Porphyridium purpureum (Rhodophyta) from red and green light: characterization of photosystem I and determination of in situ fluorescence spectra of the photosystems

Abstract

Recent investigations have shown the existence of a light-harvesting complex (LHC) of photosystem I (PSI) with a fluorescence maximum at 680 nm in the red alga Porphyridium purpureum (Wolfe et al., Nature, 367 (1994) 566–568). We have attempted to determine whether the composition of this red algal LHC is invariable or subject to light acclimatization and whether the 680–690 nm fluorescence band of intact algae, which is usually ascribed to photosystem II (PSII), could be partially due to the LHC of PSI. For this purpose, we characterized PSI core complexes and PSI holocomplexes consisting of core and LHC from algae grown under red or green light conditions and calculated the in situ spectra of the photosystems by normalizing the thylakoid spectra of red and green light algae to an identical PSI or PSII concentration. The composition of the LHC seems to be independent of the light quality, since no significant difference was found in the antenna size and the pigment composition of the PSI holocomplexes from red and green light algae; both holocomplex preparations contained 150–160 chlorophylls, 21–22 zeaxanthins and 23–25 β-carotenes per reaction centre, while the core complexes contained approximately 90 chlorophylls, 15 carotenes and no zeaxanthin. Both holocomplex preparations had a fluoroscence maximum (77 K) at 718 nm. A short-wavelength emission band, as reported by Wolfe et al., was not found in the spectra of the isolated PSI complexes or in the PSI spectrum calculated from thylakoid spectra. This indicates that the LHC does not contribute to the fluorescence spectrum of intact cells, but transfers energy very effectively to the PSI core complex.