Biotransformation mechanisms of polycyclic aromatic hydrocarbons by Raphidocelis subcapitata: The role of cytochrome P450

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants which pose significant risks to ecological environments and human health. The microalga Raphidocelis subcapitata demonstrates a remarkable capacity for PAH degradation. Given that PAHs with different molecular weights exhibit distinct physicochemical properties which may affect the activity of PAHs-degrading enzymes within microalgae. Transcriptomic analysis of R. subcapitata exposed to three PAHs - phenanthrene (PHE), pyrene (PYR), and benzo[a]pyrene (BAP) - identified a total of 14,075 differentially expressed genes across all treatments. Three genes encoding glutathione S-transferase (gSt) and two cytochrome P450 isoforms (CYPA and CYPB) were further validated by RT-qPCR; all of which are functionally associated with PAH biodegradation in R. subcapitata. Treatment with 1-aminobenzotriazole (ABT), a cytochrome P450 (CYP450) enzyme inhibitor, decreased the microalgal degradation of PHE, PYR, and BAP by 86.0%, 72.2%, and 48.3%, respectively, relative to the ABT-free algal control. Specifically, ABT primarily inhibited the formation of monohydroxylated metabolites of PHE and PYR, suggesting that monooxygenases were the primary metabolic enzymes responsible for the degradation of PHE and PYR, while the degradation of BAP likely involved a collaborative action between monooxygenases and dioxygenases. This study provided new evidence for the mechanism of microalgal PAH degradation and theoretical support for the application of R. subcapitata in remediating PAH-contaminated freshwater environments.