Biochemical aspects of the uptakes of mercury and selenium by the native British oyster (Ostrea edulis)

Wrench, John James (1977) Biochemical aspects of the uptakes of mercury and selenium by the native British oyster (Ostrea edulis). University of Southampton, Doctoral Thesis.

Abstract

Several aspects of the uptake of dissolved mercury by the oyster Ostrea edulis have been investigated.Concentration factors for these elements have been determined using the isotopes 203 Hg, in the form Hg2+, and 75 Se, in the form of Se03.Concentration factors for mercury in the digestive gland and gill were found to be in the range 1500-2000 and 400-600 respectively; comparable values for selenium in these tissues were approximately an order of magnitude lower.Investigation of uptake in the whole animal over a 24 hour period has shown that under. similar experimental conditions approximately100 times more mercury is taken up than selenium., Starvation significantly reduced the uptake of both elements by a factor of 30-40%, which corresponded to the percentage reduction, in protein levels estimated by the Biuret method. Moreover, the quantity of mercury taken up was increased by exposure to SeO2 during the starvation period. It is suggested that this effect is due to a quantitative increase in macromolecular binding sites for mercury.The LC50 values for Hg2+ and sear were found to be 10 ppm and50.1 ppm respectively; corresponding minimal lethal concentrations were estimated to be 0.2 ppm and 10 ppm. Exposure of animals to a sub-lethal concentration of SeD3 during the acclimatisation period increased the toxic response to mercury, with a reduction in the LCso to F 5.5 -ppm. All toxiat6testS bere of seven C6 & & s duration Investigation of the sub-cellular distribution of mercury and selenium has shown that a significant fraction of both elements in digestive gland is associated with the soluble protein. Comparable values are 349; of the whole homogenised tissue for mercury and 50% for selenium. I After a short period of starvation, the fraction of mercury bound to soluble protein is reduced to 20%.Investigation of the protein binding characteristics of Hg2+and Se03 have indicated that the binding of Se03 was weaker than that of Hg 2+. Moreover, this interaction could largely be supressed by raising the pH, which suggested ionic binding of Se03 to positively charged nitrogen groups. However, the majority of selenium taken up by Ostrea edulis was found to be non-dialysable under alkaline conditions; this observation suggested that Se03 was largely metabolised to some other form. The application of selective extraction procedures and column.~chromatography have enabled two of these metabolites to be identified as hydrogen selenide and as seleno-amino acids. These techniques have also been applied, with similar results, to an investigation of the forms of selenium in one species of phytoplankton, Tetraselmis tatrgthele. Despite the high concentration factor for mercury in the digestive gland, the gills have been shown to take up this element more rapidly. Moreover, the relatively low protein content of gill, 0.7 mg./gm. wet tissue compared to 49 mg./gm. wet tissue for the digestive gland, indicated that protein levels do not exert a controlling influence en the final equilibrium level of mercury in this tissue. In view of these observations an investigation of the mechanism of mercury uptake in isolated gills. has been carried out. This process has been shown to be inhibited by 2-4 dintrophenol, by deprivation of dextrose and by potassium. Further, a significant increase in the uptake of mercury has been observed in gills exposed to low concentrations of ouabain. It is suggested that the uptake of mercury by the gills of Ostrea edulis is energy, dependent and that the high concentration factor in the digestofive gland is a consequence the high protein content.

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