SURVEY OF PHOTOS YNTHE TIC BACTERIA 461 



the recent report of Eimhjellen et al, (6) implies that this interpreta- 

 tion may be inadequate; they have presented evidence for the existence 

 of two distinct bacteriochlorophylls, which they have designated as 

 bacteriochlorophyll a and b, respectively, in a strain of Rhodo- 

 pseudomonas. 



While the separation of the green from the purple bacteria rests 

 on the color of the respective organisms, the subdivision of the purple 

 bacteria into Thio- and Athiorhodaceae is based on the fact that the 

 former are predominantly autotrophic, the latter heterotrophic. The 

 distinction is not, however, as straightforward as was once believed. 

 It is true that in nature the Thiobacteriaceae, as typical sulfur bac- 

 teria, are always associated with the presence of H2S, which they 

 oxidize by way of sulfur to sulfate, while the nonsulfur purple bacteria 

 do not require H2S and depend on the presence of organic substances. 

 But some strains of small purple sulfur bacteria— and until recently 

 these were the only Thiorhodaceae that had been obtained in pure 

 culture— can also grow in media devoid of H2S but supplied instead 

 with any one of a number of organic compounds; they then behave as 

 Athiorhodaceae in this respect. Conversely, some typical nonsulfur 

 purple bacteria can use inorganic oxidizable sulfur compounds or 

 molecular hydrogen as oxidation substrates, and thus mimic the be- 

 havior of the Thiorhodaceae. 



For some time a distinction between the two groups could still be 

 defended on the ground that such Athiorhodaceae cannot be cultivated 

 in strictly inorganic media, because all members of this assemblage 

 require one or more B-vitamins for growth. But now even this cri- 

 terion can no longer be considered valid. On the one hand, Rhodomi- 

 crobium, allocated to the Athiorhodaceae, does not appear to require 

 any vitamins for growth, while on the other hand it has recently been 

 found that the growth of several Thiorhodaceae, such as the large 

 Chromatium and Thiospirillum species, is dependent on an external 

 supply of vitamin B12, so that the designation "autotrophic" cannot be 

 applied to these organisms in its strict sense. 



The position of Rhodomicrobium is rather intriguing in this con- 

 nection. The organism is occasionally encountered in elective cul- 

 tures for Thiorhodaceae, and it has been cultivated in purely mineral 

 media containing H2S (unpublished results). This suggests that it 

 might more properly have to be regarded as a member of the purple 

 sulfur bacteria. 



But apart from these difficulties it is still feasible to maintain a 

 fairly clear distinction between the Thio- and Athiorhodaceae, partly 

 on the basis of ecological and physiological behavior, partly also be- 

 cause of the striking morphological differences between the repre- 

 sentatives of the two groups. This consideration has further raised 

 the question whether there might not exist green bacteria that are 

 physiologically counterparts of the Athiorhodaceae, exhibiting an 

 essentially heterotrophic type of metabolism. 



