EFFECTS OF DISSOLVED ORGANIC-CARBON AND SALINITY ON BIOAVAILABILITY OF MERCURY

Hypotheses that dissolved organic carbon (DOG) and electrochemical cha rge affect the rate of methylmercury [CH3Hg(I)] synthesis by modulatin g the availability of ionic mercury [Hg(II)] to bacteria were tested b y using a mer-lux bioindicator (O. Selifonova, R, Burlage, and T, Bark ay, Appl, Environ, Microbiol, 59:3083-3090, 1993), A decline in Hg(II) -dependent light production was observed in the presence of increasing concentrations of DOG, and this decline was more pronounced at pH 7 t han at pH 5, suggesting that DOC is a factor controlling the bioavaila bility of Hg(LI). A thermodynamic model (MINTEQA2) was used to select assay conditions that clearly distinguished among various Hg(II) speci es, By using this approach, it was shown that negatively charged forms of mercuric chloride (HgCl3-/HgCl42-) induced less light production t han the electrochemically neutral form (HgCl2), and no difference was observed between the two neutral forms, HgCl2 and Hg(OH)(2), These res ults suggest that the negative charge of Hg(II) species reduces their availability to bacteria and may be one reason why accumulation of CH3 Hg(I) is more often reported to occur in freshwater than in estuarine and marine biota.