Mechanistic insight of fungal–microalgal pellets in photobioreactor for heavy-metal wastewater bioremediation

The high cost of harvesting microalgae limits their industrial application. Fungal–microalgal pellets can efficiently harvest microalgae and enhance heavy-metal adsorption. However, the molecular response mechanism of fungal–microalgal pellets under heavy-metal stress remains unclear. Fungal–microalgal pellets in a photobioreactor were used as a research object, and a 98 % harvesting efficiency could be achieved with adding exogenous carbon and nitrogen at pH 5.0–6.0 for 12 h of co-culture. Humic acid- and tryptophan-rich proteins in extracellular polymeric substances (EPS) participate in Cd(II) complexation. The Cd(II) response in fungal-microalgal pellets involves amino acids, glucose, lipids, energy metabolism, and antioxidant systems. The turning point was at 48 h. Proline, histidine, and glutamine synthesis and the adenosine-triphosphate (ATP) binding cassette (ABC) transport pathway play important roles in resistance to Cd(II) biotoxicity. This study provides a reference for the large-scale cultivation of fungal-microalgal symbiotic pellets and the practical application for industrial heavy-metal wastewater.