EFFECT OF OXYGEN CONCENTRATION 1915 



another "narcotic" poison, covering chlorophyll as well as the catalyst Eb; 

 this poison is burned up by the "photoperoxides" formed in light, and this 

 causes the yield to rise faster than linearly with increasing light intensity 

 (cf. chapter 33, p. 1421, for discussion of this concept). 



Experiments of this type support the conclusion — already enounced in 

 chapter 13 — that the inhibition of photosynthesis by anaerobic treatment 

 is the consequence of slow metabolic formation of a poison (or poisons), 

 and not of the necessity of oxygen for the functioning of the photosynthetic 

 apparatus. The latter view was, nevertheless, revived in the "new theory" 

 of photosynthesis of Warburg and Burk (to be described in section 4 of this 

 chapter). According to the latter, the photosynthetic cycle includes a 

 special kind of autoxidation, with a rate ten or more times that of normal 

 respiration. If this is the case, photosynthesis could not get under way 

 in complete absence of oxygen. 



Warburg and Burk's revival of the adage "no photosynthesis without 

 oxygen," has caused Allen (1954; cf. Frank 1953) to repeat earlier experi- 

 ments in Franck's and Gaffron's laboratory, in which algae showed con- 

 tinued photosynthesis under extreme anaerobic conditions (cf. chapter 33, 

 section A6) . He found that the rate of photosynthesis of Scenedesmus in a 

 nitrogen atmosphere containing < 1 X 10^^ part of oxygen can be as high 

 as in air, after two or three hours of anaerobic incubation. Even in the 

 much more sensitive Chlorella, the rate remains unaffected by the absence 

 of oxygen for 10-15 minutes, but inhibition sets in gradually afterwards. 



The same study contains also extensive measurements of the oxygen produced 

 under anaerobic conditions, by single, differently spaced light flashes, by photosynthesis 

 and by quinone reduction. These results were not published in time for detailed discus- 

 sion in chapter 34, where they belong. 



Brilliant (1940) observed that the rate of photosynthesis of Elodea in- 

 creased with oxygen concentration up to 0.2%, and remained constant 

 between 0.2 and 0.6%. 



Isotopic Discrimination. The relation between photosynthesis and iso- 

 topic composition of oxygen in the air, water, and carbon dioxide was men- 

 tioned in two places in Volume I. In chapter 1, the question was raised 

 whether the isotopic composition of atmospheric oxygen can be understood 

 in terms of its probable photosynthetic origin ; a hypothesis was suggested 

 there according to which the relative prevalence of the heavier isotope 

 0(18), in the air (compared to its content in oceanic water) is characteristic 

 of a photostationary state, resulting from indiscriminate photochemical 

 liberation of 0(16) and 0(18), and preferential fixation of 0(16) in chemical 

 oxidation (e. g., by respiration). 



This hypothesis presumed that all oxygen produced by photosynthesis 

 originated in water — an assumption which is in agreement with the inter- 



