Abstract Treatment of microalgae (Bumilleriopsis) with paraquat (1,1-dimethyl-4,4-dipyridylium dichloride) under culture conditions in the light for 20 or 160 h leads to light-induced oxygen uptake and more or less severe chlorophyll bleaching, which is accompanied by formation of malondial-dehyde. The ratio of chlorophyll to packed chloroplast volume remains about the same as that of the control, presumably indicating destruction of membranes concurrently with pigments. Unrelated to retardation of growth, degree of bleaching or to the formation of malondialdehyde quite a uniform degree of inactivation (≈ 50%) of partial redox reactions is observed in the region of photosystem II and I except for the terminal part of photosystem I (pigment 700 → NADP+) . The action of paraquat in the cell centers primarily on the photosynthetic membrane system and redox chain.
