ESSENTIAL METALLIC ELEMENTS 71 



ESSENTIAL MICRO ELEMENTS 



These elements have been called heavy-metal nutrients, trace elements, 

 micronutrients, and minor elements. The literature on this subject is 

 extensive and often conflicting. Reviews of this subject are given by 

 Perlman (1949), Foster (1939), and Steinberg (1939). A collection of 

 10,000 abstracts on the effects of the micro elements on green plants and 

 animals has been published by the Chilean Nitrate Educational Bureau 

 (1948). 



In spite of Raulin's (1869) discovery that iron and zinc are essential for 

 Aspergillus niger, there arose a school of investigators who considered tho 

 micro elements to be stimulatory rather than essential. This view is no 

 longer held. There are a number of reasons for this misinterpretation: 



(1) The failure to realize that the "chemically pure" compounds used in 

 preparing media are grossly contaminated from the biological standpoint 

 and that rigorous purification of media is essential in work of this kind. 



(2) Distilled water is often a source of metallic ions unless it has been 

 redistilled in Pyrex, or preferably quartz, stills. (3) Many kinds of 

 chemical glassware are sufficiently soluble to furnish the fungi all or a part 

 of the micro elements required. (4) The inoculum, whether mycelium or 

 spores, may introduce sufficient micro elements to obscure the need for 

 these elements. Serial transfer using media free from the element in 

 question and the use of small inocula minimizes this source of error. 



Steinberg (1936) has indicated that the optimum concentration of the 

 essential micro metallic elements for A. niger ranges from 0.3 mg. of iron 

 to 0.02 mg. of gallium per liter of medium. Lest the reader conclude that 

 these concentrations are so small as to be meaningless, it is revealing to 

 calculate the number of atoms of iron in 0.3 mg. From the atomic weight 

 of iron and Avagadro's number it may be calculated that there are about 

 3 X 10^^ atoms in 0.3 mg. of iron. If the number of cells produced by 

 A. niger under these conditions were known, the number of iron atoms 

 available for each cell could be calculated. In lieu of this information we 

 may use data from experiments on the number of yeast cells produced in 

 a liter of medium. Under favorable conditions there are roughly 500 

 billion yeast cells produced in a liter of medium (Stark et al., 1941). If 

 A. niger produces the same number of cells per liter as yeast, there would 

 be available 6.4 X 10^ atoms of iron per cell. 



The prime essential in investigations dealing with the effects of the 

 essential micro elements is a medium free from the element under study. 

 This ideal is difficult to attain in practice. Equal care is necessary in the 

 choice of culture vessels, for it is wasted effort to remove an element from 

 the medium rigorously and then contaminate it by using glassware which 



