NITRATE REDUCTION 451 



however, found values between 2/0.3 and 2/0.5, which means a much 

 lower production of ammonia than was to be expected. This was 

 assumed to depend upon an assimilation of the reduced nitrogen, which 

 thus could not be recovered quantitatively as ammonia. Davis, on the 

 contrary, obtained the stoichiometric ratio 2/1 between extra oxygen and 

 added nitrate, which could verify the net Eq. (8-5c) if it is assumed that 

 nitrate is quantitatively consumed during the time of the experiment. 

 Just as in the dark, this does not prove the validity of the formula so 

 far as a reduction to ammonia is concerned ; it only shows that the combi- 

 nation of reduction and assimilation of nitrate in the light leads to for- 

 mation of cell matter of the same average elementary composition as in 

 the dark. Myers and Johnston are the only investigators who have 

 determined the amount of consumed nitrate. It can be computed from 

 their figures (1949, Table 3) that at low light intensity the ratio between 

 extra oxygen produced and assimilated nitrate amounts to 2.9-3.1, but 

 that at high intensity values of 2.0-2.1 are obtained. These latter are 

 very near the assumed theoretical value, but Myers and Johnston logi- 

 cally explain the difference as a change in the over-all composition of the 

 cellular matter formed. 



In other experiments Myers and his collaborators obtained respiratory 

 quotients in light of 0.80 and lower, and Pirson and Wilhelmi obtained 

 around 0.70, so that the production of extra oxygen is estabhshed; in 

 these instances neither the consumption of nitrate nor the nature and 

 amount of reduced products formed were determined, so that the nitrate 

 reduction had to be inferred from the shifts in the gas exchange. It is 

 thus apparent that the situation is just the same in regard to the con- 

 ditions in light as in darkness; there are changes in the gas evolution in 

 the expected direction, but their quantitative relation to the nitrate 

 reduction is open to discussion. 



There seems to be one discrepancy in these results with Chlorella, how- 

 ever. Warburg and Negelein, using a carbon dioxide-free atmosphere, 

 obtained extra carbon dioxide production in the dark and therefore 

 postulated a correspondingly higher rate of nitrate reduction. Myers 

 states that "in the absence of CO2, however, the gas exchange is negli- 

 gible and there is no evidence of nitrate reduction." Davis has briefly 

 recorded that Chlorella in a carbon dioxide-free medium produces extra 

 oxygen and hence assimilates nitrate only in the presence of externally 

 added glucose. The same holds true for the reduction in darkness, 

 according to Myers. The contradictions might be due to different pre- 

 treatment and conditions of the plants and could be explained if War- 

 burg's algae were richer in carbohydrates than the others. Such a possi- 

 bility, however, seems to be inconsistent with the fact that Warburg's 

 algae were enormously overfed on nitrate, having been supplied with 

 0.1 A^ nitrate in 0.01 A^ nitrous acid. This may in itself have caused 



