Characterization of the Carotenoidless Strain of Scenedesmus obliquus, Mutant C‐6E, a Living Photosystem I Model*

Abstract

AbstractWhen grown heterotrophically in the dark on enriched culture medium, the pigment‐deficient strain of Scenedesmus obliquus, mutant C‐6E, is uniquely characterized by a complete deficiency in carotenoids and chlorophyll b while retaining a low level of chlorophyll a which is exclusively utilized in photosystem I‐type reactions. The strain lacks photosystem II activity but exhibits all PS‐I reactions tested, including P700 redox reactions, photoreduction of CO2 with hydrogen as electron donor, and O2 uptake following methyl viologen reduction. The mutant contains 10 times more P700 per chlorophyll than the wild type and develops the pigment‐protein complex of PS‐I, CP‐I. The action spectrum for methyl viologen reduction compares favorable to the low temperature absorption spectrum of whole cells. Both the chlorophyll fluorescence excitation and emission spectra of pigment‐protein complexes derived from cells of C‐6E show patterns typical of PS‐I. The strain lacks the LHCs and CP‐II as well as their respective apoproteins. The absence of carotenoids appears to prevent the development of the normal variety of pigment‐protein complexes and the accumulation of Chl b. This inability is also expressed by the presence of only single stranded thylakoid membranes in the chloroplast of C‐6E. When heterotrophically grown cells of this mutant are exposed to white light of 8 or 22 W m−2, 50% of its chlorophyll is lost by photooxidation within 4 or 1.5 hours, respectively.