Seasonal and Habitat-Specific Mercury Cycling in Coastal Biota Revealed by Species-Specific Isotopes

Understanding how inorganic mercury (IHg, primarily Hg(II)) and methylmercury (MeHg, primarily monomethylmercury) transform and cycle in coastal food webs is essential for evaluating Hg exposure risks, yet relevant species-specific mercury (Hg) isotope studies remain scarce. Here, we optimized a purge-trap method that enabled efficient separation and isotopic analysis of IHg and MeHg in marine organisms. Applying this method to surface and benthic organisms collected from the Bohai Sea in winter and summer, we reveal distinct seasonal and habitat-specific Hg dynamics. Surface organisms primarily accumulated MeHg that had undergone photodemethylation in the water column, while benthic organisms assimilated a mixture of photodegraded water-column MeHg (64-80%) and nonphotodegraded MeHg produced in sediments (20-36%). In vivo MeHg demethylation was substantially enhanced in surface organisms during summer, contributing ∼36% of the organismal IHg pool. Species-specific isotope signatures further suggest that methylation of IHg in sediments is a major source of MeHg to the water column, with ∼14-19% of it being photodegraded before biological uptake. Together, these findings provide new mechanistic insight into coastal Hg cycling and advance the isotopic framework for assessing ecological risks associated with marine Hg exposure.