Bioavailability of inorganic and methylmercury to a marine deposit- feeding polychaete

We measured the assimilation efficiencies (AEs) from various types of sediments and the uptake rate constants from the dissolved phase of inorganic mercury (Hg(II)) and methylmercury (CH₃Hg(II)) in the marine deposit- feeding polychaete Nereis succinea. AEs of Hg(II) ranged between 7 and 30% and were unaffected by sediment composition, whereas AEs of CH₃Hg(II) ranged between 43 and 83% and were strongly affected by sediment composition. Sediment grain size had no apparent effect on Hg(II) and CH₃Hg(II) assimilation. AEs for Hg(II) associated with anoxic sediment were slightly lower than with oxic sediment, whereas CH₃Hg(II) displayed comparable AEs for both oxic and anoxic sediment. Dissolved uptake rate constants of CH₃Hg(II) were 2.2 times those of Hg(II). A bioenergetic-based kinetic model was used to separate the pathways (solute vs sediment) and sources [Hg(II) vs CH₃Hg(II)] of Hg accumulation in N. succinea. The model predicted that, under conditions typical of coastal sediment environments, CH₃Hg(II) accumulation contributes about 5-17% of total Hg accumulation in polychaetes. Most of the Hg(II) (> 70%) accumulation is predicted to derive from sediment ingestion, whereas for CH₃Hg(II) the relative importance of dissolved vs sediment ingestion depends greatly on its partition coefficient for sediments. Uptake from the dissolved phase and sediment ingestion can be equally important for CH₃Hg(II) accumulation in N. succinea.