AUTOTROPHIC ASSIMILATION 71 



are abundant in nitrogen-fixing blue-green algae^^- ^^'^' ^^^' ^^ 

 and it appears that through them the nitrogen fixed enters 

 into general metabolism. 



Nitrogen-fixing blue-green algae appear to be of great 

 importance in contributing to the fertility of certain habi- 

 tats, tropical soils in particular and possibly some types of 

 freshwater. ^^' ^^' ^oo \ capacity for nitrogen fixation may 

 be important to certain species of Myxophyceae in enabling 

 them to colonize unpromising substrata. Blue-green algae 

 are often found in symbiotic association with other types of 

 organisms and, in certain of these at least, nitrogen fixation 

 may contribute to the success of the partnership.^^- 1^2 



THE ASSIMILATION OF NITRATE 



The majority of algae are able to utilize nitrogen supplied 

 in the form of nitrate for the synthesis of cell material. 

 Exceptions are to be found among colourless flagellates, in 

 the chemotrophic growth of certain pigmented flagellates, ^^^ 

 and in Euglena gracilis var. bacillaris, which will not grow 

 phototrophically with nitrate as the sole nitrogen source. ^'^^ 



The nitrogen of nitrate must undergo reduction before it 

 can be incorporated into amino-acids, nitrogenous bases 

 and their derivatives. This process of reduction was first 

 investigated by Warburg and Negelein using Chlorella 

 pyrenoidosa.^^^ In order to obtain a reduction of nitrate at a 

 rate comparable to those of photosynthesis and respiration, 

 these workers used an acid medium, of pH about 2-0, from 

 which undissociated nitric acid might be expected to pene- 

 trate rapidly into the cells. Chlorella in this medium in the 

 dark was found to evolve carbon dioxide extra to that pro- 

 duced by respiration under comparable conditions in the 

 absence of nitrate and at the same time to liberate ammonia 

 into the medium. The ratio of the amounts of extra carbon 

 dioxide and ammonia was not constant, evidently because 

 some of the ammonia was utilized by the cells. When the 

 cells used were not nitrogen deficient this ratio was found 

 to approach 2 : i , in agreement with the equation: 



HN03+2(CH20)->NH3+2C02+H20 . (17) 



6 



