6o THE METABOLISM OF ALGAE 



cultures. ^^^ No toxic substance inhibitory to the organism 

 when growing in the light has been demonstrated as being 

 produced in the dark.^"^* In the absence of carbon dioxide 

 no growth has been observed on any substrates either in 

 light or darkness; this seems to show that the obligate 

 phototrophy does not in this case depend on a photo- 

 chemical reaction other than that of photosynthesis. i"^* It 

 has already been noted (p. 49) that a chemolithotrophic 

 mechanism cannot replace photosynthesis in this species. 



It does not seem that substances that would otherwise be 

 suitable substrates for growth are unable to penetrate into 

 the cells of C. moewusii since acetate, pyruvate and succinate 

 are readily oxidized by the organism in the dark.^^* Thus 

 there is here no correlation between the value of a substance 

 as a substrate for respiration and its ability to support 

 growth such as has been noted for Prototheca and Chlorella 

 (p. 54). It appears, then, that energy released by oxidation 

 of organic substrates is not available for at least one syn- 

 thetic mechanism essential for the growth of C. moewusii. 

 The behaviour of an ultra-violet induced mutant of C. 

 moewusii, in which the photosynthetic apparatus is im- 

 paired,^ ^^ is interesting in this connexion. This mutant is 

 incapable of growth in purely inorganic media unless a high 

 concentration of carbon dioxide is maintained, e.g. by 

 aeration with 5 per cent carbon dioxide in air, but it will 

 grow in the light at low carbon dioxide tensions in media 

 supplemented with substances such as citrate, fumarate, 

 succinate, pyruvate, malate, glucose or glycerol, at rates 

 approaching those attained by the wild type. The mutant 

 is able to carry out the Hill reaction^^' and it may be that 

 the part of the photosynthetic mechanism which is impaired 

 is that responsible for maintaining the concentration of the 

 particular form of carbon dioxide which enters into the 

 fixation cycle, e.g. the enzyme concerned might be carbonic 

 anhydrase. If this is so, then the supplementary carbon 

 sources utilized by this mutant may act merely by being 

 oxidized to give carbon dioxide in a form in which it can 

 be fixed photosynthetically without the intervention of this 

 system. Another possibility is that oxidation of exogenous 



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