Microalgae under heavy metal stress: mechanistic insights into detoxification mechanism and antioxidant defence...

Heavy metal pollution is intensifying due to rapid industrialization, especially near industries such as mining, battery manufacturing, metal plating, tanneries, and textiles, where untreated wastewater releases toxic metals like chromium, cadmium, lead, and mercury. These metals persist in the environment and pose severe risks to aquatic life, soil health, and human populations through contamination of water and food sources. In response, microalgae have emerged as promising approach owing to their ability for heavy metal detoxification. Microalgae have shown significant potential in mitigating heavy metal pollution via extracellular adsorption, chelation, and sequestration. It employs a range of extracellular and intracellular mechanisms to reduce metal toxicity, including surface adsorption via extracellular polymeric substances , intracellular chelation by metal-binding proteins such as glutathione and metallothioneins, and sequestration through metal transporter systems. Exposure to heavy metals induces oxidative stress, prompting the activation of enzymatic and non-enzymatic antioxidant defences. Additionally, phytohormones modulate stress responses and enhance detoxification pathways. This review aims to comprehensively elucidate how microalgae interact with heavy metals, ranging from EPS- mediated adsorption at the cell surface to active intracellular chelation and sequestration. Furthermore, it presents biochemical and physiological responses underlying microalgae's responses to heavy metals, including antioxidant defence systems and phytohormonal regulation.