Multiple stressors enhance Microcystis dominance and modulate phycospheric antibiotic resistome in aquatic mesocosm.

Assessing the response of phycospheric antibiotic resistance genes (ARGs) to pollutants such as antibiotics and microplastics (MPs) under the background of climate warming is crucial for understanding ARG fate in aquatic ecosystems. In this work, we conducted mesocosm experiments to evaluate how these stressors influence phytoplankton dominance, phycospheric bacteria, and ARG evolution. Results showed that in Microcystis-dominant ecosystem, a single stressor strengthened Microcystis dominance (up to 82.37 % of total abundance), but only warming significantly enhanced phycospheric bacterial metabolic activity and promoted phycospheric ARG proliferation (1.34-fold higher on day 16). The increased propagation of Microcystis symbiotic bacteria (e.g., Roseomonas and Methylobacterium) and microcystin-degraders (e.g., Pseudomonas and Sphingomonas) drove the spread of ARGs. Though the single treatment of antibiotics (0.85- and 0.53-fold for days 16 and 30) or MPs (0.72- and 0.20-fold for days 16 and 30) decreased the abundance of ARGs, co-treatment with warming reversed this suppression (e.g., 1.55- and 1.96-fold for WA and MW groups on day 16). The results underscore the necessity of considering the combined warming-pollutant effects in ARG ecological risk assessment in natural waters, particularly under phytoplankton succession scenarios. Such insights are vital for managing antimicrobial resistance in evolving aquatic environments under global change pressures.