INORGANIC SOURCES OF NITROGEN 249 



Ammonia in high concentrations is toxic (332) and may even find 

 limited use as a fungicide (218, 388). Abnormalities in growth (458), 

 spore germination (169), and development (108) have been traced to 

 ammonia toxicity; especially in media with large amounts of organic 

 nitrogen, ammonia may accumulate to an inhibitory level and con- 

 tribute to the so-called staling effect (421). The toxicity of high am- 

 monium ion concentration to Allomyces javanicus var. macrogynus is 

 alleviated by phosphate (340). 



Nitrogen Fixation. From the earliest days of microbiology, claims 

 have been made that fungi are able, like certain bacteria, to convert 

 gaseous nitrogen to combined nitrogen. These studies have been re- 

 viewed several times in the last generation (18, 137, 163, 202, 478). 

 The evidence is overwhelmingly against the occurrence of nitrogen 

 fixation in saprophytic fungi. Results with mycorrhizal fungi, par- 

 ticularly Phoma spp., have been more often positive, but isotope re- 

 sults fail to corroborate earlier claims (601). There is, therefore, at 

 present no good evidence for fixation of nitrogen by the fungi. Nitro- 

 gen fixation occurs in root nodules of Alnns glutinosa (426), but it is 

 not yet known what microorganism is involved; the causal agent has 

 been variously assigned to the bacteria, the actinomycetes, the fila- 

 mentous fungi, and, most recently, the Plasmodiophorales (228). Un- 

 til the organism can be cultivated, both its identity and its role in 

 nitrogen fixation will remain uncertain. 



Oxidation of Inorganic Nitrogen Compounds. Heterotrophic oxi- 

 dation of inorganic nitrogen compounds is effected by a few bacteria 

 (126); this is to be distinguished from the better known autotrophic 

 oxidation of ammonium and nitrite. Among the true fungi, Asper- 

 gillus aureus and A. batatae are reported to form nitrate from nitrite 

 during growth (457). Aspergillus flavus growing on a glucose-peptone- 

 yeast extract medium produces small amounts of nitrite and rather 

 larger amounts of nitrate (468). This finding, if confirmed, has im- 

 portance to soil microbiology. Color tests suggest that A. niger oxi- 

 dizes ammonia to hydroxylamine (494). 



Among the actinomycetes, Nocardia corallina oxidizes pyruvic oxime 

 and hydroxylamine to nitrite (277, 321). An unusual species of 

 Streptomyces, S. nitrificans, has been studied by Schatz and coworkers 

 (257, 258, 464, 465). This organism utilizes urethan as the sole source 

 of both carbon and nitrogen, and forms nitrite during growth on car- 

 bamates and during metabolism of urea or ammonium carbonate. 

 Activity of cell-free preparations suggests a pyridine nucleotide-linked 

 ammonium dehydrogenase. Nitrate is not formed. 



