CHLOROPHYLL-SENSITIZED REDUCTION OF NITRATE 539 



e. g. amino acids and proteins. The reductants in this reaction are 

 organic cell constituents, e. g., carbohydrates. The process is similar to 

 respiration, with nitrate substituted for oxygen: 



(19.1) (HNO,)aq. + 2 {CHjOl > 2 COj + (NH,)aq. + HjO + 126 kcal 



Warburg and Negelein (1920) found that, in green algae (e. g., 

 Chlorella pyrenoidosa) , the reduction of nitrate is accelerated by illumi- 

 nation. These experiments were carried out in a mixture of nitric acid 

 (0.01 mole per liter) and nitrate (0.1 mole per liter) to obtain a con- 

 siderable concentration of neutral molecules HNO3, which penetrate 

 through the cell membranes much more easily than do the nitrate ions. 



Brought into such a solution, Chlorella cells produced in the dark, 

 70% more carbon dioxide than they consumed oxygen, thus indicating 

 the superposition of "nitrate respiration" (19.1) on the ordinary, or 

 "oxygen respiration" of carbohydrates. The evolution of ammonia was, 

 at first, less than equivalent to that of carbon dioxide, but approached 

 equivalency after several hours, indicating an initial amination of cellular 

 materials. (Cells starved of nitrogen develop no free ammonia at all in 

 the first few hours of nitrate assimilation.) The nitrate reduction by 

 Chlorella in the dark was exceptionally sensitive to cyanide — for example, 

 a 20% reduction in rate was brought about by less than 10~^ mole per 

 liter of HCN (as against 10~^ for normal photosynthesis and 10"^ for 

 respiration of the same algae). On the other hand, nitrate reduction 

 was less sensitive than photosynthesis to urethan poisoning — e. g., 0.013% 

 (= 0.8 X 10~^ mole per liter) phenylurethan reduced photosynthesis 

 almost to zero, but decreased nitrate reduction by not more than 30%. 

 The nitrate reduction required the presence of oxygen; under anaerobic 

 conditions, nitrite appeared as a reduction product (in addition to am- 

 monia) and acted as a poison, destroying chlorophyll and killing the 

 cells. The nitrite production was not inhibited by cyanide. 



Light affected the nitrate reduction by Chlorella in two ways: in the 

 first place, the evolution of carbon dioxide was gradually replaced, with 

 increasing light intensity, by an evolution of oxygen; and in the second 

 place, the total production of gas was increased by as much as a factor of 

 five or ten. The production of an equivalent quantity of oxygen, instead 

 of carbon dioxide, could be explained by the assumption that reaction 

 (19.1) proceeds in light in the same way as in the dark, but that carbon 

 dioxide, formed in this reaction, is consumed by photosynthesis, and thus 

 converted into oxygen; but this hypothesis could not explain why much 

 more oxygen is produced in light than carbon dioxide in the dark. This 

 relationship can be interpreted in two ways. 



One way is to assume that reaction (19.1) is accelerated by light, thus 

 producing more carbon dioxide, which is available for conversion into 



