• Riis-Pedersen, Christine; Carlsen, Eira Catharine Lødrup; Lejzerowicz, Franck Laurent Patrick; Hessen, Dag Olav & Eiler, Alexander (2026). HOW OPTICS AND THERMODYNAMICS SHAPE METHANE EMISSIONS FROM LAKES. Full text in Research Archive Show summary

  The interactions of light regime and physical stratification can be argued to dictate greenhouse gas flux through determining the thermodynamics in lake systems. Using geochemical profiling and metagenomics, we reconstructed metabolic networks in a wide range of lakes including ferruginous (iron-rich) and euxinic (sulfide-rich) systems. We observed significant methane accumulation in the anoxic monimolimnion—up to 420 µM in ferruginous lakes—yet surface levels remained low, indicating a highly efficient biofilter at the chemoclines. We identified a tight thermodynamic coupling between phototrophs and methanotrophs driven by oxidant regeneration. In ferruginous lakes, anoxygenic phototrophs likely use reduced iron (Fe2+) as an electron donor. This process regenerates oxidized iron (Fe3+), while in euxinic light energy is utilized to recycle essential electron acceptors such as SO42- (e.g., Chlorobium), required for anaerobic methane oxidation. Oxygenic phototrophs (e.g., Cyanobium) detected at and above the chemocline likely provide oxygen. This supply sustains aerobic methanotrophs like Crenothrix and Methylobacter even in suboxic waters. Hence methanotrophic capacity is ultimately constrained by optical properties selecting for low-intensity light anoxygenic or high-intensity oxygenic phototrophs. Consequently, levels of chromophoric dissolved organic matter determine methane oxidation rates. We conclude that chemoclines function as dynamic, light-powered engines for methane removal, sensitive to brownification and climate-induced shifts in stratification.

• Riis-Pedersen, Christine; Carlsen, Eira Catharine Lødrup; Hessen, Dag Olav; Lejzerowicz, Franck Laurent Patrick; Andersen, Tom & Dörsch, Peter [Show all 8 contributors for this article] (2025). Microbial drivers of methane cycling in meromictic as compared to holomictic lakes. Full text in Research Archive
• Eiler, Alexander; Riis-Pedersen, Christine; Hessen, Dag Olav & Minde, August Woldseth (2026). Under ice: The methane cycle in meromictic lakes during winter. Universitetet i Oslo. Full text in Research Archive Show summary

  Anthropogenic pressures and climate warming are increasingly driving freshwater ecosystems toward permanent stratification, yet the implications for the global carbon cycle remain poorly constrained. Meromictic lakes act as substantial reservoirs for dissolved greenhouse gases in their anoxic monimolimnia, but the efficiency of the chemocline in attenuating upward-diffusing methane (CH4) during winter ice- cover requires quantification. This study investigates depth-resolved CH4 and carbon dioxide (CO2) concentrations, alongside stable carbon isotope (δ13C) profiles, in two contrasting boreal meromictic lakes (euxinic Pollevannet and ferruginous Oppsjø) under winter conditions. High-resolution profiling revealed massive deep-water gas accumulation, with monimolimnetic CH4 concentrations exceeding 106 nM in both systems. In Pollevannet, δ13C − CH4 profiles exhibited a sharp 20‰ depletion across a 2-meter interval at the chemocline, providing isotopic evidence of a highly efficient microbial methane oxidation barrier. Furthermore, biogeochemical profiles suggest this barrier is reinforced by cryptic metabolic coupling; anoxygenic phototrophy at the low-light euxinic boundary likely regenerates sulfate (SO2−4 ), potentially subsidizing anaerobic methane oxidation in the underlying water. Oppsjø displayed a broader transition zone with more gradual isotopic shifts, reflecting its distinct ferruginous redox architecture. These findings demonstrate that stable winter stratification sustains a rigid, interdependent microbial bio-filter that effectively sequesters carbon in deep waters and decouples benthic methanogenesis from surface emissions.

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