PHOTOSYNTHETIC ACTIVITY AND CHLOROPHYLL ABSORPTION SPECTRA OF FRACTIONS FROM THE ALGA,DUNALIELLA*

Abstract— Dunaliella chloroplasts were fractionated according to C. Arntzen et al, Biochim. Biophys. Acta 256 , 85–107, 1972. The initial French-press treatment and differential centrifugation produced Fraction 1 (Fr 1) enriched in photosystem I activity and a heavier Fraction 2 (Fr 2). When Fr 2 was treated with digitonin followed by either gradient or differential centrifugation, two more fractions were recovered: Fr 1 g with a photosystem 1 activity similar to that of Fr 1, and Fr 2 g with very low photosystem II activity. Photosystem II activity was considerably lower in these Dunaliella chloroplasts and fractions than in spinach particles measured under the same conditions, but the relative activities between the fractions were similar to those for spinach. Fr 2 always had greater photosystem II activity than Fr 1, but the digitonin fractions were low and similar in photosystem II activity. Photosystem II activity was measured as the reduction of 2, 6–dichlorophenol indophenol (DCIP) with H₂O, diphenylcarbazide (DPC) or Mn²⁺ as electron donor. The results indicated that exogenous manganous ion competed with H₂O as an electron donor to photosystem II in broken chloroplasts initially, but after 10–15 s of illumination, the Mn³⁺ formed began to reoxidize DCIP and a cyclic reaction ensued. DPC and Mn²⁺ appeared to react at different sites. Computer-assisted curve analysis of the absorption spectrum of each fraction revealed four major component curves representing the absorbing forms of chlorophyll a at 663, 670, 679 and 684 nm seen in numerous other in vivo chlorophyll spectra (C. S. French et al., Plant Physiol. 49 , 421–429, 1972). However, Fr 2g had approx. 20 percent more of Ca663 and Ca670 and 10% more absorption by chl b than Fr 1 which correlated with the difference in photosystem II activity. On the long wavelength side, Fr 2 g had no Ca694 and almost no photosystem I activity. The results are not sufficient to answer the question of whether the photosystem I particle obtained from the original homogenate is significantly similar to or different from the corresponding fraction obtained from Fr 2 with digitonin.