Linking the removal of enrofloxacin to the extracellular polymers of microalgae in water bodies: A case study focusing on the shifts in microbial communities

Microalgae can promote antibiotic removal, which has attracted growing attention. However, its synergistic removal performance with bacteria in antibiotic pollutants is still poorly understood. In this study, firstly, we selected two green algae (Dictyosphaerium sp. and Chlorella sp.) and exposed them to Enrofloxacin (ENR) to observe their extracellular polysaccharides (EPS) concentration dynamic and the removal of antibiotics. Secondly, EPS was extracted and added to in situ lake water (no algae) to investigate its combined effect with bacteria. The results indicate that both Dictyosphaerium sp. and Chlorella sp. exhibited high tolerance to ENR stress. When the biomass of microalgae was low, ENR could significantly stimulate algae to produce EPS. The removal rates of Dictyosphaerium sp. and Chlorella sp. were 15.8% and 10.5%, respectively. The addition of EPS can both alter the microbial community structure in the lake water and promote the removal of ENR. The LEfSe analysis showed that there were significant differences in the microbial marker taxa, which promoted the increase of special functional bacteria for decomposing ENR, between the EPS-added group and the control group. The EPS of Dictyosphaerium sp. increased the abundance of Moraxellaceae and Spirosomaceae, while the EPS of Chlorella sp. increased the abundance of Sphingomonadaceae and Microbacteriaceae. Under the synergistic effect, Chlorella sp. achieved a maximum removal rate of 24.2%, while Dictyosphaerium sp. achieved a maximum removal rate of 28.9%. Our study provides new insights into the removal performance and mechanism of antibiotics by freshwater microalgae in water bodies and contribute to the development of more effective water treatment strategies.