Growth and photosynthesis inhibition by agricultural pesticides in three freshwater microalgae.

Abstract

Growth rate and photosynthesis of <i>Microcystis aeruginosa</i>, <i>Scenedesmus quadricauda</i> and <i>Aulacoseira granulata</i> exposed to different concentrations of the agricultural pesticides CNP (<i>p</i>-nitrophenyl 2,4,6-trichlorophenyl ether), MEP [O,O-dimethyl O-(3-methyl-4-nitrophenyl) thiophosphate], ISP [isoprothiolane (C₁₂H₁₈O₄S₂)], and TBT (tri-<i>n</i>-butyltin chloride) were determined. The effective concentration (EC₅₀) for growth and photosynthesis in each species of microalga was then calculated. Inhibition of growth and photosynthesis in the three microalgae was greatest when exposed to CNP and TBT. <i>Microcystis aeruginosa</i> and <i>A. granulata</i> showed a higher tolerance, whereas <i>S. quadricauda</i> showed a higher sensitivity. Except for MEP, the EC₅₀ values for growth obtained in the three microalgae were higher than those for photosynthesis. The growth–photosynthesis response relationship showed that, for CNP and TBT, growth of the three organisms tested were less inhibited than their photosynthesis at a lower exposure (0.001–0.05 μg/L). At a higher exposure (0.10–1.0 μg/L), the response between relative growth rates and relative photosynthesis was proportional. For MEP and ISP, a proportional response existed between relative growth rates and relative photosynthesis in all test organisms. These results suggest that the inhibition of growth and photosynthesis by agricultural pesticides differs for the three microalgae. The differences can be explained in terms of the physico-chemical properties of the four pesticides and the physiological and morphological properties of the three microalgae.