Particulate organic matter as electron acceptor for microbial respiration in peatlands

Northern peatlands store approximately 500 Pg carbon in the form of peat particulate organic matter (POM). Ombrotrophic bogs are peatlands that only receive water and nutrients through precipitation, creating anoxic, water-logged soils deprived of inorganic terminal electron acceptors (TEAs). In the absence of suitable TEAs for anaerobic respiration, methanogenesis prevails as final step in the degradation of organic matter and is expected to result in equimolar CO₂:CH₄ production ratios. However, field and laboratory studies revealed higher CO₂:CH₄ production ratios than expected based on low concentrations of canonical inorganic TEAs, suggesting the presence of a previously unrecognized TEA used in anaerobic microbial respiration. It has been hypothesized that oxidized particulate organic matter (POM_ox) functions as TEA, explaining elevated CO₂:CH₄ production ratios. Through seasonal water table fluctuations, POM gets re-oxidized abiotically, creating a microbial hotspot at the oxic-anoxic interface. To investigate these processes, incubation studies linking CO₂ and CH₄ production to the reduction of POM_ox are indispensable. Here, we present data strongly indicating that POM collected from ombrotrophic bogs in Sweden functions as TEA in anaerobic respiration, suppressing methanogenesis. We ran anoxic incubations with various initial ratios of oxidized and reduced POM and hence a range of starting electron accepting capacities, which we quantified using a novel spectrophotometric assay. Increasing contributions of POM_ox resulted in higher CO₂:CH₄ production ratios and prolonged transition times from anaerobic respiration to methanogenesis. These findings strongly support the use of POM as TEA, suppressing methanogenesis until POM_ox was depleted through respiration. Additionally, we developed an incubation system that allowed amending incubations with ¹³C-labeled substrates to selectively track their conversion to ¹³CO₂ and ¹³CH₄. Using ¹³C-glucose we successfully linked ¹³CO₂ and ¹³CH₄ formation ratios to POM redox state. Our results advance our understanding of microbial carbon turnover in peatlands in the present and future climate.