Interspecific interactions modulate atrazine and S-metolachlor toxicity on aquatic microbial autotrophs

Atrazine and S-metolachlor are herbicides frequently detected in agricultural streams, sometimes at concentrations of concern, as they may affect photosynthetic organisms. In this study, we exposed a coculture of the green alga Enallax costatus, the diatom Gomphonema parvulum and the cyanobacteria Phormidium sp. and Microcystis aeruginosa, to a gradient of atrazine and S-metolachlor (0, 10, 100 and 1000 µg·L^-1) for 7 days. The results showed that species interactions within a coculture can modulate herbicide toxicity compared to responses observed in monocultures. Green algae and cyanobacteria were more sensitive to atrazine in coculture than in monoculture, while diatoms showed better tolerance in coculture. S-metolachlor affected the chlorophyll fluorescence of organisms in coculture to a greater extent than atrazine, particularly for the green alga and the diatom. Although S-metolachlor exerts its toxicity through inhibition of the synthesis of very long chain fatty acids (VLCFAs), we did not observe any significant reduction in the abundance of these fatty acids. The changes in composition of organisms within the coculture were accompanied by a change in fatty acid profiles, reflecting the specific fatty acid profiles of each taxon. In addition, herbicide exposure may have caused increased lipid peroxidation, leading to a decrease in the relative abundance of polyunsaturated fatty acids (PUFAs) in the coculture. This study highlights the importance of considering interspecific interactions in ecotoxicology, as they can modulate herbicide toxicity on aquatic photosynthetic communities. In addition, lipid and fatty acid profiles provide sensitive information that complements 'traditional' descriptors such as cell density and photosynthesis parameters. Lipidomics can be used to study changes in coculture composition (community composition) and to assess the potential of herbicides to affect organisms at the cellular level.