462 APPENDIX 



The cultures of Chlorobium species that have thus far been studied 

 appear to be obligate sulfur bacteria. Even though they can grow at 

 the expense of acetate, such growth is strictly dependent upon the 

 simultaneous utilization of H2S; in the absence of the latter, acetate 

 is not used for growth (7), And Pfennig (personal communication) has 

 failed to obtain growth of several Chlorobium strains in ethanol- 

 containing media over and above the amount due to the H2S present. 



In this context the green bacterium, Chloropseudomonas ethylicum, 

 discovered by Shaposhnikov et al, (8,9), is of special interest. It can 

 grow in strictly mineral media or at the expense of ethanol, acetate, 

 pyruvate, sugars, and some amino acids, even in the absence of H2S, 

 and thus displays heterotrophic tendencies. To be sure, it is also a 

 typical sulfur bacterium, so that its physiological properties resemble 

 those of some small Chromatium species rather than those of Rhodo- 

 pseudomonas and Rhodospirillum, the most characteristic representa- 

 tives of the Athiorhodaceae, Nevertheless, the very fact that Chloro- 

 pseudomonas is the first green bacterium for which true heterotrophic 

 growth has been established suggests that a concerted search for other 

 heterotrophic green bacteria may be rewarding, 



DESCRIPTION OF SOME GENERA OF 

 PHOTOS YNTHE TIC BACTERIA 



The green bacteria: Chlorobacteriaceae. 



Microscopic examinations of samples from natural habitats of sul- 

 fur bacteria have suggested that there exist several types of green 

 bacteria, and these have frequently been described and named. Con- 

 sequently no less than eight different genera have been proposed, each 

 with one or two species. But nearly all the descriptions are so frag- 

 mentary that an unambiguous identification of green bacteria found in 

 nature with any of these species is practically impossible. 



Up till now, representatives of only two genera, Chlorobium and 

 Chloropseudomonas , have been isolated and studied in pure culture, 

 and in sufficient detail to permit a reasonably satisfactory delineation 

 of these entities. In addition, the availability of such cultures enables 

 students of the green bacteria to use them for comparative studies, 

 involving new isolates as well, so that the classification of this group 

 can be gradually improved as a result of accumulating experience. 



It is therefore reasonable to limit the present account to a descrip- 

 tion of the properties of the above-mentioned genera, 



Chlovobiu)n. 



This genus comprises the small, bead-shaped, nonmotile, non- 

 sporeforming, Gram-negative, green bacteria. They often grow in 

 chains resembling streptococci; under unfavorable conditions they tend 



