Steps Linking the Photosynthetic Light Reactions to the Biological Clock Require Calcium

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Abstract
The effect of modifying calcium concentration on the expression of photosynthesis circadian rhythm was examined in Fuglena gracilis, Klebs strain Z. Expression of the oxygen evolution rhythm required the presence of both extracellular and intracellular calcium. Several treatments were found to uncouple the rate of the light reactions from the biological clock. In the presence of these chemical agents, the rate of oxygen evolution increased steadily throughout the light portion of the light/dark cycle, instead of showing a peak of activity in the middle of the light cycle. Oxygen evolution was uncoupled from the biological clock when extracellular calcium concentrations were altered by the presence of EGTA or LaCl3. Uncoupling was also observed when intracellular calcium concentrations were disrupted by the use of Ca2+ channel blockers, the intracellular Ca2+ antagonist 8-(diethylamino)-octyl-3,4,5-trimethoxybenzoate, or by disrupting expression of the inositol triphosphate system. Uncoupling was also observed when the diacylglycerol signaling system, which activates kinase C, was inhibited by acridine orange. The inhibition was reversed by the presence of phorbol esters which activate the kinase. It was concluded that both the inositol trisphosphate and diacylglycerol signaling systems were required for the expression of the oxygen evolution rhythm generated by the biological clock.