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

Abstract

The effects of exogenously added CaCl₂ (0.25mM) on photopigments, photosynthetic O₂-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, Ca²⁺--supplemented cultures showed a low level reduction in Chl a but induced accumulation of carotenoids and phycocyanin under heat stress. Photosynthetic O₂-evolving capacity was maintained at a higher level in cells from Ca²⁺-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 Ca²⁺-supplemented medium. Lower levels of lipid peroxidation were recorded in Anabaena cells grown in Ca²⁺-supplemented medium in comparison to cells from Ca²⁺--deprived medium. Target cells grown in Ca²⁺-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 Ca²⁺-supplemented cultures, exposed to heat shock, again indicating a role for Ca²⁺ in stress management.

Keywords: Anabaena PCC7120; Antioxidative defense; Apoptosis; Heat shock; Photosynthetic O(2)-evolution; groEL/ES.