A correlation between intracellular calcium and incident irradiance in Nostoc 6720

Abstract

SUMMARYRecent reports of Ca2+ dependency in cyanobacterial processes, which include photosystem II, nitrogenase activity, heterocyst differentiation and trichome motility, question whether Ca2+‐mediated regulation is present in these phototrophic prokaryoles. Such regulation would require that the intracellular concentration of Ca2+ varies within an appropriate range. Several of these Ca2+‐dependent processes respond to treatments which artificially alter the intracellular Ca21+ content, but a physiological or environmental effect has not been demonstated.The effect of changes in the incident irradiance upon the proportion of heterocysts and the intracellular content of Ca2+ in Nostoc 6720 have been investigated. Both the proportion of heterocysts (heterocyst frequency) and the cellular content of Ca2+ (assessed by 45Ca24 incorporation in long labelled cultures) were negatively correlated with incident irradiance. The efflux of Ca2+ from darkened cultures was decreased suggesting that the accumulation of intracellular Ca2+ at low incident irradiance might result from decreased efflux. Orthovanadate, a non specific inhibitor of Ca2+ in Anabaena variabilis, enhanced both the cellular content of Ca24 and heterocyst frequency which suggests that direct inhibition of Ca2+ efflux is able to mimic the effects observed on decreasing irradiance.