INFLUENCE OF OXYGEN ON PHOTOSYNTHESIS 327 



photosynthesis may be the photochemical dehydrogenation of chlorophyll 

 by oxygen, leading to the formation of a photochemically active "mono- 

 dehydrochlorophyll." (This hypothesis has a certain similarity to the 

 concept of "energy dismutation" described in chapter 9.) 



However, the need for a special explanation of the role of oxygen in 

 photosynthesis, became doubtful when the reality of the phenomenon 

 itself was challenged. Harvey, in 1928, used the extremely sensitive 

 luminous bacteria to show that algae start evolving oxygen within a 

 second after the beginning of illumination, even in a medium deprived of 

 all traces of oxygen; and, more recently, Franck and Pringsheim (un- 

 published) found, by observing the quenching of phosphorescence of 

 adsorbed dyestuffs, that algae produce oxygen by the very first light 

 flash, even after two hours incubation in purest nitrogen.* After the 

 similarity of the photochemical processes in green plants and purple bac- 

 teria was established, Gaffron (1933, 1935) pointed out that many purple 

 bacteria thrive only under strictly anaerobic conditions. This too, 

 indicates that oxygen is not necessary for photosynthesis. 



As to the undoubtedly real anaerobic inhibition phenomena, Gaffron 

 pointed out that the length of the required incubation period points to 

 a slow accumulation of fermentation products rather than to dissociation 

 of a labile oxygen compound (compare the instantaneous dissociation of 

 oxyhemoglobin!). Green plants deprived of oxygen ferment, producing 

 alcohols and organic acids (c/. Chapter 6, page 130); some of these 

 products may well be poisonous to the photosynthetic apparatus. 

 Experiments of Gaffron (1935), of Noack, Pirson, and Michels (1939), 

 and of Michels (1940) showed that anaerobic inhibition is strong only in 

 acid solution, and is less pronounced, or even entirely absent, in alkaline 

 buffers. (For a detailed description of these experiments, see chapter 33 

 in volume II.) Consequently, Noack and coworkers attributed it to free 

 fermentation acids (e. g., lactic acid). 



As mentioned before, different species respond differently to anaerobic 

 treatment. In the higher plants, prolonged anaerobic incubation usually 

 causes, in addition to reversible inhibition, an irreversible injury. The 

 moss Mnium undulatum, on the other hand, was described by Briggs 

 (1933) as capable of withstanding 24 hours of anaerobic incubation 

 without permanent injury (although an induction period of two hours was 

 required afterwards for the resumption of photosynthesis). Equally re- 

 sistant are algae, which can withstand even several days of anaerobiosis. 

 In some algae, as, for instance, Chlorella, the only result of a prolonged 

 anaerobic incubation, is an extension of the induction period of normal 

 photosynthesis. In others — Scenedesmus, Ankistrodesmus, and Raphi- 



* It was too late to include here a detailed discussion of the new experiments of 

 Franck and coworkers on photosynthesis under anaerobic conditions; they will be dis- 

 cussed in volume II. 



