Spatial heterogeneity of methylmercury in plateau wetland: Driving forces from the decomposition of different aquatic plants.

Aquatic plant decline in wetlands is globally prevalent, which can affect mercury (Hg) methylation in sediment, whereas it is yet unknown whether this process depends on the specific plant species. A short-term decomposition experiment was conducted using dominant aquatic plants in a plateau wetland to assess their effects on Hg methylation in sediment. The study revealed that the decomposition of aquatic plants differentially influences the dissolved organic matter (DOM) properties, sulfate levels, and microbial composition, as well as methylmercury (MeHg) production in sediment. A two-stage MeHg pattern, similar to terrestrial plants, was shaped by aquatic plants decomposition, following an increase of 19.42-53.96% in the first stage, whereas a decrease of 10.35-57.99% in the later stage. Compared to the control group, plants decomposition resulted in a peak MeHg in the early stage, but it depended on plant species. However, during the later stages of incubation, the decomposition of Stuckenia pectinata (S. pectinata) resulted in a significant 39.24% reduction in sediment MeHg, while other plants did not result in significant differences. This contrasting outcome was attributed to the specific regulatory mechanisms of S. pectinata, including a decrease in sulfate concentrations and Hg-methylating bacteria coupled with an increase in MeHg-demethylating bacteria, complex sulfur-containing compounds and aromaticity of DOM. Given the spatial-specific colonization of aquatic macrophytes, their role in driving spatial heterogeneity of Hg methylation should be emphasized.