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A novel non-sulfur purple photosynthetic bacterium, designated Rhodospirillum centenum, was isolated from an enrichment culture designed to favor growth of anoxygenic photosynthetic N2-fixing bacteria. R. centenum grows optimally at 40-42 degrees C and has the capacity to produce cytoplasmic 'R bodies', refractile structures not observed hitherto in photosynthetic prokaryotes. The bacterium is also unusual among photosynthetic bacteria in that it forms desiccation-resistant cysts when grown aerobically in darkness with butyrate as the sole carbon source.

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Heliobacterium chlorum and Heliobacillus mobilis are closely related N2‐fixing anoxyphoto‐trophs that contain bacteriochlorophyll g (Bchl g) as the major photopigment. In the presence of O2 and light, the absorbance peak in the infra‐red (788 nm) of this novel photoreceptor disappears and absorbance at 670 nm increases simultaneously. These optical changes appear to be due to a photoisomerization reaction which converts Bchl g to a form of green‐plant chlorophyll a (in which farnesol replaces phytol). In addition to this unusual property, the Gram‐negative heliobacteria present biochemical features (16S RNA base sequence and peptidoglycan structure) indicating an evolutionary relationship with some Gram‐positive bacteria. In comparison to H. chlorum, H. mobilis grows more robustly and shows a much lower tendency to spheroplast and lyse; accordingly, H. mobilis is better suited for further investigations on the biology and biochemistry of these exceptional prokaryotes.