Barbara Klimczak

Cyanobacteria, particularly Raphidiopsis raciborskii, are known for their harmful blooms, posing significant ecological and economic challenges in aquatic ecosystems. This study investigates the strain‐specific adaptive mechanisms of five R. raciborskii strains (CS‐505, CS‐506, UAM/DH‐KmRr, UAM/DH‐BiRr, and UAM/DH‐MRr) under chill/light stress (low temperature and high light intensity), focusing on growth, pigment composition, photosynthetic activity, gene expression, and toxin production. Results revealed that growth rates were significantly reduced under stress for all strains, with the Polish strain UAM/DH‐MRr showing the highest resilience. Stress conditions caused a reduced chlorophyll a level and increased carotenoid/Chl a ratio, suggesting photoprotective adaptations. Photosynthetic quantum efficiency (Fv/Fm) declined during chill/light but was recovered post‐stress, while electron transport rates (rETR) were not significantly altered. Enhanced respiration and increased photosynthetic oxygen evolution were observed, particularly in CS‐506 and UAM/DH‐MRr. Principal component analysis (PCA) highlighted pigment content and parameters related to photosynthetic electron transport as key factors influencing response; no variable‐specific groupings were observed among strains regarding the broad range of parameters. Gene expression analyses revealed divergent transcriptional regulation of genes involved in photosynthesis and cylindrospermopsin (CYN) biosynthesis, with toxic CS‐505 upregulating cyr genes under stress, while CS‐506 down‐regulated them. Intracellular CYN levels decreased under stress, yet cell quotas increased in CS‐505, which warrants further investigation. Our results indicate the complexity of strain‐specific responses to chill/light stress and highlight the importance of understanding physiological mechanisms for predicting and managing cyanobacterial blooms in diverse climates.