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

The emission spectra collected under conditions of open (F-0) and closed (F-M) 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 F-0 and F-M emission are limited in green algae, giving rise only to a minor trough in the F-V/F-M spectrum in the 705-720nm range, irrespectively of the excitation. More substantial variations are observed in cyanobacteria, resulting in marked dependencies of the measured F-V/F-M ratios on both the excitation and the detection wavelengths. In cyanobacteria, the maximal F-V/F-M values (0.5-0.7), observed monitoring at approximately 684 nm and exciting Chl a preferentially, are comparable to those of green algae; however, F-V/F-M decreases sharply below approximately 660nm. Furthermore, in the red emission tail, the trough in the F-V/F-M spectrum is more pronounced in cyanobacteria with respect to green algae, corresponding to F-V/F-M values of 0.25-0.4 in this spectral region. Upon direct phycobilisomes excitation (i.e. >520nm), the F-V/F-M 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 F-V spectra are, in all species investigated, almost independent on the excitation wavelength. It is concluded that the excitation/emission dependencies of the F-V/F-M 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.