Total mercury, methylmercury and selenium in the livers and muscle of different fishes and a marine mammal from a tropical estuary-Brazil

摘要

In order to assess the importance of liver in the regulation of mercury accumulation, two fish species with different feeding habits, a carnivorous fish and an omnivorous fish, and a marine dolphin were collected at Guanabara Bay. Total mercury concentration (T-Hg) in liver of these organisms was generally higher than that found in muscle, presenting statistically significant difference (U test, Z= -2.2; p 0.05). In the case of the two fish species the median ratio of liver T-Hg : muscle T-Hg was 2.3 (range 1.2-3.9, n=14) for the carnivorous and 4.2 (2.9-5.9, n=14) for the omnivorous. In the dolphin, the median ratio was 15.6 (range 2.2-31.0, n=1 5). Absolute concentrations of T-Hg (fresh weight basis) in liver were greater and more variable in the dolphin (median 1.62, range 1.1-132.6 mu g g~(-1), n=3) than carnivorous and omnivorous fishes (median 0.2, range 0.02-0.4 mu g g~(-1)) and (median 0.3, range 0.2-0.6 mu g.g~(-1)). Dolphin presented lower percentage of methylmercury (percent MeHg) in liver (median 4.7 percent) than carnivorous and omnivorous fishes (median 31 percent and 22 percent respectively). Meanwhile, the three species of organisms showed similar percentage of MeHg in their muscle, around 100 percent. The dolphin presented the lowest median molar selenium: mercury ratio for its muscle and liver concentrations (0.8, range 0.5-1.8 and 1.5, range 0.5-4.6 respectively). There was a highly statistically significant difference (Z=-3.3, p= 10~(-8)) in the median molar selenium: mercury ratio in the livers of the fish species (median Se:Hg 26.4 for carnivorous and 11.7 for omnivorous). However, the median molar selenium: mercury ratio for these same species also presented a statistically significant difference (Z--2.2; p-10~(-2)) in muscles (median Se:Hg 3.9 for carnivorous and 2.7 for omnivorous). The most likely explanation for the results is efficient demethylation in the liver, suggesting the possible extensive mercury selenite formation.