THE CHEMOTROPHIC ASSIMILATION OF CARBON 55 



substrates may be assimilated appears to be much less 

 common in algae than in bacteria. Certain algae, Scenedes- 

 mus costulatiis var. chlorelloides^'^^ and Nitzschia putrida^^^ 

 for example, are nevertheless able to liquefy gelatine, a 

 property which must be due to the production of an extra- 

 cellular proteinase (see also ref. 258). 



Since ions penetrate into cells less readily than undis- 

 sociated molecules the reaction of the medium is of great 

 importance in determining whether or not ionizable sub- 

 stances can be assimilated. Thus all but the weakest organic 

 acids exist almost entirely as ions in alkaline solution and 

 are then unable to penetrate into cells, whereas penetration 

 of the same acids may occur readily from acid solutions, in 

 which the undissociated molecules predominate. If this is 

 overlooked then it may appear that an organic acid cannot 

 be utilized for chemotrophic growth. For example, from 

 the results of experiments carried out at pH 5-0 to 5-5 it 

 was concluded that pyruvic acid could not be assimilated 

 by Prototheca zopfii^^ but further work using media adjusted 

 to pH 3-0 to 4-5 showed that this substance can be meta- 

 bolized rapidly by this organism. ^^ Similarly, assimilation 

 of other acids known to be intermediates in metabolism can 

 generally be demonstrated if conditions are adjusted to give 

 a sufficient concentration of the undissociated form in the 

 medium. 1^^ The situation may be further complicated if 

 the concentration of a particular acid necessary to ensure 

 adequate penetration reaches the toxic level. Ability to 

 resist high concentrations of free fatty acids is evidently the 

 important characteristic of acetate organisms. Acetic acid 

 may play just as important a part in the metabolism of other 

 algae as it does in that of acetate organisms, but the high 

 concentrations of the free acid which the latter are able to 

 tolerate are toxic to these other forms. ^^^ 



Another possibility is that an enzyme, necessary for the 

 utilization of a particular substrate and regarded as of 

 general occurrence, may nevertheless be absent from a par- 

 ticular species. The clearest example of this is the inability 

 of certain acetate organisms to utilize glucose and other 

 sugars, which results from their lack of hexokinase, the 

 5 



