SURVEY OF PHOTOSYNTHETIC BACTERIA 465 



found in natural environments. But because the sizes reported by dif- 

 ferent investigators range from 1.7 to 3,5 by 3.5 to 7^, 1 to 1.5 by 2 

 to 6 /y, and 1 to 3 by 2 to 5 /i, respectively, for the individual species, 

 it is clear that at present a distinction between these species can only 

 be arbitrary. 



The available pure cultures are anaerobic and can be grown in 

 vitamin-free mineral media with H2S, thiosulfate, H2, or simple or- 

 ganic compounds as the oxidation substrates. In sulfide media the cells 

 may vary considerably in shape and size, depending on the sulfide 

 concentration and pH of the medium. It now seems probable that far 

 better media for their cultivation can be devised along the lines ex- 

 plored by Pfennig and that cultures in such media will provide infor- 

 mation on which a more up-to-date description and classification can 

 be based. 



Pfennig (personal communication) has now isolated pure cultures 

 of several large Chromatium species, among which Chr. okenii (5 to 

 6.5 by 10 to 20 jw) and Chr. warmingii (3.5 to 4.5 by 5 to 10 /i), the 

 latter with its characteristically bipolarly arranged sulfur globules, 

 have been definitively identified. 



Thio spirillum. 



One of the most conspicuous among the Thiorhodaceae is Thio- 

 spirillum jenense, whose spiral cells measure 3.5 to 4.5 by 30 to 40 /i . 

 It is sometimes encountered as the nearly exclusive component of 

 natural blooms, and a number of strains have recently been isolated 

 in pure culture by Pfennig (personal communication). 



Other fhiospirillum species have been described, but not yet iso- 

 lated in pure culture. 



The nonsulfur purple bacteria: Athiorhodaceae. 



In contrast to the Thiorhodaceae, the representatives of this group 

 are small and tend to a heterotrophic metabolism; most of them are 

 capable of aerobic growth. They can be cultivated at the expense of 

 any one of a variety of simple organic compounds, such as alcohols, 

 fatty acids, hydroxy acids, di- and polybasic acids, and amino acids. 

 All except RJiodomicrobium require one or more B-vitamins for 

 growth. 



Three genera can be recognized: Rhodopseudomonas , Rhodo- 

 spirillum, and Rhodomicrobium . These are differentiated on the basis 

 of cell morphology; the first two have been treated in detail elsewhere 

 (16). 



Rhodopseudomonas . 



This genus is defined as composed of small, rod- shaped, polarly 

 flagellated, nonsporeforming. Gram-negative bacteria, containing 

 bacteriochlorophyll and a variety of carotenoids. The nature of the 



