HEAVY METAL TOLERANCE AND METALLOTHIONEIN INDUCTION IN FATHEAD MINNOWS: RESULTS FROM FIELD AND LABORATORY INVESTIGATIONS

Toxicity tests were conducted to quantify the metal tolerance of two natural populations of fathead minnows (Pimephales promelas). One group of organisms was taken from a metal-contaminated flyash pond and the other population was collected from Newtown hatchery ponds. As determined in acute toxicity tests, ash pond minnows were significantly more tolerant to cadmium and copper than were hatchery minnows. At an exposure concentration of 6.0 mg Cd/l in moderately hard water, the median period of survival (LT(,50)) for flyash pond minnows was 50.0 hr compared to 6.8 hr for hatchery minnows. The LT(,50) values for ash pond and Newtown animals exposed to 0.50 mg Cu/l in moderately hard water were 17.0 and 4.5 hr, respectively. Both populations were about equally tolerant to zinc. After ash pond minnows were transferred to reconstituted water in the laboratory, tolerance to cadmium and copper decreased significantly after 7 days. Conversely, tolerance was increased in hatchery minnows following acclimation in the laboratory to sublethal concentrations of cadmium. For example, after exposing hatchery minnows for 35 days to 10 (mu)g Cd/l, acute toxicity tests gave LC(,50) values of 2.88 and 1.71 mg Cd/l for acclimated and control organisms, respectively. This induced tolerance in cadmium-acclimated hatchery minnows was lost when organisms were transferred to control water for 7 days.; Developing embryos of the fathead minnow and rainbow trout also acquired tolerance to cadmium. After eggs of the trout had been exposed to 0, 5, and 50 (mu)g Cd/l for 24 days, subsequent acute toxicity tests conducted on the newly hatched larvae gave 7-day LC(,50) values of 0.70, 1.59, and 2.02 (mu)g Cd/l, respectively.; The results support the conclusion that fathead minnows develop increased tolerance to cadmiium and copper following prolonged sublethal exposure to these metals, and that this metal-induced tolerance is not sustained once organisms are removed from toxicant stress. As determined in correlated biochemical studies, gill metallothionein concentrations correlated closely with the variations in tolerance exhibited during cadmium acclimation and deacclimation. Therefore, tolerance induction was attributed to increased production of metallothionein, a protein which selectively binds and sequesters cadmium, copper and certain other metals.