Physiological and photosynthetic characteristics associated with photo-protection in a Korean strain of the green microalga Haematococcus

Haematococcus lacustris, a freshwater microalga, is known for its environmental resilience and ability to produce the high-value ketocarotenoid astaxanthin. In this study, a novel H. lacustris strain, HYU5231, was isolated and examined for phylogenetic position, morphology, growth, and photoprotective responses. Phylogenetic analyses using 18S rRNA, internal transcribed spacer, and Rubisco large subunit sequences confirmed that HYU5231 belongs to the H. lacustris lineage. Compared with the reference strain H. lacustris UTEX 2505, HYU5231 exhibited a larger cell size, lower pigment content, and comparable biomass productivity. Under increasing irradiance, HYU5231 exhibited a consistent photosystem II maximum quantum efficiency (FV/FM > 0.5) and showed increased non-photochemical quenching (NPQ) and accumulation of astaxanthin and carotenoids. The peak in NPQ with rapid astaxanthin accumulation suggests complementary photoprotective roles during prolonged exposure to high-intensity light. To investigate the photoprotection mechanisms, photosynthetic electron transport inhibitors were used, and chlorophyll fluorescence was assessed using pulse-amplitude-modulated fluorometry. HYU5231 exhibited higher NPQ but lower electron transport rates than UTEX 2505. Unlike UTEX 2505, dithiothreitol (DTT) did not suppress NPQ in HYU5231 cells, indicating that NPQ in this strain is not predominantly dependent on a DTT-sensitive xanthophyll cycle pathway. In contrast, propyl gallate markedly suppressed NPQ in HYU5231 cells, suggesting a potential contribution of plastid terminal oxidase (PTOX)-associated electron flow to photoprotection. Furthermore, methyl viologen increased electron transport in HYU5231 cells, suggesting a limitation on the photosystem I electron acceptor side. Collectively, these results reveal a distinct photoprotective strategy in HYU5231 and suggest a potential role for PTOX-associated alternative electron transport in maintaining photosynthetic performance under intense light conditions.