Peatlands are globally important ecosystems where inorganic mercury is converted to bioaccumulating and highly toxic methylmercury, which causes severe impacts on wildlife and humans. Although biological mercury methylation has been known for decades, we still have an incomplete understanding of organisms and processes driving methylation in different environments. We hypothesize that the changes in the trophic status of peatlands across the chronosequence of peatlands will be expressed in the composition of their microbial communities and thus in their potential to form MeHg. Here we used experimental laboratory incubations of peat samples from a mire chronosequence (0-3000 years) with fundamentally different biogeochemical conditions to identify master controls of mercury methylation and demethylation. The results showed that the potential mercury methylation rates decreased with the age of the peatlands, being up to 53 times higher in the youngest peatland compared to the oldest. Mercury methylation in young mires was mainly driven by sulfate reduction, while methanogenic and syntrophic metabolism became more important in older systems. Also demethylation rates were highest in young wetlands, with a gradual shift from biotic to abiotic degradation along the chronosequence. Our findings reveal how metabolic shifts drive mercury methylation and its ratio to demethylation as peatlands age and advance our understanding of mercury dynamics in nature.

14th International Conference on Mercury as a Global Pollutant (ICMGP 2019), 8-13 September 2019, Krakow, Poland.-- 1 page