The effects of exogenously added CaCl2 (0.25 mM) on photopigments, photosynthetic O-2-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 degrees C) for up to 24 h were investigated. Heat stress lowered the level of photopigments; however, Ca2+-supplemented cultures showed a low level reduction in Chl a but induced accumulation of carotenoids and phycocyanin under heat stress. Photosynthetic O-2-evolving capacity was maintained at a higher level in cells from Ca(2+)supplemented medium. Among the antioxidative enzymes, superoxide dismutase activity was unaffected by the presence or absence of Ca2+ in contrast to increases in catalase, ascorbate peroxidase and glutathione reductase activities in cells grown in Ca(2+)supplemented medium. Lower levels of lipid peroxidation were recorded in Anabaena cells grown in Ca(2+)supplemented medium in comparison to cells from Ca2+-deprived medium. Target cells grown in Ca(2+)deprived medium developed apoptotic features in the early stages of heat shock, while Ca2+ application seemed to interfere with apoptosis because only a few cells showed such features after 24 h of heat exposure, indicating a role for Ca2+ in maintaining cell viability under heat stress. There was also continuous up regulation of two important heat shock genes (groEL and groES) in Ca(2+)supplemented cultures, exposed to heat shock, again indicating a role for Ca2+ in stress management. (C) 2016 Elsevier GmbH. All rights reserved.