Microalgae-based integrated treatment of tannery wastewater: emphasizing microbial synergy for sustainable remediation.

Tannery wastewater (TWW) presents a complex and toxic mixture of pollutants, including heavy metals such as chromium, cadmium, lead, and nickel, as well as organic compounds, surfactants, sulfides, chlorides, and dyes. Conventional treatment methods often lead to high operational costs, sludge generation, and disposal issues. This review critically examines the integration of microalgae-based systems with physical, chemical, and biological methods for advanced TWW remediation, emphasising mechanistic insights and consortia-driven treatment. Microalgae facilitate pollutant removal through biosorption, bioaccumulation, enzymatic transformation, and the mitigation of oxidative stress. Specific enzymatic mechanisms, such as chromate reductase-mediated Cr(vi) reduction and peroxidase-driven degradation of phenolics, enable precise detoxification at the cellular level. In multi-species consortia, synergistic interactions optimise nutrient uptake, broaden substrate specificity, and enhance EPS-mediated heavy metal immobilisation. This review delineates how microalgae-bacteria, microalgae-fungi, and microalgae-microalgae systems contribute distinct advantages via mutualistic nutrient exchange, quorum sensing regulation, and biofilm resilience under high pollutant stress.