312 INORGANIC NUTRITION AND METABOLISM 



ment of fungi is specific. Calcium in many organisms, including fungi 

 (117), protects against the injurious effects of certain common mono- 

 valent cations, especially hydrogen, sodium, and potassium. In bac- 

 teria, calcium and related ions also protect against zinc toxicity (127). 

 These non-nutritional effects, the wide range of optima reported, the 

 extent of replacement by strontium, and the fact that some fungi do 

 not respond at all to calcium all argue for some caution in interpreta- 

 tion of the data as reflecting an absolute and specific requirement. It 

 is noticeable that in a casamino acids medium, with high sodium con- 

 centration, calcium affects the growth of Aspergillus oryzae (222), but 

 the related A. niger in the usual low-sodium media does not respond 

 to calcium (239). The crucial experiment would be, of course, to de- 

 termine whether an apparent calcium requirement can be eliminated 

 by adjusting the concentration of other ions. 



It has been found that calcium affects the time required for perithe- 

 cial formation in Chaetomium spp. (14, 15) and the intensity of conidi- 

 ation in Trichoderma viride (27). 



Calcium, like magnesium, may be made less available if chelating 

 agents, e.g., citrate, are present in the medium, and is less available, 

 because of precipitation, at high pH than at low. 



11. COBALT 



In spite of early claims of the essentiality of cobalt for fungi, re- 

 viewed by Marston (138), there is at present no conclusive evidence of 

 a cobalt requirement; some more recent data are suggestive, however 

 (11). Cobalt is required by animals but not, apparently, by higher 

 plants (138). As mentioned elsewhere (Chapter 10), vitamin B 12 is 

 made by Streptomyces spp. — and for the synthesis cobalt must be re- 

 quired — but whether the vitamin is produced by true fungi is uncer- 

 tain. Furthermore, neither the fungi nor the actinomycetes are known 

 to require the vitamin. Although cobalt activates some enzymes, e.g., 

 the dipeptidase of Aspergillus oryzae (42), its only known essential 

 function in organisms generally is as a constituent of vitamin B 12 . In 

 view of the very great technical difficulty in establishing a cobalt re- 

 quirement if the metal is only needed for B 12 synthesis (94), it may be 

 more profitable to explore the B 12 metabolism of fungi than to attack 

 the cobalt problem directly; preliminary results (168) suggest that nei- 

 ther B 12 nor cobalt is essential to Aspergillus niger. 



Cobalt is accumulated by Neurospora crassa (12), the degree of ac- 

 cumulation — up to 23 times the external concentration — being a func- 

 tion of the amount of cobalt in the medium. Up to 40 per cent of the 



