EFFECT OF OXYGEN CONCENTRATION 1913 



oxygen should not inhibit the Hill reaction in Chlorella cells. This con- 

 clusion should be checked. 



Effects of Oxygen Deficiency. In chapter 13 (p. 326), a brief reference 

 was made to Franck and Pringsheim's phosphoroscopic measurements 

 of photosynthesis under anaerobic conditions. This study has since been 

 published (Franck, Pringsheim and Lad 1945). The method used in it 

 (quenching of trypoflavin phosphorescence by oxygen evolved by illumi- 

 nated cells into a stream of pure nitrogen) is so sensitive that it permits 

 detection of oxygen liberation by a single light flash. Many of the experi- 

 ments described in this paper had been summarized in chapter 33 ("In- 

 duction Phenomena") and 34 ("Photosynthesis in Intermittent Light"). 



f »260 



I. f'350 



1 1 1 1 r- 



3000 6000 9000 12000 15000 18000 21000 24000 27000 



Illumination (lux) 



Fig. 37D.10. Anaerobic light curves of photosynthesis (Franck, Pringsheim and Lad 

 1945). The ratio of normal to anaerobic saturation level is designated/. 



Working in a flow of gas permits measuring the weak residual photo- 

 synthesis going on under steady anaerobic conditions; Franck, Pringsheim 

 and Lad have determined light curves of this residual photosynthesis in 

 Chlorella and Scenedesmus, after varying periods of anaerobic incubation. 

 These curves (fig. 37D.10) appeared similar to the light curves of severely 

 poisoned cells, with saturation levels depressed by a factor of up to 10,000, 

 compared to aerobic conditions. Saturation was reached at consider- 

 ably lower intensity than ordinarily (e. gr., at 7 klux instead of 17 klux). 

 As seen in fig. 37D.10, the shape of the saturation curves sometimes was 

 changed from the usual hyperbolic type to a sigmoid one (commonly found 

 in purple bacteria). When this change occurred — usually after an espe- 

 cially extended anaerobic incubation — the saturating intensity again be- 

 came higher (occasionally, as high as under aerobic conditions). 



Light curves of this general type were obtained with anaerobically 



