CHAPTER III 



THE CHEMOTROPHIC ASSIMILATION 

 OF CARBON 



Many algae capable of photosynthesis are also able to 

 assimilate substances of high potential chemical energy and 

 certain forms, not possessing photosynthetic pigments, are 

 absolutely dependent upon such substances. These sub- 

 stances evidently do not contribute any chemical groups 

 that are 'essential' in the sense that vitamins are essential 

 in animal nutrition; their function is that of furnishing a 

 readily metabolized source of energy. In considering this 

 chemotrophic mode of nutrition it is again necessar\' to deal 

 v^dth the assimilation of carbon and of energy together. 



CHEMOLITHOTROPHISM IN ALGAE 



When the substance from which it derives its energy is 

 inorganic an organism is said to be chefnolithotrophic or, in 

 the older terminology, chemosytithetic. Such organisms are 

 independent alike of light and of organic substances as 

 energy sources and assimilate carbon dioxide by means of 

 energy derived by oxidation of an inorganic substrate. 



Examples of chemolithotrophy amongst the algae are less 

 abundant than amongst bacteria. The simultaneous absorp- 

 tion of hydrogen and oxygen coupled with the reduction 

 of carbon dioxide by hydrogen-adapted Scenedesmus has 

 already been mentioned on p. 30. The metabolism of 

 Scenedesmus is here similar to that of Bacillus pantotrophiis 

 or B. picnoticus, which utilize the energy released by the 

 'oxyhydrogen' reaction: 



O2+2H2— >2H20 + i37 k-cal. . . (8) 



for the reduction of carbon dioxide to bio-organic sub- 

 stances.2^'. ^e? -pj^g value of the ratio AHg/AOg for Scene- 

 desmus carrying out the oxyhydrogen reaction in the 



48 



