Toxic effects of pesticides on the marine microalga Skeletonema costatum and their biological degradation

Pesticides are considered to be toxic pollutants that are harmful to the health of humans and animals or dangerous to the environment. The pesticides chlorpyrifos, acetochlor, and dicofol, representatives of organophosphorus, acyl amide, and organochlorine pesticides, are widely used in agriculture and have been frequently detected in aquatic environments. However, the toxicity of these pesticides to marine microbes is unclear. In this study, the toxic effects of the three pesticides on a representative marine microalga, the diatom Skeletonema costatum ( S. costatum ), were investigated. Microalgal cell density and chlorophyll- a (Chl- a ) content were analyzed to investigate the toxic effects after S. costatum was exposed to the three pesticides both individually and in combination. The N/P ratio in the culture medium and the degradation of the pesticides were also analyzed during exposure. Individual acute toxicity analysis indicated that the growth of S. costatum was significantly inhibited by acetochlor, followed by the effect of dicofol and chlorpyrifos. The cell membrane of the microalgae was impaired by acetochlor. Combined toxicity analysis indicated that the presence of acetochlor increased the toxicity of dicofol and chlorpyrifos. In contrast, the presence of dicofol reduced the toxicity of acetochlor and chlorpyrifos. The N/P ratio in the culture medium was largely increased at late growth stages; however, it was significantly lowered when the microalgae were exposed to chlorpyrifos compared to acetochlor and dicofol. The degradation of the pesticides was promoted by microalgae, suggesting that microalgae might contribute to the removal of these pesticides from the marine environment. The toxic mechanism of the pesticides on the marine microalgae and the fates of pesticides in the ocean need to be further studied.