Warming effects and biogeochemical drivers on methylmercury production in Arctic soils

Abstract:

Arctic soils store not only a vast amount of soil organic carbon (SOC) but also a globally significant amount of mercury (Hg). Climate warming increases microbial activity and stimulates both the degradation of SOC and transformation of inorganic Hg to the neurotoxin, methylmercury (MeHg), which bioaccumulates and biomagnifies in the Arctic ecosystem. However, currently little is known concerning how SOC degradation influences MeHg production and what are the biogeochemical drivers of MeHg production in these soils. This study investigated the effects of warming on coupled SOC degradation and microbial Hg methylation in anoxic microcosm incubations. Mercury methylation is found to be positively correlated to the reduction of sulfate and the production of methane (CH₄) and ferrous iron, suggesting linkages among microbial pathways of sulfate reduction, methanogenesis, and methylation. Net MeHg production could increase up to 10 fold at warmer (8 °C) than colder (–2 °C) temperatures and is influenced by geochemical characteristics, such as the availability of sulfate and labile SOC. Selective microbial inhibitor experiments indicate that sulfate-reducing bacteria (SRB) are among the major contributors to Hg methylation initially when sulfate is available in the soil. However, when sulfate is depleted, methanogens equally contribute to Hg methylation. In the low-sulfate soil, both SRB and methanogens contribute to MeHg production throughout the incubation. Together, our results indicate that soil warming not only increases the emission of greenhouse gases but also fuels microbial Hg methylation, thereby increasing MeHg bioaccumulation and impacting the food web in Arctic ecosystems.