1914 KINETICS OF PHOTOSYNTHESIS CHAP. 37D 



pretreated cells of both Chlorella and Scenedesmus, in hydrogen as well as 

 in nitrogen — despite the capacity of the second species for "hydrogen 

 adaptation." The weak, but steady, residual photosynthesis was super- 

 imposed, in Scenedesmus, on photoreduction (the latter being protected 

 from "reversion," which it usually suffers in strong light, by the continuous 

 removal of oxygen by the stream of nitrogen) . The temperature coefficient 

 of the saturation rate, as well as the sensitivity to cyanide, were sharply 

 reduced in the anaerobic state; the effect of lowering the carbon dioxide 

 concentration also was much weaker than usual. 



Contrary to the observations of Noack and co-workers (c/. chapter 

 33), anaerobically incubated Scenedesmus exhibited the same inhibition 

 whether incubated at pH 6 or at pH 9. 



One factor determining the effect of anaerobic incubation proved to be 

 the density of the cell suspension. With only 1 X 10^ wet algae per milli- 

 liter, three hours of dark anaerobiosis reduced the rate only by a factor of 

 10 at 20° C, and a factor of 2 at 0° C. — many times less than was observed 

 with denser suspensions. 



It can be concluded from these experiments that anaerobic incubation 

 poisons a photosynthetic enzyme, and that the poison, produced by anaero- 

 bic metabolism, diffuses out of the cells into the medium. (It may be 

 identical with Pratt's "chlorellin," cf. p. 880.) The occurrence of sigmoid- 

 shaped curves can be taken as indication that two different poisons are 

 involved, the second one affecting particularly strongly photosynthesis in 

 low light. 



Of the three enzymes (Ea, Eb, Ec) which Franck recognized as con- 

 tributing to photosynthesis, the "carboxylase," Ea, does not seem to be 

 influenced by anaerobiosis. (The temperature coefficient of Ea, is, accord- 

 ing to Franck — cf. chapter 31, sect. 6 — higher than that of the usually 

 rate-limiting enzyme, Eb,' poisoning of Ea should therefore impose a higher 

 Qw on photosynthesis as a whole. Furthermore, since Ea is the main cy- 

 anide-sensitive component, its poisoning should increase the sensitivity of 

 photosynthesis to cyanide. The observed behavior contradicts both con- 

 clusions.) Inhibition of Eb seems to be excluded by the observations, 

 (p. 1462) that the maximum yield per flash is relatively little affected by 

 anaerobiosis (cf. chapter 32, section 4 for interpretation of this quantity as 

 a titer of Eb) • This leaves the "de-oxygenase," Ec, as the most likely object 

 of anaerobic inactivation ; the theory of the anaerobic inhibition thus be- 

 comes analogous to the Franck-Gaffron theory of the ordinary induction 

 period (cf. chapter 33, section C4). 



As to the additional, selective anaerobic inhibition of oxygen liberation 

 at low light intensities, responsible for the sigmoid-shaped light curves, 

 its cause was seen by Franck, Pringsheim and Lad in the formation of 



