This PNAS study uses mercury stable isotope analysis to demonstrate that methylmercury (MeHg) in pelagic marine food webs originates predominantly from small particles (<53 µm) in the upper 400 m of the water column, rather than from dissolved seawater or deep-water sources as previously assumed. The isotopic signatures of MeHg in marine organisms match those of marine particles rather than dissolved MeHg, fundamentally revising the mechanistic understanding of how MeHg enters pelagic fish. The finding is relevant to understanding why epipelagic and mesopelagic fish accumulate MeHg and has implications for predicting MeHg bioaccumulation in commercially important seafood species.
Key numbers
Isotopic mass-independent fractionation (MIF) signatures of MeHg in marine organisms matched particle-bound Hg, not dissolved Hg, at depths of 0–400 m in the North Pacific. MeHg methylation inferred to occur preferentially in small particles (<53 µm) in the upper ocean, a distinct mechanism from the previously assumed in situ methylation in deep anoxic zones. This has implications for the depth zone at which commercially harvested pelagic fish bioaccumulate MeHg.
Methods (brief)
Mercury stable isotope analysis (mass-dependent and mass-independent fractionation of 202Hg, Δ199Hg, Δ201Hg) on marine organisms, filtered particles, and water from the North Pacific Ocean. ICP-MS/multi-collector ICP-MS for Hg isotope ratios.
Implications
Certification: The particle-source mechanism explains why MeHg bioaccumulation in pelagic fish is linked to surface-ocean particle flux; this is relevant to predicting which fish species carry higher MeHg loads. Courses: Key mechanism paper for understanding why MeHg is the dominant Hg species in pelagic fish and what drives inter-species and inter-region variation in fish Hg. App: Supports the mechanistic framing for MeHg in seafood ingredients; not a direct concentration measurement but informs which fish categories warrant higher MeHg flags. Microbiome: MeHg exposure route through seafood consumption has gut-microbiome interaction implications; relevant for mercury-gut-axis if that page is created.