Photomicrobial Visible Light-Induced Magnetic Mass Independent Fractionation of Mercury in a Marine Microalga

Methylmercury (MeHg), a highly neurotoxic substance, accumulates in aquatic food webs, and is enriched in odd isotopes (i.e., 199 Hg and 201 Hg), purportedly as a result of abiotic photodegradation in surface waters. Here, we highlight the potential role of phytoplankton in the mass independent fractionation (MIF) of MeHg in marine food-webs by providing evidence of (1) degradation of intracellular MeHg and reduction of intracellular inorganic mercury (Hg(II)) in the marine microalga, Isochrysis galbana; (2) a large, positive MIF (Δ 199 Hg reactant – Δ 199 Hg product ∼ 5–10‰) during intracellular degradation of MeHg in cells exposed to visible light with no UVB, consistent with the accumulation of odd isotope-enriched MeHg in marine food-webs; and (3) a negative MIF (−1‰) during intracellular reduction of Hg(II) in the presence of UVB light. If representative of the photochemical reactivity of MeHg in marine phytoplankton, our results indicate that algal cell-mediated demethylation of MeHg by visible light could account for 20 to 55% of the total photochemically driven demethylation in the open ocean and transparent freshwater ecosystems with deep euphotic zones. Thus, our results extend the importance of phytoplankton (and possibly other light permeable microorganisms) in mercury biogeochemistry beyond their role as accumulators of MeHg and/or reducers of Hg(II) at the base of the food chain to include MeHg degradation and MIF of Hg in sunlit layers of the ocean and other aquatic systems.