Light-to-dark transitions trigger a transient increase in intracellular Ca2+ modulated by the redox state of the photosynthetic electron transport chain in the cyanobacterium Anabaena sp PCC7120

Light-to-dark transitions represent one of the most crucial environmental stresses that photosynthetic organisms must cope with, since substantial metabolism adaptations are required in order to utilize alternative energy and carbon sources. Although signal transduction systems for changing light regimes are not sufficiently understood, calcium has been implicated in plants as a second messenger in light-on and light-off events. Much less is known about light signalling in cyanobacteria, but it has been shown that calcium probably performs similar signalling roles in these organisms and other prokaryotes. Herein it is reported that light-to-dark transitions trigger a calcium transient in aequorin expressing Anabaena sp. PCC7120. The magnitude of this transient depends on the fluence rate previously irradiated and can reach a peak height over 2 mu<smallcapitals>m</smallcapitals> free calcium when the fluence rate of light is around 400 mumol photons s(-1) m(-2). The use of increasing calcium concentration, ethylene glycol-bis (beta-aminoethylether) N,N,N',N'-tetraacetic acid (EGTA), verapamil and trifluoperazine indicated that these transients are originated by a calcium influx probably through verapamil-sensitive Ca2+ channels and are probably modulated by calcium-binding proteins. Experiments with different light spectral qualities and the photosynthetic inhibitors 3-(3,4 dichlorophenyl)1,1,dimelthylurea (DCMU) and 3,5-dibromo-3-methyl-b-isopropyl-p-benzoquinone (DBMIB) indicate that the calcium transient triggered by the light-to-dark transition is not coupled to a specific photoreceptor but rather to changes in the redox state of photosynthetic electron transport chain components other than the plastoquinone pool.