Bioavailability of methylmercury to sacramento blackfish (Orthodon microlepidotus): Dissolved organic carbon effects

The effect of dissolved organic carbon (DOG) on methylmercury (MeHg) uptake across the gills of Sacramento blackfish (Orthodon microlepidotus) was investigated using the Hg-203 radioisotope (half life = 46.9 d). The efficiency of fish gills in extracting MeHg (1.4 ng/L) from water was measured using a McKim-type fish respirometer that separated exposure (inspired) water from expired water. Blackfish gill ventilation and oxygen consumption rates remained constant, while (MeHg)-Hg-203 uptake was decreased significantly (p < 0.05) in the presence of DOC (2 and 5 mg C/L). Mean (MeHg)-Hg-203 extraction efficiency, uptake rate constant, and blood to inspired water ratio decreased 78%, 73%, and 63%, respectively, with 2 mg C/L of DOG, and 85%, 82%, and 70% with 5 mg C/L DOG, compared to the (MeHg)-Hg-203 reference treatment group. Because respiratory parameters remained unchanged, reductions in (MeHg)-Hg-203 uptake indicate strong interactions between DOC and (MeHg)-Hg-203. Methyl(203)Hg levels in fish gills, kidney, and spleen from 2 and 5 mg C/L were significantly lower (p < 0.05) than those observed from the reference treatment group. These reductions in uptake (bioavailability) support the hypothesis that trans-gill transport of (MeHg)-Hg-203 is inhibited when it is complexed by DOC in the aqueous medium, decreasing (MeHg)-Hg-203 uptake and accumulation in fish organs.