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Phosphorus is one of the important factors to successfully establish the microalgal-bacterial symbiosis (MABS) system. The migration and transformation of phosphorus can occur in various ways, and the effects of phosphate on the MABS system facing environmental impacts like heavy metal stress are often ignored. This study investigated the roles of phosphate on the response of the MABS system to zinc ion (Zn 2+ ). The results showed that the pollutant removal effect in the MABS system was significantly reduced, and microbial growth and activity were inhibited with the presence of Zn 2+ . When phosphate and Zn 2+ coexisted, the inhibition effects of pollutants removal and microbial growth rate were mitigated compared to that of only with the presence of Zn 2+ , with the increasing rates of 28.3% for total nitrogen removal, 48.9% for chemical oxygen demand removal, 78.3% for chlorophyll-a concentration, and 13.3% for volatile suspended solids concentration. When phosphate was subsequently supplemented in the MABS system after adding Zn 2+ , both pollutants removal efficiency and microbial growth and activity were not recovered. Thus, the inhibition effect of Zn 2+ on the MABS system was irreversible. Further analysis showed that Zn 2+ preferentially combined with phosphate could form chemical precipitate, which reduced the fixation of MABS system for Zn 2+ through extracellular adsorption and intracellular uptake. Under Zn 2+ stress, the succession of microbial communities occurred, and Parachlorella was more tolerant to Zn 2+ . This study revealed the comprehensive response mechanism of the co-effects of phosphate and Zn 2+ on the MABS system, and provided some insights for the MABS system treating wastewater containing heavy metals, as well as migration and transformation of heavy metals in aquatic ecosystems., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Microalgal-bacterial symbiosis (MABS) system performs synergistic effect on the reduction of nutrients and carbon emissions in the water treatment process. However, antimicrobial agents are frequently detected in water, which influence the performance of MABS system. In this study, triclosan (TCS) was selected to reveal the effects and mechanisms of antimicrobial agents on MABS system. Results showed that the removal efficiencies of chemical oxygen demand, NH 4 +-N and total phosphorus decreased by 3.0%, 24.0% and 14.3% under TCS stress. In contrast, there were no significant decrease on the removal effect of total nitrogen. Mechanism analysis showed that both the growth rate of microorganisms and the nutrients retention capacity of extracellular polymeric substances were decreased. The intracellular accumulation for nitrogen and phosphorus was promoted due to the increased cytomembrane permeability caused by lipid peroxidation. Moreover, microalgae were dominant in MABS system with ratio between microalgae and bacteria of more than 5.49. The main genus was Parachlorella , with abundance of more than 90%. Parachlorella was highly tolerant to TCS, which might be conductive to maintain its survival. This study revealed the nutrients pathways of MABS system under TCS stress, and helped to optimize the operation of MABS system. [Display omitted] • The MABS system with or without EPS exfoliation were used to reveal TCS' effects. • The removal of COD, NH 4 +-N and TP decreased while TN removal was not affected. • Microbial assimilation, EPS adsorption and bio-accumulation were affected by TCS. • The dominant microalgal and bacterial species were Parachlorella and Rhodococcus. • Microalgae played leading roles on assimilation and bio-accumulation. [ABSTRACT FROM AUTHOR]