The antagonistic effects of selenium on mercury accumulation by biota in aquatic systems.

摘要

Mercury (Hg) contamination of fish and other biota is a widespread problem throughout the Boreal Shield Ecozone of Canada. Several previous studies have shown that selenium (Se) has an antagonistic effect on Hg accumulation in aquatic life however, many questions remain regarding the mechanisms and the effective chemical forms and concentrations of Se that are responsible for the antagonistic effects. To answer these questions, both mechanism exploration and more surveys are needed. These were the aspects that I addressed in a combined lab and field study.In the survey study, I examined the relationship between Hg and Se in the various tissues (muscle, liver and brain) of walleye, one of the most common and widely distributed sportfish species of the Boreal Shield and a species known to exhibit elevated Hg concentration in the majority of lakes. The fish were sampled from selected lakes with different distances (4 to 107 Km) from the smelters (Se pollution source) in Sudbury, Ontario. My results showed that concentration of total Hg in the water did not significantly vary from lake to lake, but that of total Se did decrease with increase of distance from the smelters. Both Hg and Se concentrations in the fish tissues however did vary from lake to lake significantly (p 0.05). When the concentration of tissue Se was low, both total and methyl Hg varied from low to high in fish tissues. In this study, an apparent threshold level was detected: when the concentration of tissue total Se were higher than 6.2, 12.0 and 3.5 mg Kg-1 dry weight, for muscle, liver and brain, respectively, tissue total and methyl Hg were very low. These results suggested that Se had an antagonistic effect on Hg accumulation in walleye, but the effect took place only when Se concentration were above certain thresholds, possibly because when above these thresholds Se triggered a significant excretion of MeHg from the fish body. It was also possible that Se in an aquatic system needed to reach a certain level before it could exert dominant effects to reduce Hg accumulation in the food chain.As for the possible mechanisms, I tested the hypothesis, that the interaction of Hg and Se in aquatic environment would lead to the formation of mercuric selenide (HgSe), which was very inert and therefore sequestered Hg from being methylated and thus prevented it from being subsequently accumulated in the aquatic food chain. My results showed that HgSe was indeed very inert even in strong acid, alkaline and high sulfidic environments, indicating that Hg in such a form could not be readily methylated in an aquatic system. I also found that interaction of Hg and Se species commonly found in aquatic systems, such as elemental Se (Se0), selenite (SeO3 2-), elemental Hg (Hg0) and mercuric Hg (Hg 2+), could lead to the formation of HgSe. For example, Se0 could absorb Hg0 to form HgSe SeO3 2- could turned Hg2+ into HgSe under the mediation of a bacterium, Pseudomonas fluorescens, and possible other similar microorganisms in aquatic systems. The mechanism study showed that the formation of HgSe could explain the observed Hg -- Se antagonism.