ELECTROCHROMIC ABSORBENCY CHANGES IN THE CHLOROPHYLL-C-CONTAINING ALGA PLEUROCHLORIS-MEIRINGENSIS (XANTHOPHYCEAE)

Flash-induced absorbance changes were measured in the Chl-c-containing alga Pleurochloris meiringensis (Xanthophyceae) between 430 and 570 nm. In addition to the bands originating from redox changes of cytochromes, three major positive and two negative transient bands were observed both 0.7 and 20 ms after the exciting flash. These transient bands peaking at 520, 480 and 451 nm and 497 and 465 nm, respectively, could be assigned to an almost homogeneous shift of the absorbance bands with maxima at 506, 473 and 444 nm, respectively. The shape of the absorbance transients elicited from PS I or PS II was identical, and the two photosystems contributed nearly equally to the absorbance changes. Furthermore, the decay transients were sensitive to the preillumination of the cells. These data strongly suggest that the absorbance transients originate from an electrochromic response of carotenoid molecules. The pigment species responsible for the 506 nm absorption band, probably heteroxanthin or diatoxanthin, transferred excitation energy to both photosystems as shown by the aid of 77 K fluorescence excitation spectra.