NITROGEN METABOLISM 215 



been some variation of opinion as to the fixation of 

 nitrogen by yeasts. About the beginning of the century 

 it was claimed that species of Torula, Saccharomyces, 

 Oidium and Monilia were able to use gaseous nitrogen 

 when grown on artificial media containing only carbo- 

 hydrates and tap -water. Then followed a period during 

 which fixation of nitrogen was denied, the growth observed 

 in the earlier experiments being ascribed to traces of 

 nitrogenous impurities in the sugars and to ammonia and 

 nitrates in the water. It was shown that very small 

 quantities of nitrogen, of the order of 0-01 per cent., 

 would serve to support growth. Later Fulmer and Nelson 

 showed that if Sacch. cerevisice is grown for a long period 

 on sucrose -phosphate solutions freed from ammonia and 

 oxides of nitrogen, fixation does occur. The gain in 

 nitrogen is only apparent after about six weeks, an actual 

 loss being observed during the early stages, probably 

 due to conversion of some of the nitrogen in the yeast 

 into undetectable compounds. This time-lag may well 

 be the cause of the failure of earlier workers to detect 

 fixation. 



Nitrites and Nitrates. — These substances can be used 

 as nitrogen source by bacteria of the Azotohacter species, 

 ammonia probably being formed as an intermediate. 

 The question is still not settled in the case of the 

 yeasts, but the balance of opinion is that nitrites and 

 nitrates are not utilised, especially under aerobic con- 

 ditions. Their presence in a medium stimulates spore 

 formation. 



Ammonium Salts and Amines. — These substances are 

 usually readily assimilable, ammonium phosphate being 

 a good source of nitrogen for all micro-organisms except 

 Vibrio comyna. Soil bacteria of the N itrosomonas group 

 oxidise ammonia to nitrite. Free ammonia can be used 

 by many organisms, but if it is present in any but very 

 low concentrations it retards growth owing to its toxicity. 

 The same applies to the amines, some of which, especially 



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