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Rhodospirillum tenue sp. n. differs from the rhodospirilla described so far by its small size, ultrastructure, and physiological and biochemical characteristics.

The fine structure of several strains of green bacteria belonging to the genus Chlorobium has been studied in thin sections with the electron microscope. In addition to having general cytological features typical of Gram-negative bacteria, the cells of these organisms always contain membranous mesosomal elements, connected with the cytoplasmic membrane, and an elaborate system of isolated cortical vesicles, some 300 to 400 A wide and 1000 to 1500 A long. The latter structures, chlorobium vesicles, have been isolated in a partly purified state by differential centrifugation of cell-free extracts. They are associated with a centrifugal fraction that has a very high specific chlorophyll content. In all probability, therefore, the chlorobium vesicles are the site of the photosynthetic apparatus of green bacteria.

A succinate-mineral salts medium of pH 5.2 provided selective enrichment conditions for Rhodomicrobium vannielii and for a new species belonging to the Athiorhodaceae, described herein as Rhodopseudomonas acidophila. Seven strains of the new species have been isolated from different sources in the United States and Germany. The cells are rod-shaped or ovoid, 1.0 to 1.3 mum wide and 2 to 5 mum long, and motile by means of polar flagella. Multiplication occurs by budding. The photopigments consist of bacteriochlorophyll a and carotenoids of the spirilloxanthin series, together with new carotenoids. All strains can grow either under anaerobic conditions in the light or under microaerophilic to aerobic conditions in the dark. No growth factors are required. The range of simple organic substrates photo-assimilated resembles that characteristic of Rhodomicrobium. Good photolithotrophic growth is possible at the expense of molecular hydrogen; thiosulfate and sulfide are not utilized.