THE CHEMOTROPHIC ASSIMILATION OF CARBON 57 



5 per cent of carbon dioxide and with an ammonium salt 

 as the nitrogen source it attains excellent growth upon this 

 substrate after a lag phase. ^^^ Certain substances appear 

 sometimes to be utilized more readily in the dark than in 

 the light. The strain of Chlorella vulgaris mentioned above, 

 for example, grows better in the dark than in the light when 

 provided with lactose, cellobiose or methyl-j5-D-glucoside, 

 as substrate. These three compounds are all /5-glucosides 

 and it appears that adaptive enzymes are formed for their 

 utilization and that the rate of adaptation is slower in the 

 light when the photosynthetic products are available as an 

 alternative source of carbon."^^' 



THE RELATIONSHIP BETWEEN PHOTOSYNTHESIS AND 

 CHEMOTROPHIC ASSIMILATION 



In an organism which is capable of both, phototrophic 

 and chemotrophic assimilations proceed at rates which are 

 mutually dependent. For Scenedesmus costulatus var. Morel- 

 hides it has been found that at high light intensities a 

 maximum rate of growth is attained which cannot be in- 

 creased by the addition of glucose.-^^ With reduced light 

 intensity the alga absorbs glucose to supply the deficiency 

 due to retarded photosynthesis but only sufficiently to bring 

 the growth rate up to the maximum. At still lower light 

 intensities the growth rate falls, approaching the value 

 attained in complete darkness on glucose (see Table 4). 

 The same relationships have been demonstrated in Chlorella 

 pyrenoidosar^^ There are many reports of the growth of 

 algae in light being accelerated by the addition of organic 

 substrates to the medium, but in none of these instances 

 is there satisfactory evidence that the provision of carbon 

 dioxide was sufficient to maintain maximum rates of photo- 

 synthesis. No case has yet been found in which an organic 

 carbon source accelerated growth under conditions of light- 

 and carbon dioxide-saturation of photosynthesis. ^^^ It thus 

 appears that the photosynthetic mechanism is generally 

 capable of saturating with its products the synthetic systems 

 involved in growth so that under optimum conditions for 



