Effects of Zn pre-exposure on Cd and Zn bioaccumulation and metallothionein levels in two species of marine fish.

Zinc is an essential trace metal but also a potential toxicant to aquatic organisms. In this study, two juvenile marine fish species, the black sea bream Acanthopagrus schlegeli and the grunt Teraponjarbua, were pre-exposed to Zn either from waterborne (0.74-170 microg L-1) or dietary (39-5926 microg g-1) Zn for 1 or 3 weeks. The concentrations of Zn and metallothionein (MT) in the whole body of the black sea bream and in the gills, viscera and carcass of the grunt were then measured during this pre-exposure. Following the pre-exposure, both fish species were then exposed to 109Cd and 65Zn labeled food or water to quantify the dietary assimilation efficiency (AE) and the uptake rate of dissolved Cd and Zn. Zn concentrations in both fish species were enhanced after pre-exposure, but the increases were much less than the increase of ambient Zn pre-exposure concentration. Following Zn pre-exposure, MT concentrations in the viscera and carcass were significantly elevated, whereas the MT levels were not significantly elevated in the gills. Waterborne and dietary Zn exposure enhanced the uptake rates of dissolved Cd and Zn in both fish. The maximum increases of uptake rate constants of dissolved Cd and Zn were up to 1.9-2.8 and 2.1-2.6 times, respectively, in the seabream and grunt. In contrast, dietary assimilation efficiency of Cd and Zn was not significantly enhanced following Zn pre-exposure. A positive linear relationship was found between the uptake rate constants of dissolved metals and Zn or MT concentrations in the fish. The results suggested that Zn pre-exposure increased the potential of metal uptake from ambient water, but had little effect on dietary metal uptake. Furthermore, the Zn body concentration and metal uptake from the dissolved phase were significantly dependent on the fish body size.