Rapid methylmercury demethylation in phytoplankton during algal blooms: insights from mercury isotopes and dynamic modeling.

Methylmercury (MeHg) is a persistent toxic pollutant of global concern. Phytoplankton plays a pivotal role in the bioaccumulation and transformation of MeHg in aquatic ecosystems, with several algal species demonstrating the capacity for MeHg demethylation. However, the pathways and contributions of such demethylation remain poorly understood in natural ecosystems due to extremely low MeHg concentrations. By integrating compound-specific stable isotope analysis and dynamic modeling, this study traced MeHg transformation within phytoplankton in a natural lake. The results indicate that rapid growth dilution, combined with active demethylation, reduces MeHg concentrations in phytoplankton over time. A considerable proportion (32.8%) of MeHg is degraded via light-dependent demethylation, leading to a significantly increased mass-independent fractionation (MIF, Δ¹⁹⁹Hg). Notably, we estimated the MIF of MeHg in the water column to be ∼1.0‰, which is significantly lower than that in food webs (∼3.3‰). This contrast confirms that phytoplankton, rather than abiotic processes in the water column, serves as the main driver of photo-demethylation in aquatic ecosystems. In addition, demethylation, especially light-independent demethylation, constitutes a notable source of inorganic Hg (IHg) in phytoplankton, contributing up to 20.7% of total IHg. These findings highlight the critical yet previously overlooked role of phytoplankton-mediated demethylation in the Hg biogeochemical cycle. Given the increasing incidence of algal blooms under climate change, this study provides novel insights into MeHg dynamics and calls for refining models of MeHg bioaccumulation and associated risks.