Abstract: A laboratory test was conducted to investigate the toxic effects of surfactants cetanecyl trimethyl ammonium chloride (CTAC) and stearyl trimethyl ammonium bromide (STAB) on the growth, protein, chlorophyll, and malondialdehyde (MDA) contents, and superoxide dismutase (SOD) activity of Scenedesmus quadricauda, with the toxicological mechanism of the surfactants analyzed. Both CTAC and STAB inhibited the growth of S.quadricauda ,and the toxicity of STAB was higher than that of CTAC. In the first 4 days CTAC-and STAB exposure, the protein and chlorophyll contents and SOD activity of S. quadricauda decreased after an initial increase, while the MDA content decreased constantly. According to the dosage-effect relations, it was deduced that the toxic effects of the surfactants on S. quadricauda were probably due to the changes of cell membrane permeability and the overload of reactive oxygen species.

Key words: Biomarkers, Lysosomes, Stress protein, Mettllothioneins