Fluorescence spectra of chlorella in the 295–77°K range

A relation of fluorescence spectra of Chlorella (in the 295–77°K range) to the changes in the phase of the ice is suggested in this paper.

When Chlorella cells are first cooled to 77°K and then warmed slowly, the change of the fluorescence intensity as a function of temperature is different for different bands as earlier reported in spinach chloroplasts. In the 80–150°K range, the F698 decreases rapidly, F687 remains almost unchanged, and f680 increases; above 150°K (the temperature at which ice changes from vitreous to cubic phase), F687 decreases more rapidly than any other band. The F725, that decreases smoothly in the 77–295°K range, is composed of several bands (F717 and F725).

If Chlorella cells are first cooled to 77°K, and then warmed to 260°K, and recooled to 77°K before melting, f685, f698 and f730 — but not f717 — are lowered; this fluorescence decrease is not due to changes in the reabsorption of fluorescence or to major changes in the cellular structure due to freezing.

The F698 band is greatly influenced by the aqueous environment; the phase of the ice and the addition of polar solvents (10% dimethylsulfoxide) greatly influence it. These data are consistent with the hypothesis that F698 is from an energy trap (at low temperatures) of System II.