Metallothionein and bioaccumulation of cadmium in juvenile bluegills exposed to aqueous and sediment-associated cadmium

I evaluated metallothionein (MT), free (unbound) hepatic cadmium and whole body cadmium as indicators of cadmium exposure in juvenile bluegills Lepomis macrochirus in laboratory tests. Two types of cadmium exposure were tested; aqueous and sediment-associated. In the aqueous tests, fish were exposed to cadmium (0.0 to 32.3 {dollar}mu{dollar}g/L) in an intermittent-flow diluter. The experimental design for the aqueous exposure was completely randomized and included two replicates in each of eight treatments (seven cadmium exposures and one water control). In the sediment-associated cadmium test, fish were exposed to resuspended river sediment containing 1.3 to 21.4 {dollar}mu{dollar}g Cd/g (dry weight) at a nominal total suspended solids concentration of 1000 mg/L in revolving, circular glass exposure chambers. The experimental design for the sediment exposure was a randomized block and included three replicates in each of six treatments (five sediment exposures and one sediment-free control). In both the aqueous and sediment bioassays, 25 juvenile bluegills per replicate were exposed for 28-d. Total cadmium concentrations were measured in various bluegill liver fractions, whole bluegill, water, and resuspended sediment to assess the partitioning and bioaccumulation of cadmium after the tests. Mean concentrations of MT and free cadmium in bluegill livers and concentrations of cadmium in whole bluegills were positively correlated with aqueous cadmium concentration and were equally suitable as indicators of aqueous cadmium exposure. Sediment-associated cadmium was biologically available, but to a lesser extent than aqueous cadmium. Cadmium concentrations in whole bluegills exposed to resuspended river sediment were 1.5- to 3.5-fold the concentrations in bluegills in sediment-free controls. Free cadmium and MT concentrations in bluegill liver and whole-body cadmium concentrations in bluegills were positively correlated with the cadmium concentrations in filtered water, resuspended sediment, and bulk river sediment; however, whole-body cadmium concentrations were a more sensitive indicator of exposure to sediment-associated cadmium than either free cadmium or MT concentrations in liver.