Notes the Natural History of Rhodopseudomonas palustris 585 



the latter apart as a group. The present occurrence of Bi^ as a 

 "temperature factor" for some members of the Athiorhodaceae 

 and the recent report (8) that Thiorhodaceae sucli as Chroma- 

 tium okenii and Thiospirillum jenense have absolute requirements 

 for Bi2 help bridge this gap. 



In nature, anaerobic growth of photosynthetic bacteria is 

 limited by the light requirement. Bacterial chlorophylls absorb 

 maximally in the far red at 800-900 mu where algal and higher 

 plant chlorophylls no longer absorb light effectively. When com- 

 peting for light with photosynthetic organisms that utilize visible 

 and near-red light, the purple bacteria absorb far-red light with- 

 out interference. Since water absorbs strongly in the red to infra- 

 red, photosynthetic bacteria are largely confined to the shallow 

 bays, tidal marshes, and mud flats where sufficient red light 

 penetrates to satisfy their requirements. Schlegel and Pfennig 

 (9) found photosynthetic sulfur bacteria rich in ponds thickly 

 covered over with duckweed, and used near infra-red for en- 

 richments to avoid overgrowth by green and blue-green algae. 

 As a consequence of their dependence upon liglit, photosynthetic 

 bacteria are largely restricted to shallow aquatic environments. 

 These may be subjected to extremes of temperature. We have 

 frequently observed purple bacteria in hot springs at tempera- 

 tures of 40-50 C; there are reports of their occurrence as high as 

 70 C at Searles Lake, California ( 1 ) . Although knowledge of the 

 basis of thennal tolerance is still fragmentary, the enhanced re- 

 quirements for vitamins at elevated temperatures suggest that 

 such induced growth-factor requirements set limits to the de- 

 velopment of photos\'nthetic forms in nature. 



Anaerobic use of organic substrates by photosynthetic bacteria 

 is strictly light-dependent. Tliere has been an idea that liydrogen 

 acceptors other than molecular oxygen would not suffice for 

 aromatic oxidations, derived from demonstrations that molecular 

 oxygen was an obligatory oxidant for enzymes that cleave the 

 benzene ring (10). On general grounds however, one would think 

 that substrates for microorganisms under aerobic conditions 

 should serve in the same capacity anaerobically if a suitable oxi- 

 dant is available. Aromatic compounds such as benzoate no longer 

 appear to be an exception to the rule: their utilization by purple 



