Carotenoid transformations during chloroplast development in Scenedesmus obliquus PG1 demonstrated by deuterium labelling

Abstract

THE unicellular green alga Scenedesmus obliquus (Chlorophyta), when grown photosynthetically, contains pigments (chlorophylls and carotenoids) similar to those of the chloroplasts of higher plants¹. Several mutant strains¹ have abnormal pigmentation¹. One of them, PG1, when grown in the dark accumulates the acyclic carotenoids phytoene (7,8,11,12,7′,8′,11′, 12′-octahydro-ψ,ψ-carotene), phytofluene (7,8,11,12,7′,8′-hexahydro-ψ,ψ-carotene) and ζ-carotene (7,8,7′,8′-tetrahydro-ψ,ψ-carotene) in place of cyclic carotenes and xanthophylls and cannot make more than trace amounts of chlorophylls. The pigmentation of light-grown PG1, however, is virtually identical to that of the wild type. When dark-grown PG1 is illuminated, the content of acyclic carotenoids decreases and that of cyclic carotenoids increases². Time-course studies³ agree with the hypothesis that the cyclic carotenoids produced on illumination are formed from the acyclic precursors that accumulate in dark-grown cultures. It has not been possible to confirm this by radioisotopic labelling, but we have now done so using deuterium labelling. Our procedure also provides a general method for following the kinetics of formation of important chloroplast constituents during chloroplast development in this and other organisms.