The effects of two widely used pesticides in paddy fields, 2, 4-D, dimethylamine and endosulfan, on growth, photosynthetic rate and the photosynthetic pigments of Chaetoceros sp. and Nannochloropsis sp. were assessed in this study. The 48 hour 50% inhibitory concentration (IC50) value for 2, 4-D dimethylamine on Chaetoceros sp. and Nannochloropsis sp. were 142.2 mg l−1 and 211.8 mg l−1, respectively, and the 48 hour IC50 value for endosulfan on Chaetoceros sp. and Nannochloropsis sp. were 21.9 μ g l−1 and 45.8 μ g l−1, respectively. Endosulfan was much more toxic to the microalgae than 2, 4-D dimethylamine. Both pesticides reduced photosynthetic pigments and oxygen production rate of the microalgae. After 48 hours exposure to the pesticides, the microalgae were transferred to a pesticide-free medium to investigate the post-exposure effect. 2, 4-D dimethylamine and endosulfan did not cause irreversible damage on the microalgae. However, there was a prolonged lag phase and the maximum specific growth rate (μ) of the microalgae was significantly retarded.

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