The role of carotenoids in the photoadaptation of the brown-colored sulfur bacterium Chlorobium phaeobacteroides

The brown-colored sulfur bacterium Chlorobium (Cb.) phaeobacteroides 1549 (new name, Chlorobaculum limnaeum 1549) contains many kinds of carotenoids as well as bacteriochlorophyll (BChl) e. These carotenoids were identified with C18-high-performance liquid chromatography, absorption, mass and proton nuclear magnetic resonance spectroscopies and were divided into two groups: the first is carotenoid with one or two phi-end groups such as isorenieratene and beta-isorenieratene and the second is carotenoid with one or two beta-end groups such as p-zeacarotene, beta-carotene and 7,8-dihydro-beta-carotene. The latter 7,8-dihydro-beta-carotene was found to be a novel carotenoid in nature. OH-gamma-Carotene glucoside laurate and OH-chlorobactene glucoside laurate were also found as minor components. The distribution of BChl e homologs in Cb. phaeobacteroides cultivated under various light intensities did not change, but the carotenoid to BChl e ratio changed markedly: carotenoid with the phi-end group maintained the same ratio to BChl e, whereas that with the beta-end group increased with increasing light intensity. The cells cultured under low-light intensity contained more phi-end carotenoids than beta-end. In Cb. phaeobacteroides the wavelength of the Qy band of BChl e aggregates did not change. We suggested that Cb. phaeobacteroides photoadapts to light intensity by changing the carotenoid composition.