Influence of ploidy on radiation resilience in extremophilic alga Galdieria sulphuraria under extreme ionizing conditions

Ionizing radiation is one of the key physicochemical factors affecting living organisms, but its impact on unicellular algae remains poorly understood. Cyanidiophyceae are a class of extremophilic microalgae capable of thriving under the highly inhospitable conditions of volcanic hot springs. Among them, Galdieria sulphuraria is a unique species capable of mixotrophy, sexual reproduction, and exists in both haploid and diploid stages depending on environmental conditions. This study investigates, for the first time, the effects of ionizing radiation (2–6 kGy) on a representative extremophilic Cyanidiophyceae, focusing on radiation-induced damage and recovery in relation to ploidy. Our findings reveal that Galdieria sulphuraria in the diploid state exhibits extraordinary radiation resistance, surviving exposure to enormous doses as high as 6 kGy, making it one of the most radiation-tolerant photoautotrophic organisms known. Furthermore, diploids exhibit significantly higher tolerance than haploids, as evidenced by their superior survival, shorter duration of radiation sickness, enhanced synthesis of protective carotenoids, reduced oxidative damage, and high photosynthetic efficiency during recovery. These results provide novel insights into the role of ploidy in radiation resistance in algae and contribute to a broader understanding of extremophile adaptations. Given the relevance of ionizing radiation in astrobiology and space exploration, Galdieria sulphuraria emerges as a promising model for studying eukaryotic survival in extraterrestrial environments.