Glutamate and Glutamine Enable Rapid Acclimation of <i>Haematococcus lacustris</i> to High‐Light During Culture Phase Shift by Pre‐Activating Photoprotective Pathways

Microalga Haematococcus lacustris is known for its ability to produce high-value product astaxanthin. The abrupt shift from weak-light in the vegetative phase to high-light in the astaxanthin induction phase triggers severe photodamage during the large-scale cultivation of H. lacustris for astaxanthin production in two phases, causing photobleaching with resultant biomass loss. Current mitigation relies primarily on physical shading. This study found that glutamate/glutamine supplementation before phase shift mitigated the photodamage caused by sudden high-light stress in H. lacustris. Glutamate rapidly enhanced CO₂ fixation while simultaneously inducing various photoprotective pathways, including non-photochemical quenching, cyclic electron flow, chlororespiration and photorespiration. This maintained the balance between light absorption and energy utilization in H. lacustris during abrupt environmental shifts, thereby preventing damage on both the donor and acceptor sides in photosystem II. Pre-incubation with glutamate in the dark pre-activated these photoprotective pathways, thus enabling seamless acclimation to high-light stress following abrupt shifts. Meanwhile, glutamine exhibited equivalent efficacy in inducing multiple photoprotective pathways. These findings establish a novel strategy based on dark pretreatment for enhancing photosynthetic capacity under high light and protecting algal cells against photobleaching. The study provides fundamental insights into photoprotective acclimation mechanisms in microalgae and higher plants facing rapid environmental shifts.