ESSENTIAL METALLIC ELEMENTS 67 



The elements are in the form of chemical compounds, some of which 

 are relatively simple, Avhile others are complex. With the exception of 

 oxygen the essential elements are usually utilized in the form of com- 

 pounds or ions. An essential element may exist in a chemical compound 

 and be unavailable. The properties of a chemical compound are deter- 

 mined by all the atoms that compose it and by the way in which atoms 

 are joined together in the compound. It is convenient to consider the 

 essential elements one by one, but this is done only to simplify the 

 approach to a complex subject. These separate factors must be con- 

 sidered in relation to the organism as a whole. 



A fungus is no more capable of growth on an iron-free medium than on 

 a carbon- or nitrogen-free medium. Yet, in a balanced medium the ratio 

 of iron to carbon is in the neighborhood of 1 to 50,000. The essential 

 metallic elements function in conjunction with enzyme systems (Chap. 4). 

 This accounts for the small amounts of these elements required. If a 

 vital enzyme system lacks an essential metal ion, it will not function. It 

 appears that in processes such as growth a suboptimal amount of an 

 essential metal will stop growth because the apoenzymes or coenzymes 

 synthesized will lack the necessary activating metal. The ratios as well 

 as the amounts of the various essential metallic ions affect certain 

 metabolic processes other than growth. 



The absolute amounts of the essential metallic elements required differ 

 widely. Raulin (1869) found that Aspergillus niger required 1 g. of 

 potassium to produce 64 g. of mj^celium, while 1 g. of magnesium sufficed 

 for the synthesis of 200 g. of mycelium. Recent work of Steinberg (1946) 

 with .4. niger indicates still higher yields per gram of these two elements. 

 The yield of mycelium per gram of iron and zinc was in the neighborhood 

 of 55,000 g. 



The list of metallic elements knoTvni to be essential to fungi has in- 

 creased over the years. The list now includes potassium, magnesium, 

 iron, zinc, copper, calcium, gallium, manganese, molybdenum, vanadium, 

 and scandium. Others will probably be added as cultural methods 

 become more refined and more species are studied. It is unfortunate that 

 only a few fungi have been investigated thoroughly with respect to mineral 

 nutrition. In stating that the above elements are essential, the reserva- 

 tion must be made that they are essential for some fungi under certain 

 conditions. WTiile it may be assumed that all fungi require the 

 same essential elements, experimental evidence is lacking for most 

 species. 



For the purpose of discussion the essential metallic elements will be 

 divided into two groups, macro and micro metallic elements. This 

 grouping is made solely for convenience and on the basis of the amounts 

 ordinarily employed in culturing fungi under laboratory conditions. 



