Calcium signals from heterocysts of Anabaena sp. after UV irradiation

The effects of UV plus PAR or PAR irradiation only on the cytosolic calcium concentration of the heterocysts of a filamentous cyanobacterium Anabaena sp. have been studied using the fluorescence microscopy technique and Calci um Crimson as a calcium indicator. After UV + PAR irradiation a pronounced calcium signal, indicating an increase in the cytosolic calcium concentrati on, was observed in heterocysts in comparison to control cultures irradiate d with PAR only. No increase in the calcium signal could be observed after UV + PAR irradiation of cells in Ca2+ free medium, but shortly after washin g the cells in tap water the calcium signal reappeared in the heterocysts. This indicates that the signal is due to the uptake of external Ca2+. Simil arly, no increase in the calcium signal could be recorded in the heterocyst s when the cells were irradiated with UV in the presence of lanthanum, whic h is known to block calcium channels. Washing the cells in tap water restor ed the calcium signal, indicating that the uptake of calcium is mediated by specific membrane channels, the opening of which is triggered by UV.