Action spectrum, kinetics and quantum requirement of phosphopyridine nucleotide reduction and cytochrome oxidation in the blue-green alga Anacystis nidulans

Abstract

By means of fluorescence and absorption-difference spectrophotometry the kinetics were measured of phosphopyridine nucleotide reduction and cytochrome oxidation in intact cells of the blue-green alga Anacytis nidulans upon illumination with light of various wavelengths. The action spectrum of phosphopyridine nucleotide reduction was found to be proportional to that of cytochrome oxidation; both action spectra showed activity of chlorophyll a and of phycocyanin, but a relatively higher activity of chlorophyll a than the action spectrum of photosynthesis. 3-(3,4-Dichlorophenyl)- i,i-dimethylurea, known as a very potent inhibitor of photosynthesis, did only slightly affect the initial rate of phosphopyridine nucleotide reduction upon illumination. The efficiency of cytochrome oxidation was appreciably increased by 3-(3,4-dichlorophenyl)- i,i-dimethylurea in light of 620 mμ, mainly absorbed by phycocyanin, but not or only slightly in light of 680 mμ, mainly absorbed by chlorophyll a. The number of quanta, required for the reduction of a phosphopyridine nucleotide molecule, was found to be about 5 at 620 mμ; the number needed for the oxidation of a molecule of cytochrome was 7–10 at 680 mμ. The results, together with other evidence, support the hypothesis that in algal photosynthesis two different photochemical oxidation-reduction reactions occur, mediated by two different pigment systems. The two systems operate in series: the first one reduces phosphopyridine nucleotide and oxidizes the cytochrome, the other one reduces the cytochrome and produces O 2 by means of the oxidation of water. Possibly in both light reactions one quantum is needed for the transfer of one hydrogen equivalent.