Role of Ca as protectant under heat stress by regulation of photosynthesis and membrane saturation in Anabaena PCC 7120.

The present study was aimed at understanding the effects of heat stress on selected physiological and biochemical parameters of a model cyanobacterium, Anabaena PCC 7120 in addition to amelioration strategy using exogenous Ca. A comparison of the cells exposed to heat stress (0-24 h) in the presence or absence of Ca clearly showed reduction in colony-forming ability and increase in reactive oxygen species (ROS) leading to loss in the viability of cells of Ca-deficient cultures. There was higher level of saturation in membrane lipids of the cells supplemented with Ca along with higher accumulation of proline. Similarly, higher quantum yield (7.8-fold) in Ca-supplemented cultures indicated role of Ca in regulation of photosynthesis. Relative electron transport rate (rETR) decreased in both the sets with the difference in the rate of decrease (slow) in Ca-supplemented cultures. The Ca-supplemented sets also maintained high levels of open reaction centers of PS II in comparison to Ca-deprived cells. Increase in transcripts of both subunits ((rbcL and rbcS) of RubisCO from Ca-supplemented Anabaena cultures pointed out the role of Ca in sustenance of photosynthesis of cells via CO fixation, thus, playing an important role in maintaining metabolic status of the heat-stressed cyanobacterium.