Response of mercury methylation to algal bloom decomposition or elevated CO2 in surface sediments from the East China Sea.

Methylmercury (MeHg), a neurotoxin bioaccumulated in aquatic food webs, poses risks to ecosystems and human health, with marine products mainly contributing to global human MeHg exposure. The East China Sea (ECS), influenced by anthropogenic Hg inputs and environmental changes like algal boom and elevated CO2, is critical for understanding MeHg dynamics. This study investigated how algal decomposition and elevated CO2 (1000 ppm) affect Hg methylation in the ECS sediments. Surface sediments showed average MeHg concentrations of 0.291 ± 0.069 ng g-1 and a MeHg/THg ratio of 0.67 %. Input of algal biomass from Enteromorpha prolifera increased MeHg levels by 11 %-47 % and the abundance of Hg-methylating Deltaproteobacteria-hgcA gene by 140 %-1119 %, linked to enhanced labile organic matter. Conversely, elevated CO2 suppressed MeHg production by 54 %-77 % and reduced Deltaproteobacteria-hgcA gene abundance by 67 %-99 %, despite the total organic matter content remaining unchanged and only slight decreases (3 %-5 %) in bioavailable components (p > 0.05). These findings highlight that algae-derived organic matter stimulates microbial methylation via supporting methylator growth, while elevated CO2 inhibits it by reducing populations. The study underscores the complex impacts of human-induced environmental changes on coastal Hg biogeochemistry, providing insights for predicting MeHg risks in vulnerable marine systems under future changing scenarios.