It therefore stands to reason that this spectral domain should be avoided in selleck kinase inhibitor fluorescence induction measurements where Chla fluorescence is used as a proxy of energy flowing through PSII. Long wavelength (>690 nm) fluorescence from PSI is also relatively strong in cyanobacteria. Regardless of the excitation band that

is used we therefore find that narrow (10-nm) wavebands centred at the PSII Chla emission band (680–690 nm) yield best results (Fig. 11). The efficiency of energy transfer from the PBS to reaction centres is considered very high (Sidler 1994 for a review), but not all harvested energy is transferred to the PSII core. Our results show PBS fluorescence in the BMS202 chemical structure order of 22% of F o in the Chla emission band. This emission is absent in algae (with exceptions) and theoretically leads to a lowered reading of F v/F m in cyanobacteria and in communities

with a high cyanobacterial biomass (Campbell et al. 1996, 1998). We find, however, that a variable component to PBS fluorescence can alleviate the theoretical Cell Cycle inhibitor dampening of F v/F m considerably (Fig. 10). Indeed, the peak of F v/F m in the excitation–emission spectrum is found in the order of 0.65–0.75, for several cyanobacteria species (Fig. 3), despite an average dampening by 6.2% of F v/F m due to the overlapping fluorescence of PBS pigments and Chla. Such high F v/F m values for cyanobacteria

have been reported in very few other studies (Raateoja et al. 2004; Suggett et al. 2009), which used FRRF. Variable fluorescence from PBS is surprising; it has been assumed that these pigments do not exhibit variable fluorescence at all. These findings that are reflected in some recent studies using different fluorescence induction techniques (Küpper et al. 2009; Kana et al. 2009) challenge the idea of a constant, highly efficient resonance transfer from PBS pigments to the reaction centres. Our fluorescence data provide insufficient means to explore the relation between the rise of PBS fluorescence and closing of PSII reaction centres, or to see how illumination or nutrient conditions might influence PBS F v/F m. Nevertheless, Lck it is notable that F v/F m from the PBS at 650 nm showed a fair correlation with cyanobacterial PSII Chla F v/F m (Fig. 8c). In a pilot experiment that is not presented here, we exposed N. spumigena with saturating light flashes (590 nm) and observed induction of PBS fluorescence (650 nm), suggesting that the present result is neither merely an artefact of DCMU treatment nor to prolonged exposure to light in our spectrofluorometer. If the mechanism behind phycobilisomal variable fluorescence can be explained in terms of PSII kinetics, this may open up the way to study the physiology of cyanobacteria in natural communities.