Role of calcium in the mitigation of heat stress in the cyanobacterium Anabaena PCC 7120.

The effects of exogenously added CaCl (0.25mM) on photopigments, photosynthetic O-evolution, antioxidative enzyme activity, membrane damage, expression of two heat shock genes (groEL and groES) and apoptotic features in Anabaena 7120 under heat stress (45°C) for up to 24h were investigated. Heat stress lowered the level of photopigments; however, Ca--supplemented cultures showed a low level reduction in Chl a but induced accumulation of carotenoids and phycocyanin under heat stress. Photosynthetic O-evolving capacity was maintained at a higher level in cells from Ca-supplemented medium. Among the antioxidative enzymes, superoxide dismutase activity was unaffected by the presence or absence of Ca in contrast to increases in catalase, ascorbate peroxidase and glutathione reductase activities in cells grown in Ca-supplemented medium. Lower levels of lipid peroxidation were recorded in Anabaena cells grown in Ca-supplemented medium in comparison to cells from Ca--deprived medium. Target cells grown in Ca-deprived medium developed apoptotic features in the early stages of heat shock, while Ca application seemed to interfere with apoptosis because only a few cells showed such features after 24 h of heat exposure, indicating a role for Ca in maintaining cell viability under heat stress. There was also continuous up regulation of two important heat shock genes (groEL and groES) in Ca-supplemented cultures, exposed to heat shock, again indicating a role for Ca in stress management.