The effect of extreme waterborne cadmium exposure on the internal concentrations of cadmium, calcium, and sodium in Chironomus riparius larvae

Chironomus riparius larvae (3rd–4th instar) were extremely resistant to waterborne Cd with 48 h LC50s of 331 mg Cd/L in soft water (10 mg/L CaCO3) and 1106 mg Cd/L in moderately hard (140 mg CaCO3/L) water. Unexposed larvae had whole body Ca and Na concentrations of 11.2(0.3) and 84.5(3.0) μmol/g, respectively. The larvae exposed through acute toxicity tests accumulated massive amounts of Cd, reaching >50 μmol/g in larvae exposed to 437 mg Cd/L, though burdens were lower at higher exposure concentrations. These Cd burdens were approximately fivefold greater than whole-body Ca concentrations. Cd exposure also had a significant negative effect on internal Ca: whole-body Ca declined by over 70% in larvae exposed to Cd above the LC50 concentration. The effect of Cd exposure on whole-body Na was much less dramatic as levels dropped by 10–28% in the acutely exposed larvae. Time series exposures (up to 72 h) across a range of Cd concentrations (0.1–865 mg/L) revealed that internal Ca dropped within the first hour of exposure regardless of the concentration of Cd. In all but the highest (865 mg Cd/L) exposure, internal Ca eventually recovered to the control level. Cd resistance in C. riparius may lie in its ability to maintain internal Ca balance even when exposed to extreme (>100 mg/L) levels of Cd, coupled with remarkable capacities for storage–detoxification and excretion of Cd.