74 THE METABOLISM OF ALGAE 



reaction. Since nitrate reduction can also occur in the dark 

 in Chlorella it is necessary, if it is accepted that photo- 

 chemical reduction occurs, to suppose that there are two 

 distinct mechanisms for the reduction of nitrate. Confirma- 

 tion that some organisms are able to utilize nitrate under 

 phototrophic conditions but not when growing chemo- 

 trophically would give strong support to this idea. 



Reduction of nitrate to ammonia takes place in plants 

 in several stages. Under certain conditions nitrite is liber- 

 ated instead of ammonia when Chlorella^^^ or Ankistrodes- 

 mus^^^ reduce nitrate in an acid medium. Both nitrite and 

 hydroxylamine appear to be intermediates in the reduction, 

 the sequence being: 



NO3-— >N02-— >NH3(OH)+— >NH4+ . (20) 



It may be noted here that nitrite can serve as a source of 

 nitrogen for some algae, although it is toxic except in low 

 concentrations, ^^^' ^^° and that, in low concentrations, 

 hydroxylamine is available as a nitrogen source for certain 

 organisms and may, indeed, be the form in which the 

 nitrogen of nitrate enters into the general metabolism of 

 the organism. 2^^' ^^^ 



Nitrate assimilation by green algae is inhibited by low 

 concentrations of cyanide, an indication that heavy metals 

 are concerned in the process. ^^^' ^^^' ^^® As in higher plants, 

 manganese appears to play a specific part in the reduction 

 of nitrate by algae^^" and traces of molybdenum have been 

 found to be essential for the achievement of maximum 

 rates of nitrate assimilation by Anabaena.^^^ 



THE ASSIMILATION OF AMMONIUM NITROGEN 



A mmonium salts can be utilized as sole source of nitrogen 

 by most algae and are generally more readily assimilated 

 than is nitrate. If the two forms are supplied together, the 

 ammonium nitrogen is utilized preferentially and the nitrate 

 is onlv consumed when the ammonia is exhausted.^"*' ^^°' ^^^ 

 If Chlorella growing in a medium containing nitrate is 

 supplied with an ammonium salt the rate of gas exchange 

 alters immediately, showing no adaptation to this latter 

 source of nitrogen to be necessary. ^^ The obvious explana- 



