Putrescine Mitigates the Biomass–β-Carotene Conflict in Dunaliella salina Under Thermal Stress

Heat-induced β-carotene synthesis in <i>Dunaliella salina</i> typically compromises biomass accumulation, resulting in a biomass-β-carotene trade-off. This study demonstrates that exogenous putrescine (Put) alleviates this conflict through temperature-dependent mechanisms. At 28 °C (optimal for growth), 10^-6 M Put increased biomass by 9.52% and β-carotene yield by 10.72%, probably by accelerating electron transport and relatively mitigating the loss of photosynthetic function. At 34 °C (optimal for β-carotene synthesis), 10^-7 M Put enhanced biomass by 9.68% and β-carotene yield by 35.71% through a process associated with nitric oxide (NO) accumulation, involving antioxidant synergy and controlled reactive oxygen species (ROS) signaling, which activated photoprotective carotenogenesis. At 40 °C (extreme thermal stress), 10^-7 M Put maintained β-carotene levels 44.99% above the control despite a 2.50% biomass reduction, reflecting a shift toward photoprotection via elevated non-photochemical quenching (NPQ) and sustained electron transport beyond photosystem II (δ_RO). Put's hierarchical modulation of redox homeostasis, photosystem plasticity, and NO signaling underpinned its temperature-dependent efficacy. Peak NO levels correlated with β-carotene yield, while thermodynamic enzyme denaturation at 40 °C limited protection. These findings establish a temperature-concentration framework for Put application that alleviates the biomass-β-carotene trade-off under climate variability.