The "Fe-S wheel": A new perspective on methylmercury production dynamics in subalpine peatlands

The cycling of iron (Fe) and sulfur (S) in peatland ecosystems plays a pivotal role in modulating methylmercury (MeHg) formation. This study integrates data on Fe and S fractions, geochemical factors, microbial communities, and statistical modeling to propose the novel "Fe-S wheel" conceptual framework. This framework explores the coupled cycling of Fe, S, and Hg in the Dajiuhu peatland (DJH), an exemplary natural laboratory in central China. Through this framework, we demonstrate that the "Fe-S wheel" exerts a strong direct inhibitory effect on MeHg formation. However, when the S/Fe molar ratio is less than 0.25, the "Fe-S wheel", influenced by microbial communities, can indirectly enhance MeHg generation by promoting humic acid-bound Hg. Conversely, when the S/Fe molar ratio exceeds 0.25, the "Fe-S wheel", under the influence of dissolved oxygen, suppresses MeHg formation by inhibiting strong-complexed Hg and sulfide-bound Hg. Global peatland data corroborate these findings, showing a significant negative correlation between the S/Fe ratio and MeHg concentrations. Given the uncertainties in the interaction and transformation mechanisms between Fe and S, the S/Fe molar ratio is likely to serve as a key parameter reflecting their coupled effects on MeHg. This study highlights the critical role of Fe and S interactions in regulating MeHg generation within peatland ecosystems.