MEPS 267:133-143 (2004)  -  doi:10.3354/meps267133

ABSTRACT: To study the effects of sublethal doses of mercury on corals, colonies of Porites astreoides (Lamarck) were exposed to nominal concentrations of 0.01, 0.1 and 0.5 mg Hg l^-1 using semi-static, chronic bioassays for up to 15 d, with HgCl₂ administered by pulses every 3 d (mean concentration in the water was 0.004, 0.037 and 0.180 mg Hg l^-1, respectively). While total Hg in the corals was directly proportional to Hg exposure, analysis of the different coral compartments (polyps, zooxanthellae and skeleton) showed that zooxanthellae and the skeleton accumulated Hg in direct relation to Hg exposure, but polyp tissue accumulated more Hg at 0.1 than at 0.5 mg Hg l^-1. This suggests saturation of Hg only in polyps and/or activation of mechanisms of detoxification. Within a colony, the Hg concentration per unit area of coral surface differed between compartments as follows: zooxanthellae > polyp > skeleton. Colonies exposed to the highest Hg concentration accumulated 1.738 µg Hg cm^-2, 89% of which was found in zooxanthellae, 7% in polyps and 4% in the skeleton. Polyp biomass (dry weight and protein content), zooxanthellae biomass (cell density and protein content), and pigment concentration per unit area of coral surface decreased with Hg exposure. The bioconcentration factor ([Hg] in organism/[Hg] in water) was inversely related to the Hg concentration in water. The capacity of zooxanthellae and the skeleton to concentrate Hg and the decrease in zooxanthellae density support the hypothesis that polyps may divert Hg to these 2 coral compartments as a detoxifying mechanism.

KEY WORDS: Mercury · Bioconcentration · Bioassays · Sub-lethal exposure · Hard corals · Porites · Caribbean