Phosphate adaptive regulation in microalgae from phosphorus-rich livestock wastewater: Enhanced metabolic pathways contrasting with impaired transport.

Elevated phosphate (PO43-) levels in livestock wastewater (LSWW) significantly inhibit microalgal bioremediation, though the underlying mechanisms remain poorly characterized. This study investigated the physiological and molecular responses of microalgae to high PO43- stress at concentrations relevant to LSWW systems. Results demonstrated significant physiological stress and impaired nutrient uptake, with a 96.83% reduction in PO43- removal efficiency and a 62.28% reduction in removal rate. These responses resulted from a coordinated feedback inhibition, with enhanced intracellular phosphorus (P)-related metabolism but suppressed active PO43- transport. Specifically, stress adaptation was mediated through the coordinated upregulation of pentose PO43- pathway (e.g., genes encoding RBKS, rpiA, G6PD), glycolysis/gluconeogenesis (e.g., genes encoding talA, PFK, PFP) and phosphatidylinositol metabolism (e.g., genes encoding PIP5K, SAC1, IMPA, mmsA), collectively promoting NADPH regeneration, energy homeostasis, membrane transport, and cell signaling. ATP production was elevated through oxidative phosphorylation (e.g., genes encoding ppa and PMA1) whereas ATP conservation was achieved by downregulation genes encoding ABC transporters, despite concomitant oxidative stress and membrane destabilization. High PO43- suppressed key PO43- transporters genes (PHT1, PHT4, PHT5) and inhibited PO43--dependent enzymatic activities, including acid phosphatase and ADP-glucose pyrophosphorylase. Photosynthetic integrity was maintained via carotenoid-mediated photoprotection, which mitigated oxidative damage through singlet oxygen quenching and radical scavenging. Metabolic profiling revealed a shift from protein and polysaccharide synthesis towards lipid accumulation, with notable production of odd-chain fatty acids exhibiting favorable biodiesel properties. These findings decipher the molecular regulatory networks underlying P stress in microalgae, providing practical strategies to enhance nutrient recovery and biomass valorization in microalgae-based LSWW treatment.