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English: Emission and excitation fluorescence spectra of Anacystis nidulans at low temperatures. (A) Emission spectra of fluorescence in the 4 K (liquid helium temperature) to 77 K (liquid nitrogen temperature) range in cells of A. nidulans; wavelength of excitation was at 620 nm (absorption by phycocyanin; see Figure 2B). For comparison, a room temperature (295 K) spectrum (black dashed curve) is shown. (B) Emission spectra of fluorescence in the 4–77 K range in cells of A. nidulans; wavelength of excitation was at 440 nm (absorption by Chl a; note that cyanobacteria have most of their Chl a in PSI). For comparison, a room temperature (295 K) spectrum is also shown. (C) Excitation (or action) spectra of fluorescence, as a function of temperature (4–77 K) in cells of A. nidulans, measured at 685 nm (mostly PSII fluorescence). (D) Excitation (or action) spectra of fluorescence as a function of temperature (4–79 K) in cells of A. nidulans, measured at 715 nm (mostly PSI fluorescence). All panels of the figure were modified and reproduced from Cho and Govindjee (1970).