Light-induced changes in the fluorescence yield of chlorophyll a in Anacystis nidulans II. The fast changes and the effect of photosynthetic inhibitors on both the fast and slow fluorescence induction'

(1) The intensity dependence and spectral variations during the fast transient of chlorophyll a (Chi a) fluorescence have been analyzed in the blue-green alga Anacystis nidulans. (Unlike the case of eukaryotic unicellular green or red algae, the fast fluorescence induction characteristics of the prokaryotic blue-green algae had not been documented before.) (2) Dark adapted cells of Anacystis exhibit two types of fluctuations in the fluorescence yield when excited with bright orange light (absorbed mainly in phycocyanin). The first kinetic pattern called the fast (sec) fluorescence transient exhibits a characteristic original level O, intermediary hump I, a pronounced dip D, peak P and a transitory small decline to a quasi steady state S. After attaining S, fluorescence yield slowly rises to a maximum level M. From M, the decline in fluorescence yield to a terminal T level is extremely slow as shown earlier by Papageorgiou and Govindjee (#). As compared with green and red algae, blue-green algae seem to have a small PS decline and a very characteristic slow SM rise, with a M level much higher than the peak P. (3) A prolonged dark adaptation and relatively high intensity of exciting illumination are required to evoke DPS type yield fluctuations in Anacystis. At low to moderate intensities of exciting light, the time for the development of P depends on light doses, but for M, this remains constant at these intensities. (4) Fluorescence emission was heterogeneous during the induction period in Anacystis; the P and the M levels were relatively enriched in short-wavelength system II Chi a emission as compared to D and S levels. (5) The fast DPS transient was found to be affected by electron transport cofactor (methyl viologen), and inhibitors (e.g., DCMU, NH2OH) in a manner suggesting that these changes are mostly related to the oxido-reduction level of intermediates between the two photosystems. On the other hand, the slow SM changes in fluorescence yield, as reported earlier (5, 15), parallel oxygen evolution. These changes were found to be resistant to a variety of electron transport inhibitors (O-phenanthroline, HOQNO, salicylaldoxime, DCMU, NH2OH and Antimycin a). It is suggested that, in Anacystis,