AUTOTROPHIC ASSIMILATION 75 



tion of these facts is that nitrate must be reduced before its 

 nitrogen can be utiHzed in cell synthesis whereas the nitro- 

 gen in ammonia, being already at the same level of reduc- 

 tion as that in bio-organic compounds, can participate 

 immediately in metabolism. The fact that an organism able 

 to utilize nitrate must possess some mechanism extra to 

 that required for ammonia assimilation is recognized in the 

 distinction drawn between autotrophs sensu stricto, which 

 are able to reduce oxidized inorganic nutrients, and meso- 

 trophs, which require one or more reduced inorganic 

 nutrients such as ammonia (see p. 16). 



Secondary effects of ammonia absorption make it a less 

 favourable source of nitrogen than nitrate. Ammonium 

 nitrogen enters living cells most readily as undissociated 

 ammonium hydroxide, with the result that the medium 

 surrounding the cells becomes more acid and may eventu- 

 ally reach a pH sufficiently low to inhibit growth.^^s While 

 absorption of nitrate causes a medium to become more 

 alkaline, the change is relatively less and this form of nitro- 

 gen is generally used in preference to ammonium salts in 

 making up media for algae. Only if pH changes are reduced 

 by buffering or by some other means, is growth with 

 ammonium salts as good as that with nitrate. Ammonium 

 nitrogen, even in slight excess, may have a profound influ- 

 ence on the course of metabolism and can exert effects, 

 which may or may not be deleterious, apart from those 

 attendant upon changes in hydrogen ion concentration. 

 Even concentrations of the order of 0-5x10-4 M of an 

 ammonium salt suppress the formation of heterocysts by 

 Anahaena cylindrical^ and although growth of Prorocentrum 

 spp. and Peridinium spp. is possible in the presence of 

 0-2x10-5 M ammonium chloride, it is inhibited by a con- 

 centration of 0-2 X iQ-^ M.^^ ' 



As with nitrate, the uptake of ammonium nitrogen by 

 algal cells depends on the previous history of the cells and 

 on the conditions to which they are exposed.^^s The addi- 

 tion of an ammonium salt to a suspension of nitrogen- 

 starved Chlorella vulgaris is followed immediately by a rapid 

 assimilation of ammonia which continues until either the 



