74 PHYSIOLOGY OF THE FUNGI 



If a fungus is grown on a medium, it will absorb and utilize the essential 

 elements present in the medium. The success of this procedure depends 

 upon having a low initial concentration of the essential micro elements, 

 which soon become exhausted so that the culture liquid no longer supports 

 growth. Removal of the mycelium will thus remove the elements which 

 have been taken up. The culture filtrate may then be used as a medium 

 relatively free of micro elements. However, fungi excrete various com- 

 pounds which may affect the results. MacLeod and Snell (1947) have 

 recently utilized this method in studying the mineral nutrition of some 

 lactic acid bacteria. 



Iron. Raulin's claim that iron was essential for fungi was questioned 

 at first, but his findings were soon confirmed. So far as is known, iron is 

 essential for all fungi. It may be noted that, in the absence of another 

 essential element in the medium, iron alone may cause little or no response. 

 If the zinc content of a medium is low, the addition of iron to an iron-free 

 medium will have little effect. This situation is true of any essential 

 nutrient. Only one element may be studied at a time, but all the other 

 essential nutrients must be present before the effect of the nutrient under 

 investigation can be studied. Some results of Steinberg (1919) with 

 Aspergillus niger on media purified by the calcium carbonate method are 

 given in Table 11. Neither iron nor zinc alone had much effect on the 

 growth of A. niger, since both of these elements are essential for this 

 fungus. 



Table 11. The Effect of Iron and Zinc, Singly and in Combination, on the 



Amount of Growth of Aspergillus niger 



(Steinberg, Am. Jour. Botany 6, 1919.) 

 Essential Micro Element Mg. Mycelium 



Added 



Control (none added) 18 



Iron 44 



Zinc 40 



Iron plus Zinc 731 



Little interest has been shown in recent years in proving iron to be an 

 essential element for a large number of fungi. In view of the almost uni- 

 versal occurrence of a group of iron-containing enzymes (catalase, the 

 cytochromes, cytochrome oxidase, etc.), the essential role of iron is taken 

 for granted. 



The most obvious effect of suboptimal iron concentrations upon fungi 

 is decreased growth. This result is probably due to the decreased and 

 limited amounts of iron-containing enzymes formed under these condi- 

 tions. It was shown by Yoshimura (1939-1940) that the amount of 

 catalase produced by Aspergillus oryzae increased as the amount of iron 

 in the medium increased. Lilly and Leonian (1945) showed that a rela- 



