Evaluation of Roundup® effects on Chlorella vulgaris through spectral changes in photosynthetic pigments in fresh and marine water

International audienceGlyphosate is a broad-spectrum herbicide whose use is now restricted because of its potential toxicity to human and environmental health. Its presence is observed even in aquatic ecosystems where this emerging contaminant affects primary producers such as phytoplankton. This study focuses on the cosmopolitan green microalga, Chlorella vulgaris Beijerinck, 1890. The ecotoxicological effects of the commercial herbicide, Roundup®, were evaluated in freshwater (FW) and seawater (SW) using ultraviolet/visible spectroscopy over a five-day experiment. A dose-response relationship was investigated on a wide range of concentrations (0.1 to 25,000 µg L−1). The population of FW-Chlorella was stimulated when exposed to concentrations from 10 to 5,000 µg L−1 and inhibited otherwise. Chlorophylls and carotenoids exhibited their highest peak area values after four days, before their signal was depleted by day 5, regardless of Roundup® concentration. Conversely, cells of SW-Chlorella were stimulated at the highest concentrations (> 1 mg L−1), as if glyphosate was acting as a source of phosphorous nutrient. The peak areas of chlorophylls and carotenoids increased in the presence of the contaminant, especially on day 3. The defense mechanisms activated during its adaptation to marine salinity (e.g., decrease in the chlorophyll/carotenoid peak area ratio) could have favored its resilience. Exposure to Roundup® caused a shift in the wavelength at maximum absorbance, which could introduce a bias when measuring peak heights at a single wavelength. The analysis of absorption spectra and the use of peak areas appear to be a promising approach for monitoring changes in photosynthetic pigments