Effect of irradiance level on distribution of chlorophylls between PS II and PS I as determined from optical cross-sections

The number of antenna chlorophyll molecules transferring the energy, of absorbed photons to the reaction center of either photosystem (PS) II or PS I (i.e., the 'size' of the PS), the numbers of RCII and I, and the Chl b/a ratio vary with growth irradiance level to optimize the efficiency of energy harvesting and utilization. The effects of photoadaptation on structural organization are best studied by methods measuring size-dependent properties in vivo. The effective optical cross-section, sigma, or area available for photon capture, determined from the light-saturation characteristics of PS-specific signals following single-turnover flashes, provides this information. In these experiments, sigma of PS II was calculated from the light-saturation of O2 evolution, and that of PS I from the amplitude of a post-illumination respiratory oscillation. Cells grown under different irradiance levels were probed at wavelengths where both Chl a + b or only Chl a absorb. RC stoichiometries, Chl b/a ratios, and the in vivo absorption sigma per chlorophyll were also measured. At all wavelengths studied, cross sections of both PS II and PS I were fit to a single size; deviation from Poissonian curves in regions of strong absorption were the result of absorption of actinic radiation within a single cell and not of heterogeneity of PS size. The yield of O2 evolution following flashes at 723 nm was within 10%, of that at shorter wavelengths. Photoadaptation to high light resulted in a greater decrease in the number of RCI than of RCII and in a decreased Chl b/a ratio in PS II. For all irradiance levels, the sum of the number of RC's times the number of chlorophylls per PS for PS II and PS I equals (+/- 15%) the total extractable Chl/cell; therefore, to our knowledge for the first time, and by optical means, all chlorophyll can be assigned to the two photosystems.