Physiological responses and phosphorus transformation of Chlorella vulgaris under tricresyl phosphate stress

Organophosphate esters (OPEs) are emerging contaminants widely distributed in aquatic environments. Chlorella vulgaris can effectively remove tricresyl phosphate (TCP), a representative aromatic OPE. TCP exerts a concentration-dependent effect on algal growth and phosphate metabolism. Low TCP concentrations (<30  mg/L) induce a hormesis effect, enhancing algal growth by 6-8 %, whereas high TCP concentrations (≥30  mg/L) inhibit growth by 16-19 % and double the level of reactive oxygen species (ROS), with severe growth inhibition occurring above 50  mg/L. Under tolerable TCP stress, Chlorella exhibits divergent regulation of two key phosphatases: alkaline phosphatase activity (APA) displays a U-shaped response to increasing TCP concentrations, while 5'-nucleotidase (5'-NT) peaks at 20-40  mg/L TCP. A comparative experiment with conventional inorganic phosphate (supplied at a phosphorus concentration equivalent to that of 25  mg/L TCP) revealed that TCP-exposed microalgae experienced a "high total phosphorus (TP), low phosphate phosphorus (PO₄-P)" stress state, characterized by elevated APA and 5'-NT activities. A total of 32.3 % of TCP was removed within 7  days, and 45.3 % of the removed TCP-derived phosphorus was recovered as biomass-bound organic phosphorus, with no significant polyphosphate accumulation detected under TCP stress. This study provides a theoretical basis and technical support for applying microalgal systems to the remediation of point-source OPE contamination, coupled with phosphorus resource recycling.