Effects of acetochlor on the interaction between Scenedesmus and Microcystis: Integrated perspectives on toxicity, biotransformation, and competition strategies.

To reveal the disruption caused by herbicides and the mechanisms of algal interactions on interspecific competitive strategies at a metabolic and population level, this study established short-term (7 d) and long-term (21 d) Scenedesmus-Microcystis competition coculture systems and investigated the toxicity of acetochlor (ACT) on algae competition. Scenedesmus obliquus (EC 6.586 μg/L) is three orders of magnitude more sensitive to ACT than Microcystis aeruginosa (EC 19,539 μg/L), placing it at a competitive disadvantage in environments with ACT pollution. Short-term coculture tests (ACT concentrations from 0 to 12.5 μg/L) showed that ACT suppresses S. obliquus growth and competition, while M. aeruginosa initially showed compensatory growth, which was negated by ACT. Metabolomics revealed that interspecies competition and ACT affect fatty acid synthesis and nitrogen assimilation metabolism of both microalgae, suggesting species differences in the mode of action (MOA) of ACT toxicity and resource competition strategies, respectively. ACT weakens the ability of M. aeruginosa to compete for nitrogen and synthesize microcystin under competitive stress. ACT biotransformation can be conducted across species. In an algal culture system with equal initial biomass, the 7 d ACT degradation rate increased by 24.9 % and 123.8 % with coculture of the two algae compared with monocultures of S. obliquus and M. aeruginosa, respectively. In long-term experiments, the degradation rate increased by 19.0 % and 8.9 % in cocultures compared with the monocultures. Lotka-Volterra models showed that competitive inhabitation was alleviated, implying that the competition interspecies relationship is beneficial for the coexistence of both algal populations under ACT stress.