BIOASSAY OF MINERALS 313 



cobalt so accumulated is bound to a protein. Cobalt accumulation in 

 N. crassa is affected indirectly by the iron concentration (11). 



Cobalt at appreciable concentrations is toxic to fungi and other or- 

 ganisms* Toxicity is reduced by histidine (181, 182, 212), which pre- 

 sumably forms a complex with the cobalt ion. The uptake of cobalt 

 by Aspergillus niger is sharply reduced by the addition of relatively 

 small amounts of magnesium (3). A third complication in cobalt tox- 

 icity is the observation that, although iron and cobalt do not simply 

 compete, the enzymatic symptoms of cobalt toxicity in Neurospora 

 crassa resemble closely the symptoms of iron deficiency (80). 



12. OTHER INORGANIC NUTRIENTS 



Aspergillus niger responds slightly to vanadium (17, 18, 19) and 

 fleshy fungi take up vanadium from the soil (20), but satisfactory proof 

 of an essential role is not at hand. Vanadium is essential, however, 

 for the alga Scenedesmus obliquus (6). 



Steinberg has reported growth increases from addition of gallium 

 (231) and — only with glycerol as carbon source — of scandium (233). 

 Results on gallium have not, however, proved reproducible (237). 



Claims of an essential or growth-promoting role of boron (256) and 

 organic compounds of silicon (87) have not as yet been confirmed; 

 Winfield (255) reports negative results with boron. Chlorine appears 

 to have no necessary role in the nutrition of fungi (60) although chlorin- 

 ated metabolites are not uncommon (Chapter 6), and chlorine is es- 

 sential to higher plants (28). Other metals, e.g., aluminum and chro- 

 mium, often exert physiological effects and may even under peculiar 

 circumstances improve growth, but it has not been suggested that they 

 are essential. 



13. BIOASSAY OF MINERALS 



The response of fungi to nutrients is sensitive enough so that a num- 

 ber of bioassays for inorganic elements in soil have been developed to 

 the point of practical utility. Almost all the work has been with 

 Aspergillus niger, although a radial growth assay for phosphorus with 

 Cunninghamella spp. was devised in 1934 (139, 140). 



Bioassays for minerals are more laborious than chemical determina- 

 tions and are intrinsically less accurate; precision is especially low in 

 those assays based on visual estimates of growth or spore color (151). 

 The principal advantage of the bioassay method, and the reason for its 

 popularity, is that it may more nearly approximate a determination of 



