In this study, two strains of symbiotic bacteria (SOB-1 and SOB-2) were isolated from Scenedesmus obliquus, and various algal-bacterial mutualistic systems were established under acetochlor (ACT) stress conditions. Following exposure to varying ACT concentrations from 2.0 to 25.0μg/L, the capacity for co-cultured bacteria to degrade ACT was enhanced in 7 days by up to 226.9% (SOB-1) and 193.0% (SOB-2), compared with axenic algae, although bacteria exposed to higher ACT concentrations exacerbated algal metabolic stress, oxidative states, apoptosis and cellular lysis. ACT reduced carbohydrates in the phycosphere by up to 31.5%; compensatory nutrient plunder and structural damage by bacteria were the potential exploitation pathways determined based on the inhibition of bacterial infection using a glucanase inhibitor. The ACT-induced reduction in algal antimicrobial substances, including fatty acids and phenolics (by up to 58.1% and 56.6%, respectively), also facilitated bacterial exploitation of algae. ACT-dependent interspecific interaction coefficients between algae and bacteria generated from long-term symbiosis cultures implied that bacteria moved from mutualism (0 and 2.0μg/L ACT) to exploitation (7.9 and 25.0μg/L ACT). The population dynamic model under incremental ACT-concentration scenarios inferred that theoretical systematic extinction may occur in algal-bacterial systems earlier than in axenic algae. These outcomes provide interspecific insights into the distortion of algal-bacterial reciprocity due to the ecotoxicological effects of ACT. Environmental Implications Acetochlor (ACT) is a widely used herbicide in agriculture that is distributed throughout aquatic environments worldwide. It is highly toxic to green algae. Here, ACT disruption of the symbiotic relationship between microalgae and bacteria was investigated, evaluating nutrient exchange, algal structural defenses and antibacterial substance levels. This work provides mechanistic and population dynamic insights into the systemic response within microalgae and symbiotic bacteria exposed to ACT, assessing its adverse effects in aquatic planktonic communities beyond its algal toxicity. These insights are helpful to further the ecological understanding of ACT application and pesticide management. Display omitted •Two strains of symbiotic bacteria were isolated from microalgae.•Acetochlor (ACT) altered nutrient exchange and algal antibacterial defenses.•Symbiotic bacteria accelerated ACT degradation but aggravated its algal toxicity.•The interaction of symbionts changed from being mutualistic to exploitative.•Population dynamic simulations predicted the fates of the binary community.