The 590-nm excitation configuration featured in Fig. 8b is representative of configurations with excitation in the 590–630 nm range, check details which are not individually shown here. At longer excitation wavelengths >630 nm, fluorescence in both cyanobacteria and algal groups is increasingly excited so that the signal becomes less specific to the cyanobacterial subpopulation. Moving the excitation source from 590 towards 650 nm increases the fluorescence yield in both groups (Fig. 7c), which can be explained by the presence of phycocyanin in all cyanobacterial cultures and the accessory chlorophylls b and c in the
algal cultures. The absorption shoulder of Chla around 625 nm and the main red peak of Chla at 675 nm also increasingly absorb light when the excitation waveband is moved beyond 600 nm (Sathyendranath et al. 1987; Bidigare et al. 1990; Ficek et al. 2004). The relatively high affinity for illumination >600 nm in both algae and cyanobacteria implies that the light source need not be as bright to fully saturate PSII in all organisms, and error properties
of the F v/F m measurement improve slightly, compared to illumination around 590 nm. At the same time, shorter wavebands prevent crosstalk between excitation and emission bands, an important consideration in fluorometer design. Results for a fluorometer with broad-white (400–650 nm, spectrally neutral) illumination are given in Fig. 12a. This ‘cool white’ excitation light resulted in a weak representation of cyanobacterial F v/F m against improved
results for algal cultures compared to λex = 590 nm (Fig. 8b). PI3K inhibitor Fig. 12 Simulated community F v/F m as a function of algal and cyanobacterial F v/F m, for fluorometers with different light source configurations and a 10-nm wide emission band centred at 683 nm. a A BMN 673 concentration neutral white light source (400–650 nm), b a broad-green light source (535–585 nm) Excitation in the 535–585 nm domain should lead to approximately equal representation of algae and cyanobacteria, based on the data shown thus far. Figure 12b shows the result for such a ‘broad-green’ light source. The configuration is still more sensitive to algae than cyanobacteria, but the difference in regression slopes and offsets could at least in part be attributed PAK5 to the presence of more cases with low F v/F m in the group of cyanobacteria, while scatter is approximately equal for both groups. Cultures of cyanobacteria with low F v/F m (and F 0) had limited influence on community F v/F m, especially when paired with healthy algae. For the purpose of identifying community photosynthetic capacity rather than differentiation of algal and cyanobacterial subpopulations, this is not a poor result: phytoplankton that contributes little to community photosynthesis has a proportionally lower impact on community F v/F m.