Abstract
Growth of cultures of the dinoflagellate Prorocentrum micans Ehrbg. was slowed by parathion > 1 ppm. Parathion also decreased chlorophyll content and perturbed cellular ultrastructure, eliciting espeically plastoglobuli in their chloroplasts. Toxicity of this organophosphorous insecticide is unlikely to be due to its anticholinesterase activity since P. micans appears not to contain cholinesterase. Fluorescence kinetics show that parathion affects the photosynthetic system, particularly photosystem II.