Notes the Natural History of Rhodopscudomonas palustris 581 



were obtained by shake-culture isolation. When green algae in- 

 terfered, the tubes were incubated under Bausch & Lomb inter- 

 ference filters transmitting far-red light (700-900 m/j.). Selective 

 jBlters within this range were used to discriminate between purple 

 and green photosynthetic bacteria. In selecting for Athiorhoda- 

 ceae, sulfate was kept minimal. After development of colonies, 

 the agar column was ejected into a sterile Petri dish. 



Growth and Manometric Experiments 



Stock cultures were maintained as stabs on the enrichment 

 medium or yeast extract 0.3 per cent supplemented with 1.5-2.0 

 per cent agar. Cells were grown in completely filled bottles, or 

 in screw-capped culture tubes or micro-fernbach flasks under 

 aerobic conditions (3). Growth was measured as optical density 

 or dry weight. For manometry, cells were incubated at 25-30 C 

 in the light, harvested by centrifugation and washed twice in 

 0.05 M phosphate buffer at pH 7.0. Oxygen consumption by 

 resting cells suspended in bufi^er was determined at 30.0 C in 

 the dark. 



Several strains of purple bacteria typical of the genus Rhodo- 

 pseudomonas and occasionally of RhodospiriUimi ruhrum were 

 isolated from benzoate enrichment cultures. Chemically defined 

 media in which yeast extract was replaced by PABA (10 /xg%) 

 proved more selective and invariably yielded R. palustris by the 

 second transfer. Of more than 40 strains of R. palustris, all grew 

 well with PABA as sole growth factor, as compared with ten out 

 of fifteen strains isolated from non-aromatic substrate enrich- 

 ments. 



