Methylmercury production across an organic carbon gradient in Fe-rich subarctic soils

Arctic and subarctic soils accumulate anthropogenic inorganic mercury (Hg) transported from lower latitudes. Under reducing conditions, it can be converted to the neurotoxic methylmercury (MeHg). However, the extent and biogeochemical drivers of MeHg production in these soils remain unclear. Here, we investigated MeHg production in various types of Icelandic soils rich in iron (Fe, 40-110 g kg -1 ) and presenting varying organic carbon content (OC, 40-250 g kg -1 ), classified herein as mineral, intermediate or organic soils. Six topsoils (0-25 cm) and three subsoils (70-130 cm) were incubated anoxically for 18 weeks during which MeHg production and release to the aqueous phase were followed together with ancillary parameters (e.g., Fe(III) and sulfate reduction) and microbial community composition. Large MeHg production (up to 11% of total Hg) was observed across topsoils having different characteristics, with topsoils generally exhibiting greater MeHg production than subsoils. A larger release of MeHg to the aqueous phase was observed for mineral soils, indicating a higher potential risk for connected aquatic ecosystems. Total OC content was a poor predictor of MeHg production. In contrast, MeHg concentrations were correlated with (i) the abundance of hgcA , a key gene required for Hg methylation, in a desulfobacterotal group and (ii) geochemical and microbial indicators of both Fe(III) and sulfate reduction. Overall, this study shows that not only organic but also mineral soils are crucial to assess MeHg risks in (sub)arctic environments. Moreover, the extent of MeHg production was better explained by temporal changes in microbial and geochemical processes, including hgcA abundance, Fe(III) and sulfate reduction, than the initial biogeochemical characteristics of the soils.