Fluorescence of the reaction centre of photosystem II in cells of green alga Chlamydomonas reinhardtii

Abstract

Cells of green alga Chlamydomonas reinhardtii, mutant ASS66, which only exhibit the spectra and polypeptide composition of photosystem II, possess a weak fluorescence at about 673 nm along with two main emission bands at 686 and 698 nm, after cultivation in the dark. The 673 nm fluorescence exhibits an excitation spectrum similar to the absorbance of the isolated reaction centre of photosystem II (D1/D2 protein complex). It contains chlorophyll a, pheophytin a, and carotenoid bands with the chlorophyll:pheophytin ratio of about 3. Under illumination, the intensity of the 673 nm emission band decreases with concomitant increase in the emission band at 698 nm, which is thought to belong to the core antenna of photosystem II. Under illumination, in the presence of dithionite (strong reducing conditions), the relative intensity of 673 to 686 nm emission does not change, but the intensities of the pheophytin bands in its excitation spectrum were reduced two-fold. That the relative intensity of the 673 nm band in the emission spectrum under illumination in the presence of dithionite does not change indicates that it originates from an emitter other than pheophytin (possibly reaction centre chlorophyll a).