Analysis of the fast fluorescence rise kinetics in saturating light allows determination of the wavelength- and sample-specific functional absorption cross-section of PS II, Sigma(II)λ, with which the PS II turnover rate at a given incident PAR can be calculated. Sigma(II)λ is defined for a quasi-dark reference state, thus differing from σPSII used in limnology and oceanography. The spectral behavior of Sigma(II)λ of Chlorella vulgaris and cyanobacteria Synechocystis PCC 6803 is shown in Fig. 1.
Vastly different light response curves for electron transport rates (rel. ETR) of Chlorella are obtained with light of different colors (440 and 625 nm) when the usual PAR-scale is used (Fig. 2).
Based on Sigma(II)λ, the PAR in units of μmol quanta/(m2 · s) can be converted into PAR(II) in units of PS II effective quanta/s. Then, a fluorescence-based electron transport rate ETR(II) = PAR(II) · Y(II)/Y(II)max can be calculated where the Y(II) and Y(II)max are the PS II quantum yields in the light-exposed and the dark state, respectively. Plots of ETR(II) vs PAR(II) for Chlorella are almost identical using either 440 nm or 625 nm light (Fig. 3).
Clearly, ETR(II) in contrast to relative ETR qualifies for quantifying the absolute rate of electron transport in optically thin suspensions of unicellular algae and cyanobacteria.
All data from:
Schreiber U, Klughammer C, Kolbowski J (2012)