THE CHEMOTROPHIC ASSIMILATION OF CARBON 6l 



substrates can be used as a supplementary source of energy 

 for growth, the capacity for photosynthesis of the mutant 

 being then sufficient for an essential step that in obligate 

 phototrophs is specifically associated with the photochemical 

 reaction but which in facultative chemo-autotrophs can 

 be by-passed by dark reactions. 



It may be noted that Euglena gracilis, although a faculta- 

 tive chemo-organotroph, responds sluggishly to added 

 organic substrates in the dark after having been cultured 

 phototrophically.^^ Possibly this condition represents a 

 stage in the development of obligate phototrophy. 



OBLIGATE CHEMO-ORGANOTROPHY 



The strains of Chlorella generally used in experimental 

 work are facultative chemo-organotrophs which during 

 growth in darkness produce pigments that are qualitatively 

 the same as those produced in the light and which remain 

 capable of beginning photosynthesis without any markedly 

 prolonged induction period on exposure to light.^**' ^^^ In 

 these cases the photosynthetic mechanism is stable. Chlor- 

 ella variegata, however, if cultured in the presence of 

 organic substrates in the dark becomes yellow and only 

 slowly recovers its capacity for phototrophic growth when 

 returned to the light. Some cells may become white and 

 in them the capacity for producing chlorophyll is irrevoc- 

 ably lost.2' Similarly, Euglena gracilis, if cultured in the 

 dark loses chlorophyll and on return to the light slowly 

 becomes photosynthetic once more.^^^ With certain strains 

 of this species completely colourless individuals arise during 

 culture in the dark and these do not regain their green 

 colour in the light and are thus permanently chemo- 

 organotrophic.^*^ 



The loss of photosynthetic pigments which may occur 

 when algae are cultured in the dark is reversible so long as 

 leucoplasts, i.e. the decolorized chromatophores, are re- 

 tained by the cells. Irreversible loss of pigments, or apo~ 

 chlorosis, takes place when a cell containing no leucoplasts 

 is produced. Production of such cells does not depend on 

 the fortuitous exclusion of leucoplasts from one of the 



