THE CHEMOTROPHIC ASSIMILATION OF CARBON 51 



Hexose sugars and acetic acid are the substrates most 

 generally utilized by algae, but individual species differ 

 considerably in the substances which support growth best. 

 For example, fructose supports vigorous growth in the dark 

 of Cystococcus (Trebouxia) sp. but allows only poor growth 

 under the same conditions of Scenedesmus costulatus var. 

 chlorelloides?^^^ 251 'pj^g range of substrates available to 

 Nostoc punctiforme is very similar to that for members of 

 the Chlorophyceae except that this organism is able to 

 assimilate polysaccharides such as starch and inulin whereas 

 most green algae do not appear able to do this.^^^ Only 

 glucose among a large number of substances tested suffices 

 to support growth in the dark of Navicula pelliculosa.^'^^ 

 Flagellates belonging to the Euglenineae, Cryptophyceae 

 and Volvocales grow best when provided with acetate, 

 whereas they grow poorly or not at all upon sugars, and 

 are hence often known as 'acetate organisms'.^^^' ^^^ Such 

 organisms are abundant in situations in which the decom- 

 position of organic matter liberates fatty acids and alcohols 

 in relatively high concentrations, e.g. in water contaminated 

 with sewage. An extreme example of this type is afforded 

 by Chlorogonium sp., which cannot utilize any organic 

 energy source other than acetic acid.^^* Other acetate 

 organisms, however, are able to make use of a wider variety 

 of substrates. Euglena gracilis can grow in the dark if pro- 

 vided with a salt of a lower fatty acid such as acetic or 

 butyric. Other types of organic acid, tartaric, lactic, pyruvic, 

 succinic and phosphoglyceric for example, are unsuitable. 

 Among the lower fatty acids only those with an even number 

 of carbon atoms and less than seven carbon atoms in all 

 give good growth.18^ It is to be noted that not all acetate 

 organisms conform to this pattern. For example, other 

 strains of Euglena gracilis are able to utilize succinic acid.^^^ 

 The rule regarding the utilization of fatty acids is not of 

 general application among algae since Prototheca zopfii 

 utilizes fatty acids with even and odd numbers of carbon 

 atoms with equal readiness.^^ 



Determinations of the final populations attained in cul- 

 tures supplied with organic substances cannot give much 



