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

The FV/FM parameter is routinely used estimate the maximal quantum efficiency of Photosystem (PS) II, ?PSII,M [1]. In land plants FV/FM exceeds 0.7. Similar values, albeit in general smaller, are observed in green algae. In these organisms, having transmembrane light harvesting antenna complexes, the FV/FM ratio shows only a limited dependence on the excitation and detection wavelengths (e.g. [2,3]). In cyanobacterial strains, however, significantly lower (two to three folds) than 0.7 are often reported (e.g. [4,5]). To seek more details on the FV/FM of cyanobacteria, a comparative investigation of its excitation and emission spectral dependence in model strains of green algae (C. reinhardtii and C. sorokinina) and cyanobacteria (Synechocystis sp. PPC6803 and Synechococcus sp. PCC7942) was performed. It is shown that, whereas the emission spectra collected near F0 and at FM conditions are close-toindependent from the excitation wavelength in the C. reinhardtii and C. sorokinina, a pronounced dependence is observed in Synechocystis and Synechococcus instead. Moreover, 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 Synechocystis, the maximal FV/FM values (0.5-0.63), observed monitoring at ~684 nm and exciting Chl a preferentially, are comparable to those of C. sorokiniana. However, in Synechocystis, differently from Chlorella, FV/FM decreases sharply below ~670 nm. Furhermore, in the red emission tail, the trough in the FV/FM spectrum, centred at ~715 nm, is more pronounced than in green algae, corresponding to minimal values of 0.27-0.4. Upon direct PBS excitation (i.e. >510 nm), the FV/FM value detected at 684 nm decreases to 0.4-0.5 and is close-tonegligible (0.1-0.2) when detected below 650 nm. At the same time, the FV spectra are, also in cyanobacteria, almost independent on the excitation wavelength. Parallel lifetime analysis indicates a weak emission dependence of PSII associated components. It is therefore concluded that the excitation/emission dependencies of the FV/FM ratio are not due to intrinsic variations of PSII quantum yield. It is instead proposed to arise from overlapped contributions from three independent emission of PSI, PSII and a fraction of energetically uncoupled PBS antenna, excited in different proportion depending on the respective optical cross-section. Impact on fluorescence-based parameters estimation, and correction methods will be discussed.