BIOACCUMULATION OF MERCURY AND METHYLMERCURY

The factors controlling the accumulation of mercury in fish are poorly understood. The oft invoked lipid solubility of MMHg is an inadequate explanation because inorganic Hg complexes, which are not bioaccumula ted, are as lipid soluble as their MMHg analogs and, unlike other hydr ophobic compounds, MMHg in fish resides in protein rather than fat tis sue. We show that passive uptake of the lipophilic complexes (primaril y HgCl2, and CH3HgCl) results in high concentrations of both inorganic and MMHg in phytoplankton. However, differences in partitioning withi n phytoplankton cells between inorganic mercury - which is principally membrane bound - and MMHg - which accumulates in the cytoplasm - lead to a greater assimilation of MMHg during zooplankton grazing. Most of the discrimination between inorganic and MMHg thus occurs during trop hic transfer while the major enrichment factor is between water and ph ytoplankton. As a result, MMHg concentrations in fish are ultimately d etermined by water chemistry which controls MMHg speciation and uptake at the base of the food chain.