Controls of Dissolved Organic Matter and Chloride on Mercury Uptake by a Marine Diatom

The effects of natural dissolved organic carbon (DOC) from different origins (estuarine, coastal, and diatom decomposed) and chloride (CI) on the uptake of inorganic mercury [Hg(II)] and methylated mercury (MeHg) by the marine diatom Thalassiosira pseudonana was investigated using radiotracer techniques. We first developed a new method to remove the surface adsorbed mercury and quantified the intracellular mercury uptake by the diatoms. The dominant mercury species (DOC or chloride complexes, based on the mercury speciation phase diagrams) was controlled by the concentrations of DOC and Cl-, which could explain the effects of DOC and Cl- on mercury uptake. DOC complexes dominated Hg(II)'s speciation and reduced its uptake in most seawater examined. DOC complexes dominated MeHg's speciation only at relatively high DOC levels (>100 AM), but it could affect MeHg uptake even when MeHg-Cl complexes dominated. In a mercury-DOC complex dominated system, both the origin and quantity of DOC greatly influenced mercury uptake by the diatoms. Although DOC generally inhibited the uptake of Hg(II) or MeHg, DOC resulting from diatom decomposition enhanced Hg(II) uptake. Under conditions dominated by chloride complexation, neutral mercury chloride species (HgCl2 or MeHgCl) may control the uptake.