Dissolved organic matter-mediated metabolic shifts in Chlorella sorokiniana enhanced bioremediation of multi-ionic stress

Industrial wastewater contaminated with multiple toxic ions, such as cadmium and selenite, poses a significant environmental threat. This study investigated the ability of Chlorella sorokiniana to simultaneously biomineralize cadmium and biotransform selenite in real industrial wastewater. The results demonstrated that C. sorokiniana achieved 95% cadmium removal (from 2 mg/L to 0.1 mg/L) and 92.2% selenite removal (from 4.9 mg/L to 0.38 mg/L) within 36 h at an optimal Cd(II)-to-Se(IV) ratio of 1:2 with the presence of natural dissolved organic matter (DOM), particularly humic acid (HA). Concurrently, this condition also enhanced lipid accumulation to 40.53% of dry cell weight and increased biomass productivity to 1.54 g/L over a 12-day cultivation period, with a lipid yield of 52.0 mg/g biomass/day. Mechanistic studies revealed that the multifaceted roles of DOM in activating nitric oxide signaling, and upregulating calcium-dependent proteins (CaM, CDPK, and Ca²⁺-ATPase), thereby enhancing antioxidant enzyme activities (GPx, SOD, and CAT) and stabilizing CO₂ assimilation efficiency. This study provides valuable insights into the biochemical pathways utilized by microalgae under multi-ion stress, paving the way for the development of innovative, eco-friendly solutions for environmental restoration and sustainable biofuel production.