This paper describes the photosynthetic response of a Roseobacter strain of marine aerobic anoxygenic phototrophic bacteria to an organic substrate limitation. In batch cultures, higher values of the spheroidenone/bacteriochlorophyll a ratio were observed under substrate-deficient conditions. Interestingly, the maximum photochemical quantum efficiencies of the photosystem under substrate-deficient conditions using blue or green excitation were significantly higher than those under substrate-replete conditions. These results indicate that spheroidenone, which can absorb green light, may play an important role in their photosynthesis as a light-harvesting antenna pigment, and the photosynthetic competence of the Roseobacter strain can increase in an organic substrate-deficient environment.