Influence of light intensity on the toxicity of herbicides, alone or in mixture, to freshwater phytoplankton

Light intensity directly affects phytoplankton and can alter the toxicity of phytotoxic pollutants present in natural water bodies. Light fluctuation in aquatic ecosystems often occurs as a function of water turbidity, water movement, cloud cover, and seasonality. Atrazine and simazine are commonly used herbicides that inhibit photosynthesis, posing significant risks to aquatic primary producers, and may be found simultaneously in aquatic ecosystems. The interactions between light and herbicide mixtures on phytoplankton growth and physiological state are poorly understood. Therefore, we addressed the toxicity of the herbicides, atrazine and simazine (individually and mixed), on the growth and photosynthetic activity of three freshwater phytoplankton under three light intensities. We found that the toxic effects of single and mixed herbicides are species-specific and significantly modulated by light intensity, with synergistic effects observed for herbicide mixtures under high light conditions. Atrazine and simazine (individually and mixed) toxicities on photosynthesis were greater for the three species grown under low light than under very low light. However, high-light adapted strains of M. aeruginosa were less sensitive to single and mixed herbicides than those adapted to low- and very low-light conditions. Under low- and high-light conditions, the photoprotective ability was extremely sensitive to the inhibitory effects of atrazine and simazine, individually and when mixed. Understanding these interactions is important because microalgae form the base of aquatic food webs and their impairment can have cascading effects on ecosystems. These findings underscore the importance of considering multiple environmental stressors in assessing the ecological risks of herbicides, highlighting potential impacts on aquatic primary productivity. • The sensitivity to herbicides (alone or mixed) differs among freshwater algae. • The toxicities of single and binary herbicides are modulated by light intensity. • Two cyanobacteria adapted to high light are sensitive to herbicides. • High-light adaptation decreased the toxicity of single and mixed herbicides.