Photoprotection comes at a cost: growth-productivity trade-offs associated with enhanced NPQ in Haematococcus lacustris.

Non-photochemical quenching (NPQ) dissipates excess absorbed light energy and protects photosynthetic organisms from photodamage, but its role in regulating the balance between growth, stress tolerance, and astaxanthin accumulation in Haematococcus lacustris remains unclear. Here, we investigated the relationship between NPQ and photosynthetic performance, stress-induced differentiation, and productivity in this astaxanthin-producing microalga. We isolated and characterized an NPQ-enhanced mutant line, A116, using cultivation assays under different stress conditions, analysis of photosynthetic parameters, pigment profiling, and whole-genome resequencing. A116 displayed stronger and faster NPQ induction, associated with increased LHCSR accumulation. This phenotype was accompanied by a decreased photosynthetic electron transport and lower photochemical efficiency under moderate-to-high light. Under high light, A116 showed a delayed the transition to astaxanthin-rich cysts under high light, allowing greater biomass accumulation under CO2-limiting conditions. However, under high CO2 availability, where carbon fixation relieved excitation pressure supporting efficient photosynthesis, the enhanced NPQ phenotype of A116 mutant was associated with reduced growth and astaxanthin productivity compared with the wild type. The contrasting performance of A116 under different carbon regimes suggests that increased energy dissipation may support high-light tolerance under carbon limitation but impose a cost when absorbed light can be efficiently used for carbon assimilation. These findings suggest that optimal algal productivity may require tuning photoprotective capacity to environmental conditions rather than maximizing NPQ.