Effects of Triphenyl phosphate (TPHP) on Microcystis aeruginosa: Growth stimulation and adaptability mechanisms.

As a representative organophosphate flame retardant (OPFR), triphenyl phosphate (TPHP) is ubiquitously detected in aquatic environments. The aquatic ecotoxicity of TPHP is mostly focused on fish, with fewer studies on microalgae, a key producer that affects the aquatic ecological balance. This study systematically deciphered the dose-dependent effects of TPHP (0.1-2 mg/L) on Microcystis aeruginosa (M. aeruginosa) through integrated physiological assays, stable isotope (C) test, transcriptomics, and microcystin-LR (MC-LR) profiling. The concentration distribution analysis demonstrated strong algal-mediated TPHP degradation (26-63 % degradation), with 28-35 % of the residual TPHP adsorbed on cell surfaces and 30-51 % accumulated intracellularly. While inducing concentration-dependent membrane alterations, TPHP exposure maintained algal growth and photosynthetic competence. Notably, at elevated concentrations (1-2 mg/L), TPHP enhanced photosynthetic performance and biomass production through coordinated upregulation of core metabolic pathways encompassing photosynthesis, carbon metabolism, oxidative phosphorylation, and protein biosynthesis. Concurrently, M. aeruginosa activated multiple defense mechanisms against TPHP stress, characterized by extracellular protein overproduction, tryptophan enrichment in extracellular polymeric substances, and intensified MC-LR synthesis/release. These findings reveal a dual role of TPHP as both a metabolic stimulant and toxic stressor, potentially reshaping algal community dynamics and exacerbating harmful algal bloom risks. This work provides critical mechanistic insights for refining OPFR risk assessment protocols in aquatic systems.