NITROGEN METABOLISM 221 



hydrogen evolved during the breakdown of ghicose. 



Besides such anaerobic processes of nitrogen fixation, 

 aerobic organisms are known which effect the same 

 reaction. Beijerinck isolated organisms from soil and 

 canal water which, when grown in a nitrogen-free medium 

 containing an adequate carbon source, actively fixed 

 nitrogen. The chief organism responsible is Azotobacter 

 chroococcum, It is usually accompanied in nature by 

 an organism, Alcaligenes radiobacter, which lives in 

 symbiosis with it, but is itseK not capable of fixing nitrogen. 

 A. chroococcum ferments glucose to give mainly carbon 

 dioxide together with lactic, acetic and formic acids and 

 some alcohol. Stoklasa showed that when A. chroococcum 

 is grown anaerobically on a medium containing nitrate it 

 reduces the latter to nitrite and ammonia, but gives only 

 feeble growi^h compared with that under aerobic condi- 

 tions ; a certain amount of nitrogen fixation also occurs 

 anaerobically. When grown aerobically on nitrate media 

 good gro\\i;h occurs, the nitrate is reduced mainly to 

 nitrite, very little ammonia being found since it is probably 

 used up in the synthetic processes accompanying the in- 

 creased growth ; fixation of nitrogen, in this case, occurs 

 only to a very limited extent. Ale. radiobacter seems to 

 gro'w equally well aerobically or anaerobically and is in 

 each case an active denitrifier, rapidly converting the 

 nitrate to free nitrogen, which is lost from the system. 

 When A. chroococcum and Ale. radiobacter are grown in 

 symbiosis on nitrate media the nitrogen set free by the 

 latter is fixed by the former and converted into cell 

 constituents. When low concentrations of nitrate are 

 present considerable amounts of atmospheric nitrogen are 

 fixed, but this does not occur when high concentrations of 

 nitrate are present in the medium. 



It has been shown by other workers that when Azoto- 

 bacter is grown on synthetic media containing glucose or 

 mannitol as the carbon source, the organism fixes nitrogen 

 in four to six daj^s, and that ammonia and amino -nitrogen 



