TOXIC CELL CONCENTRATIONS OF 3 POLYCHLORINATED BIPHENYL CONGENERS IN THE GREEN-ALGA SELENASTRUM-CAPRICORNUTUM

Algal growth inhibition tests were performed with the unicellular gree n alga Selenastrum capricornutum and three C-14-labeled polychlorinate d biphenyl (PCB) congeners. Toxicity was related to external aqueous c oncentrations and additionally to internal algal bound PCB concentrati ons. Estimates of the concentrations at 50% effectiveness (EC50s) for the three PCB congeners ranged within a factor of 17 (14-241 nmol/L) w hen based on measured aqueous concentrations. When based on internal t oxicant concentrations (toxic cell concentrations, TCC50) the correspo nding range was 6.7 to 14.3 mmol/kg wet weight. Thus, changing the bas is from external to internal concentrations reduced the range by almos t one order of magnitude. Additional toxic cell concentrations of five monoaromatic compounds and S. capricornutum were calculated from lite rature data to be in the same order of magnitude as the experimental t oxic cell concentrations for the PCBs, whereas EC50 values for all sub stances ranged by more than four orders of magnitude. The experimental and calculated data indicate that observed differences in the estimat ed EC50 values were mainly due to differences in bioconcentration beha vior rather than to different intrinsic toxicities. These findings are in agreement with the concept of baseline toxicity, meaning that a nu mber of hydrophobic organics exerts their acute toxicity by one relati vely nonspecific mode of action.