Comparative excitation-emission dependence of the FV /FM ratio in model green algae and cyanobacterial strains.

The emission spectra collected under conditions of open (F0 ) and closed (FM ) photosystem II (PSII) reaction centres are close-to-independent from the excitation wavelength in Chlamydomonas reinhardtii and Chlorella sorokiniana, whereas a pronounced dependence is observed in Synechocystis sp. PCC6803 and Synechococcus PCC7942, instead. The differences in band-shape between the F0 and FM emission are limited in green algae, giving rise only to a minor trough in the FV /FM spectrum in the 705-720 nm range, irrespectively of the excitation. More substantial variations are observed in cyanobacteria, resulting in marked dependencies of the measured FV /FM ratios on both the excitation and the detection wavelengths. In cyanobacteria, the maximal FV /FM values (0.5-0.7), observed monitoring at approximately 684 nm and exciting Chl a preferentially, are comparable to those of green algae; however, FV /FM decreases sharply below approximately 660 nm. Furthermore, in the red emission tail, the trough in the FV /FM spectrum is more pronounced in cyanobacteria with respect to green algae, corresponding to FV /FM values of 0.25-0.4 in this spectral region. Upon direct phycobilisomes excitation (i.e. >520 nm), the FV /FM value detected at 684 nm decreases to 0.3-0.5 and is close-to-negligible (approximately 0.1) below 660 nm. At the same time, the FV spectra are, in all species investigated, almost independent on the excitation wavelength. It is concluded that the excitation/emission dependencies of the FV /FM ratio arise from overlapped contributions from the three independent emissions of PSI, PSII and a fraction of energetically uncoupled external antenna, excited in different proportions depending on the respective optical cross-section and fluorescence yield.