PLANT METABOLISM 225 



deaminations o£ a variety of types play natural roles in addition to 

 the reversals of the reactions noted above. Most of the known cases 

 involve microorganisms rather than ordinary plants and animals with 

 one major exception, urea. Urea is a major product of animal 

 metabolism and is undoubtedly widespread elsewhere. It is attacked 

 in solution by a common enzyme according to 



H2NCONH2 + H2O > 2NHa + CO2 



urease 



As oxygen began to appear in the atmosphere during the evolution 

 of the plant, microorganisms appeared which could obtain the energy 

 they needed by the oxidation of ammonium ion. The mechanism of 

 this reaction is not known but in NifyosomoiKis may he written in the 

 overall form 



NH4+ + fO. + H.,0 -^ NO2- + 2H:,0+ 



and is known to occur in soils, especially in neutral or alkaline soils 

 as might be predicted from the appearance of hydrogen ions. Once 

 nitrite ion is formed, another group of bacteria called Nitrobacter 

 complete the oxidation. 



NO2- + i02 -> NOr 



Neither nitrite nor nitrate is especially useful for plants and animals 

 in this form, although plants probably reduce appreciable quantities 

 of nitrate and utilize the products. Both ions are somewhat toxic to 

 animals, and nitrite is especially dangerous even to plants. Under 

 certain abnormal environmental conditions, large enough concentra- 

 tions of potassium nitrate accumulate in oat straw, for example, to 

 be fatal to livestock. Sometimes local concentrations of nitrites in 

 poorly located farm wells have reached levels toxic to children. 



Therefore, if ammonia were converted irreversibly to nitrate, the 

 supply of available nitrogen would tend to dwindle. However, other 

 bacteria called denitrifiers have evolved and reduce nitrate to nitrite 

 and on to molecular nitrogen or to ammonia. These soil organisms 

 must, of course, employ energy from natural reducing substances in 

 the process. The inorganic compounds involved may include free 

 sulfur, sulfide, thiosulfate, hydrogen. Various organic materials may 

 serve, but there is usually some species specificity. Ordinarily the 

 denitrification occurs in the absence or near absence of oxygen, and 

 the nitrate and nitrite ions function as the oxidizing agents. 



This process introduces a new complication, for in spite of scattered 

 reports to the contrary, animals and most plants cannot employ molec- 

 ular nitrogen in any of their metabolic processes. Hence, denitrifica- 



