Evaluation of biophysical and optical determinations of light absorption by photosystem II in phytoplankton

Fast repetition rate (FRR) fluorescence can provide highly resolved estimates of light absorption by photosystem II ( PSII), a variable that is critical to bio-optical determinations of phytoplankton productivity. We compared estimates of chlorophyll a - specific light absorption by PSII, a(PSII)(chl), using both biophysical ( FRR) and optical ( chlorophyll a - specific light absorption coefficient, a(chl)) techniques on cultures of phytoplankton from diverse taxa. Biophysical determinations of achl PSII were obtained from the product of the effective light absorption cross-section of PSII (sigma(PSII)), measured by FRR fluorescence, and the ratio of PSII reaction centers to chlorophyll a (n(PSII)), measured by oxygen flash yields. Both parameters were highly variable between individual taxa. In mixtures of algae, the estimates of sigma(PSII) were largely determined by the taxon that dominated fluorescence intensity. Independent optical estimates of achl PSII were obtained by weighting the light absorption spectra of photosynthetic pigments by the fluorescence excitation spectra. The biophysical and optical estimates of achl PSII were highly correlated (r(2) = 0.94) with a slope that was not significantly different from 1 and an intercept of 0. Estimates of productivity using biophysical or optical absorption measurements should therefore be comparable when the latter are adjusted to account for the proportion of light provided for photochemistry into both photosystem I and II. Finally, we show how simultaneous measurements of sigma(PSII) and a(PSI)(chl)I can be used to derive n(PSII) where flash- yield measurements are impractical, as is almost universally the case in field measurements.