A new genus of marine budding phototrophic bacteria, Rhodobium gen. nov., which includes Rhodobium orientis sp. nov. and Rhodobium marinum comb. nov.

Strains of a previously undescribed species of purple nonsulfur phototrophic bacteria were isolated from coastal seawater in Japan. These new isolates were gram-negative, motile, budding rods that contained lamellar intracytoplasmic membranes and produced pink to red cultures. Cell extracts of photosynthetic cultures exhibited absorption maxima at 377, 468, 500, 530, 591, 802, and 870 nm, indicating that bacterio-chlorophyll a and carotenoids of the spirilloxanthin series were present. The new isolates were halophilic, facultatively aerobic photoheterotrophs that grew anaerobically in the light or aerobically in the dark. Maximum growth occurred in the presence of 4 to 5% NaCl. Anaerobic growth in the dark with nitrate as a terminal electron acceptor also occurred. Various organic compounds were used as photosynthetic electron donors and carbon sources. Sulfate was used as a sulfur source. Both menaquinone 10 and ubiquinone 10 were produced; these quinones were the major quinones. A phylogenetic analysis based on 16S rRNA gene sequences revealed that strain MB312T (T = type strain), a representative of the new phototrophs, was a member of a lineage that was distinct from members of the genus Rhodopseudomonas; Rhodopseudomonas marina was the closest relative. On the basis of the data described above, we propose the name Rhodobium orientis gen. nov., sp. nov. for the new isolates. We also propose that Rhodopseudomonas marina Imhoff 1983 should be transferred to the genus Rhodobium as Rhodobium marinum comb. nov.