72 THE METABOLISM OF ALGAE 



where (CHgO) represents a carbohydrate or similar sub- 

 strate. 



Chlorella placed in the acid nitrate solution in the light 

 was found by Warburg and Negelein to evolve, not extra 

 carbon dioxide, but oxygen extra to that produced by 

 photos3^nthesis in the absence of nitrate but under other- 

 wise comparable conditions. This was explained by sup- 

 posing that the carbon dioxide evolved in reaction 17 is in 

 illuminated cells used for the production of more (CH2O): 



2C02+2H20->2(CH20)+202 . . (18) 



However, the discrepancy was noticed that the amount of 

 extra oxygen produced in the light was greater than the 

 amount of extra carbon dioxide evolved in the dark. 



The possibility must be borne in mind that, under the 

 acid conditions used by Warburg and Negelein, the meta- 

 bolism of Chlorella may have been abnormal. However, 

 subsequent work with less extreme treatment of the alga 

 has confirmed their principal results. It appears that in 

 media of low pH the penetration of nitrate into the cells 

 is not increased but that the liberation of ammonia is 

 favoured. ^^ In media of pH 4-5 no ammonia is produced by 

 Chlorella supplied with nitrate but the gas exchanges taking 

 place are similar to those observed by the earlier workers. 

 For example, the photosynthetic quotient of Chlorella in 

 the presence of nitrate has been found to be 1-47 as com- 

 pared with the value of i-o6 obtained with an ammonium 

 salt as the nitrogen source but under otherwise similar con- 

 ditions.^^ The situation is complicated, however, since the 

 extent of nitrate reduction and consequently the gas ex- 

 changes observed are dependent on the previous history of 

 the cells employed in the experiment and on the conditions 

 to which they are exposed (Table 5). At high light inten- 

 sities there is proportionately less nitrate assimilation and 

 the value of the photosynthetic quotient approaches unity 

 indicating that the chief product of photosynthesis is carbo- 

 hydrate. The same effect is achieved if the cells are pre- 

 viously depleted of carbohydrate by starvation. If, on the 

 other hand, the concentration of carbohydrates in the cells 



