Diminution des effets toxiques de mixtures d'herbicides vis-à-vis de microalgues d'eau douce, en présence de matière organique dissoute naturelle

International audience; As primary producers, microalgae are the basis of aquatic food webs. Thus, they can be directly impacted by herbicides which are, in turn, affected by complex and changing conditions of freshwater environments. For instance, dissolved organic matter (DOM) may interact with pesticides and affect their fate and ecotoxicity. Therefore, the present study aimed to investigate whether the natural DOM influences the toxicity of herbicide mixtures towards two freshwater microalgae. To that aim, a diatom (Gomphonema gracile) and a chlorophyte (Sphaerellopsis sp.) isolated from a pristine river were exposed to a mixture of three herbicides, with or without natural DOM. Herbicide concentrations were selected to represent environmentally realistic conditions (0.05µg/L for irgarol and diuron, 0.5µg/L for metolachlor) and a 10-times higher exposure. The responses of the microalgae were assessed over a growth cycle, by monitoring daily their growth and photosynthetic efficiency. On the last day, the intracellular relative lipid content was quantified, as well as the proportion of bacteria in the microcosms to ensure no bloom had occurred. In the absence of DOM, photosynthetic activity decreased in Sphaerellopsis at both concentrations of herbicides, whereas Gomphonema was only impacted at the highest one. DOM alone had no influence on the parameters measured for Gomphonema, but significantly stimulated the growth of Sphaerellopsis. Surprisingly, combined exposure to DOM and herbicides removed the toxic impacts observed without DOM for both species. Our results demonstrate species-specific responses to DOM and herbicides, and highlight a mitigation of pesticides impacts towards microalgae in the presence of natural DOM.