Toxic Effect Assessment Of Atrazine On Chlorella Pyrenoidosa Using Biochemical And Molecular Biomarkers

Growth rate is a common biomarker used in evaluating toxic effects of the pesticides on microalgae. It is simple and convenient to assess the integrative toxicity of the pesticides to algae. However, it is not able to reflect the specific toxic effect of a certain pesticide. To assess the specific toxicity of the pesticides, new toxicity endpoints should be taken into account at biochemical and molecular levels. These new toxicity endpoints will be helpful to build the multi-level system of toxicity effect evaluation, and contribute to a comprehensive understanding of the toxicity mechanism of pesticide to algae.In the present work, the green alga Chlorella pyrenoidosa was chosen to assess the acute toxicity of atrazine under autotrophic and mixotrophic conditions. Cell growth, antioxidant enzymes (superoxide dismutase and catalase) and transcription of photosyntheis-related genes were monitored after 48 and 72 h exposure to atrazine. The cellular level of malondialdehy was also determined. The photosyntheis-related genes included chlL (L subunit of light-independent protochlorophyllide reductase), psbA(D1 protein of photosystem II), and rbcL (large subunit of ribulose-1,5-bisphosphate carboxylase oxygenase). Our results showed that atrazine inhibited the growth of C. pyrenoidosa under both autotrophic and mixotrophic conditions. Superoxide dismutase and catalase activity and malondialdehyde content increased with the atrazine concentration increasing from 0.0 to 3.0 mg/L under autotrophic condition. After 72 h exposure to 3.0 mg/L atrazine, superoxide dismutase and catalase activity were increased by 49.8% and 27.3%, respectively, and malondialdehyde content were increased by 109.3%. Under mixotrophic condition (addition of 0.4% glucose), the activity of superoxide dismutase and catalase and the content of malondialdehyde increased with atrazine expose, and reached the maximum values at the atrazine concentration of 2.0 or 3.0 mg/L, while they reduced with the further increase of atrazine concentration. The maximum activities of superoxide dismutase and catalase were increased by 46.7% and 49.4%, respectively, and the maximum malondialdehyde content was increased by 25.7%. Compared with autotrophic condition, superoxide dismutase activity was lower than that under mixotrophic condition, while catalase activity and malondialdehyde content were higher. The same thing happened in the transcript abundances of three target genes. The transcript abundances of chlL, psbA and rbcL gene increased in autotrophic condition, and increased before reduced in mixotrophic condition. After 72 h exposure to 3.0 mg/L atrazine, the transcript abundances of three target genes were increased in autotrophic condition by 22.1%,86.0% and 72.7%, respectively, and they were increased in mixotrophic condition by 149.3%,122.4% and 111.0%.Our results demonstrate that atrazine exposure affected antioxidant enzymes and gene expression in C. pyrenoidosa. Compared with traditional biomarker (cell growth) normally detected after 72 h, biochemical and molecular biomarkers was able to detect earlier effects of 48 h. The biochemical and molecular toxicity endpoints have great potential as toxicity biomarkers. On the other hand, the data on toxicity effects at biochemical and molecular levels would improve the understanding of the mechanism of atrazine toxicity. At present, autotrophically grown algal cells were used to assess the toxicity of the pesticides. In this study, we found it possible to use mixotrophically grown algal cells to monitor the toxicity of the pesticides faster.