THE CHEMOTROPHIC ASSIMILATION OF CARBON 59 



appears to be no conclusive evidence for a stimulatory effect 

 of light upon the growth of algae apart from that due to 

 photosynthesis. However, photochemical reactions other 

 than those concerned in photosynthesis are known to occur 

 in plants, e.g. the formation of chlorophyll in higher plants 

 involves such a process, and it is possible that when a greater 

 range of forms has been examined examples of these will 

 be found among the algae. 



OBLIGATE PHOTOTROPHY 



From the account that has been given above it would be 

 expected that all photosynthetic algae would also be capable 

 of chemo-organotrophic nutrition. However, although 

 many algae are able to grow in the dark if provided with a 

 suitable organic substrate, there are some which have not 

 so far been grown under these conditions and which appear 

 to be obligate phototrophs. A strain of Chlorella vulgaris, ^^ 

 Chlamydomonas spp.,^'^* Prorocentrum micans, Peridinium 

 sp.,^^ Anahaena cylindrica,^^ and perhaps the majority of 

 diatoms, 2*^' ^'^ for example, are apparently of this type. An 

 analogous situation is met with in the chemolithotrophic 1 

 bacterium Thiohacillus thio-oxidans in which sulphur and 

 carbon dioxide are required for growth and can be replaced 

 by no other energy or carbon source respectively.^®^ The 

 occurrence of obligate phototrophism is evidently sporadic 

 as far as systematic position is concerned. 



Most information about obligate phototrophism is avail- J 

 able for Chlamydomonas moewusii}'^^ Some sixty-four | 

 organic materials, including acids, alcohols, sugars, phos- 

 phate compounds (including phosphoglyceric acid), nitro- 

 gen compounds and preparations rich in growth factors, 

 have been tested and found ineffective as substrates for the 

 chemo-organotrophic growth of this flagellate. Cell extracts 

 and hydrolysates of the organism itself and filtrates from 

 light-grown cultures likewise do not support growth in 

 darkness. A most curious feature is that whereas control 

 cultures without substrate grow on being returned to the 

 light, cultures with substrates are found to be dead although 

 the substrate concentrations used are not toxic in light 



