Salt-induced alterations of the fluorescence yield and of emission spectra in Chlorella pyrenoidosa

(1) The addition of salts to the suspending medium induces a decrease in the yield of chlorophyll a fluorescence in normal and DCMU-poisoned intact algal cells of Chlorella pyrcnoidosa. Potassium and sodium acetate cause a pronounced lowering of the fluorescence at relatively low concentrations (0.01-0.1 M). MgClj and KC1 cause a similar lowering of fluorescence but at much higher concentrations (0.1-0.4 u). In contrast to sodium acetate, ammonium acetate does not cause any significant change in the fluorescence transient. (2) Unlike the case in isolated chloroplasts, MgClj decreases the ratio of short wavelength (mainly system 2) to long wavelength (mainly system 1) emisiion bands in both DCMU poisoned and normal cells. Since these salt-induced changes do not appear to be related to the redox reactions of photosynthesis, the salts might have caused a decrease in the mutual distance between the two photosystems by changing the microstructure of the chloroplasts in vivo thereby facilitating the spillover of excitation energy from strongly fluorescent system 2 to weakly fluorescent system 1. (3) The light induced turbidity changes in intact algal cells, as measured by the increase in optical density at 540 nm, is reduced in the presence of these salts. However, MgClj produces the greatest reduction while Na acetate the least, even though both of these salts (at the concentrations used) cause large suppression of the fluorescence transient. Moreover, the light induced turbidity changes were, essentially irreversible. (4) As high concentrations of salts increase the osmotic potential of the bathing medium, it seems that the osmotic changes as well as the ionic changes in the intact algal cells are responsible for the fluorescence quenching and changes in the mode of excitation transfer observed in this study. In the case of low concentrations of salts (e.g., 0.1 u Na or K acetate) the effects are predominantly ionic, and in the case of very high concentrations of MgCl2 (0.4 M), die osmotic effects play a much larger role.

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