Methylmercury (MeHg) is a neurotoxin that bioaccumulates and biomagnifies in marine food chains. Photodemethylation is a major sink of MeHg in euphotic seawater, resulting in lower levels of MeHg in surface seawater than in deep seawater, and limitation of MeHg uptake by marine organisms. The role of dissolved organic matter (DOM) is important in photodemethylation of MeHg because DOM can produce free radicals including reactive oxygen species (ROS) and form photosensitive MeHg−DOM complexes. There are four potential pathways in MeHg photodemethylation: (1) direct photodemethylation of MeHg, (2) photodemethylation of MeHg by free radicals including ROS (1O2, 3DOM*, e-aq, ●OH), (3) photodemethylation of MeHg-DOM complexes by free radicals including ROS, and (4) direct photodemethylation of MeHg-DOM complexes via intramolecular charge transfer. In order to investigate the major pathways of MeHg photodemethylation in euphotic seawater, we measured demethylation rates of MeHg in artificial seawater (salinity 35) under UV-A, in the presence, and absence of DOM and specific ROS scavengers. The fluorescence quenching was measured by excitation-emission matrix to confirm the complexation of MeHg by DOM and to identify DOM components reacting with MeHg. The initial experimental results showed that approximately 60% MeHg was demethylated in the presence of DOM (Pony Lake Fulvic acid, PLFA) under UV-A radiation (120 h), whereas only 10% of MeHg was demethylated under dark conditions. In the absence of DOM, MeHg concentration did not decrease at all under UV-A irradiation. The demethylation kinetics for various types of DOM and specific ROS scavengers in artificial seawater are currently under investigation.

Role of dissolved organic matter in photodecomposition of methylmercury in seawater

251st National Meeting (2016)

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