yo THE METABOLISM OF ALGAE 



It is evident that in its physiology nitrogen fixation by 

 blue-green algae resembles that in other organisms. Thus 

 nitrogen fixation can only occur concurrently with growth 

 and is suppressed in the presence of readily available com- 

 bined nitrogen such as an ammonium salt.^' Nitrate in- 

 hibits nitrogen fixation in Anahaena cylindrica only if the 

 alga is adapted to utilize this form of combined nitrogen. ^^ 

 Traces of molybdenum are essential for the achievement 

 of maximum rates of fixation in blue-green algae as in other 

 nitrogen-fixing organisms. '*^' ^^' ^^^ In Nostoc niuscorum assi- 

 milation of free nitrogen does not occur in media of pH 

 below approximately 5-7.^^ Nitrogen fixation in blue-green 

 algae may occur either during phototrophic growth or in the 

 dark if a suitable carbon source is provided. ^^' ^® In Nostoc 

 muscorum growing in the dark, 10 to 12 mg. of nitrogen 

 have been found to be fixed per gm. of glucose utilized.^^ 



The properties of the nitrogen-fixing enzyme system in 

 Nostoc muscorum appear to be very similar to those of 

 Azotohacter and of the RhizobiumAegnme system. ^^ The 

 half maximum rate of fixation occurs at a partial pressure 

 of nitrogen of about 0-02 atmosphere, a value of the same 

 order as those for Azotohacter (0-02 atmosphere) and Tri- 

 folium pratense (0-05 atmosphere). Like those of other 

 nitrogen-fixing organisms the enzyme system of Nostoc is 

 specifically inhibited by hydrogen and by carbon monoxide. 

 In bacteria there appears to be some correlation between 

 the presence of hydrogenase and the capacity to fix nitro- 

 gen^'^ but this correlation does not hold in the algae, for 

 Nostoc muscorum cannot be adapted to hydrogen (Table 3) 

 and those algae possessing a hydrogenase system seem to 

 be incapable of nitrogen fixation.^^^ 



The mechanism by which nitrogen fixation is accom- 

 plished in living organisms is still unknown, but it is gener- 

 ally agreed that ammonia is the first recognizable product 

 although in some organisms in certain circumstances this 

 may be replaced by hydroxylamine.^^^ Nitrogen fixed in 

 experiments of short duration accumulates principally in 

 glutamic and aspartic acids in Nostoc muscorum, as it does in 

 other nitrogen-fixing organisms. ^°^« These two amino-acids 



