THE CHEMOTROPHIC ASSIMILATION OF CARBON 63 



but in which scarcely any indication of affinity is to be 

 found. 2*^ Among the morphologically more complex algae 

 certain parasitic species of Rhodophyceae, e.g. Harveyella, 

 show complete absence of photosynthetic pigments. ^°^ 



There is no evidence of any significant alteration in 

 gro\\th or dark metabolism occurring in apochlorosis. The 

 requirements for chemotrophic nutrition and the storage 

 products are the same in corresponding colourless and pig- 

 mented forms. 240, 242 



OXIDATIVE ASSIMILATION 



The organic substrate used in chemo-organotrophic 

 growth must serve as a source both of energy and of carbon 

 and therefore is partly oxidized in respiration and partly 

 built up into cell material. The mechanism of such oxidative 

 assimilation of organic substances must now be considered. 



Oxidative assimilation was first studied in ProtothecaP^ ^3 

 Under aerobic conditions the supply of glucose to this 

 organism stimulates respiration, but the amount of oxygen 

 consumed for a given amount of glucose is sufficient to 

 account for complete oxidation to carbon dioxide and water 

 of only part of the glucose, the rest, about 30 per cent, 

 apparently being converted to cell material. Other sub- 

 stances, including glycerol, ethyl alcohol, acetic acid and 

 propionic acid, can be partly oxidized and partly assimilated 

 by Prototheca in a similar manner. Under anaerobic con- 

 ditions glucose cannot be used for building cell material 

 but is fermented, yielding lactic acid in almost exactly the 

 amount required by the equation: 



CeHi206->2CH3.CH(OH).COOH . . (12) 



Oxidative assimilation has also been investigated in Chlor- 

 ella pyrenoidosa,^^^ C. vulgaris^"^ and Scmedesmus quadri- 

 cauda.^^^ 



In investigations with Prototheca the amounts of sub- 

 strate decomposed and oxygen consumed have been found 

 to be in agreement with the hypothesis that a carbohydrate 

 is the principal product of oxidative assimilation, being 

 formed according to a simple over-all equation such as: 



CH3.COOH+02->(CH20) + C02+H20 . (13) 



