AUTOTROPHIC ASSIMILATION 69 



fixation by these organisms was published."^ In the many 

 subsequent studies in which a capacity for nitrogen fixation 

 has been demonstrated in Myxophyceae the Kjeldahl 

 method has generally been used for the determination of 

 combined nitrogen but nitrogen fixation has also been 

 established for Nostoc muscorum^^ and Calothrix parietina^^^ 

 using the heavy isotope of nitrogen as a tracer. 



There is now satisfactory evidence of a capacity for nitro- 

 gen fixation in some twenty-one species distributed among 

 the following genera of Myxophyceae: Nostoc, Anahaena, 

 Cylindrospermum, Aulosira (for references see 89), Calo- 

 thrix,'^^''^ ^^^ Tolypothrix, Anabaeniopsis,^^'^ Mastigocladus.^^ 

 Further research will no doubt reveal many more nitrogen- 

 fixing species in this class but it is certain that not all blue- 

 green algae possess the property. Evidence that species 

 belonging to the Chroococcales and to the Oscillatoriaceae 

 are nitrogen-fixing is unsatisfactory^^ and certain forms, 

 e.g. Phormidium spp.,^®' ^^^ Gloeocapsa spp., Microcystis 

 (Diplocystis) aeruginosa and Aphanizomenon flos-aquae,^^ 

 have been found to be incapable of growth in media free 

 from combined nitrogen. 



Work with pure cultures of members of the Chloro- 

 phyceae has generally shown that these algae cannot fix 

 nitrogen. A report that a number of species of unicellular 

 green algae, including a Chlorella, possess the property^®^ 

 has been shown to be based on results obtained with an 

 unsuitable analytical technique.** Nevertheless, the sugges- 

 tion that Chlorella may, under certain circumstances, fix 

 elementary nitrogen has been revived as a result of an 

 observation that old cultures of this alga with a high lipide 

 content contained substantially more combined nitrogen as 

 determined by the absolute method of Dumas than that 

 originally supplied.^^* So far this finding has not been 

 substantiated.^^^ 



While there is no evidence to suggest that algae belong- 

 ing to classes other than the Myxophyceae and Chlorophy- 

 ceae include species capable of nitrogen fixation it is well 

 to remember that very few forms have as yet been investi- 

 gated critically from this point of view. 



