Osmotic stress alters UV-based oxidative damage tolerance in a heterocyst forming cyanobacterium

Cyanobacteria are successful in diverse habitats due to their adaptation strategies. Their mechanisms to cope with individual stresses have been studied. However, the response to combined stress conditions as found in nature remains unclear. With this aim, we studied the dual effect of 24h-osmotic and 3h-UV irradiation on the cyanobacterium Calothrix BI22. Our approach included the study of redox homeostasis, oxidative damage, reactive oxygen species production-consumption processes and photosynthetic activity. Superoxide in vivo determination with confocal image processing showed the highest accumulation under UV. However, no lipoperoxidation occurred due to a high SOD activity. This cyanobacterium was less prepared to cope with the osmotic stress assayed. Under this condition, O₂ photoevolution decreased abruptly and oxidative damage was produced by reactive species other than superoxide. In this situation the cellular control of the amount of ROS failed to prevent oxidative damage and photosynthesis was seriously disturbed in spite of maximum quantum photosynthetic efficiency remained unchanged. Calothrix BI22 presented the more severe oxidative damage when both stressors were applied. The osmotic stress disentangled the mechanisms developed by this cyanobacterium to deal with 3h-UV irradiation alone.