90 THE METABOLISM OF ALGAE 



compounds of the Chlorophyceae has not been determined 

 but Anabaena cylindrica has been found to Uberate extra- 

 cellular pentose in amounts of up to 1-4 per cent of its dry 

 weight as well as nitrogenous substances which may repre- 

 sent a considerably greater proportion of the total carbon 

 fixed by the alga.^^ It is to be noted that the dissolved 

 organic matter to be found both in freshwater and seawater 

 consists principally of substances of a nitrogenous nature 

 and of pentosans. ^•'^ Organic acids, alcohols and other 

 extracellular products commonly found in cultures of bac- 

 teria and fungi have never been found in appreciable quan- 

 tities in filtrates from algal cultures. 



Extracellular nitrogenous products in cultures of nitrogen 

 fixing algae have been noted frequently^^' ®^' ®'^' ^^' ^*^and it 

 appears that the liberation of such substances may be char- 

 acteristic of most members of the Myxophyceae, whether 

 they fix nitrogen or not.^^ For Anabaena cylindrica the total 

 amount of extracellular substances is relatively greatest in 

 young cultures, in which they may account for over 50 per 

 cent of the nitrogen assimilated, and the substances con- 

 cerned are evidently not produced by autolysis of dead cells. 

 The liberation of extracellular combined nitrogen appears 

 invariably to accompany growth although its relative extent 

 varies according to culture conditions. The substances 

 concerned are principally polypeptides but in young cul- 

 tures a considerable proportion of the nitrogen may be 

 present as amide. ^'^ It is not known whether similar sub- 

 stances are liberated by algae belonging to other groups 

 but such excretion easily escapes notice in the usual culture 

 media made up with ample amounts of nitrate or ammonium 

 salts. 



It may be agreed that 'conservation of carbon and a mini- 

 mal level of excretion is probably generally characteristic 

 of the green algae' and that in addition to their role in 

 raising the energy level of carbon 'they also provide the 

 mechanism of carbon accumulation and can ill afford waste- 

 ful luxuries of excretion'. ^^^ This generalization, however, 

 does not necessarily apply to species belonging to other 

 groups of algae and even the low levels of excretion reported 



