Function of Carotenoids in Photosynthetic Bacteria

Abstract

A RELATION between photosensitivity and the loss of carotenoid pigments has been observed in Rhodopseudomonas ¹, Chlorella ², Chromatium ³, Chlamydomonas ⁴, Rhodospirillum ⁵, Corynebacterium ⁶ and Zea mays ⁷. Sistrom et al. have suggested that photosensitization in the carotenoidless mutant of Rhodopseudomonas spheroides ‘… is a specific consequence of the replacement of carotenoid pigments with a colourless polyene.’ and that ‘… the primary function of carotenoid pigments in phototrophs is to act as chemical buffers against photo-oxidation of other cell constituents by chlorophyll, thus conferring a high degree of immunity to endogenous photosensitization.’⁸ The carotenoid may carry out this buffering function by acting as a preferentially photo-oxidizable substrate. The effectiveness of such a photochemical buffering system would depend on the existence of an enzymatic mechanism which could reduce the photo-oxidized carotenoid. Inhibition of this enzymatic reduction should result in photosensitization. Accordingly, the effect of low temperature on the photosensitivity of wild type R. spheroides (2.4.1) and Rhodospirillum rubrum was investigated.