218 BIOLOGICAL ACTIVITIES OF MOLDS 



of classification need not be altogether discarded, however, because 

 the nitrogen requirements of the fungi could be based on comparative 

 responses with an identical carbon source. 



That heavy metals play an essential role in the nutrition of molds 

 has been recognized since the earliest investigations dealing with the 

 cultivation of these fungi in synthetic media. In addition to phos- 

 phorus, sulphur, magnesium, and potassium, certain other elements 

 are not only desirable but also often necessary to obtain the maxi- 

 mum yield of fungi from synthetic media. Originally it was held 

 that elements added to the media in minute quantities were beneficial 

 because these substances acted as chemical stimulants. It was postu- 

 lated that the accelerated and increased growth of fungi in media 

 containing these elements was due to the physiological response of 

 the organism to the toxic properties of these elements, now known 

 to be essential. This concept was based on the notion that poisons 

 when added in minute quantities act as stimulants. It was assumed, 

 of course, that the control media were free of trace elements. Hence 

 growths on such media were considered normal and increases in 

 growth were thought to be due to the "stimulating effect" of the 

 added heavy metals. Steinberg ^^ has presented evidence to disprove 

 this chemical stimulation theory. By using extremely efficient meth- 

 ods of purification, this investigator was able to prepare media free 

 of traces of heavy metals. The growth of Aspergillus niger was so 

 scanty in such media and such large increases in yield were obtained 

 when zinc and iron were added that he regarded the chemical stim- 

 ulation theory as untenable. He considered these two elements just 

 as essential to the nutrition of the mold as carbon and nitrogen. 

 In addition to zinc and iron, elements such as copper, manganese, 

 molybdenum, and gallium are now considered not only requisite for 

 maximum growth but also absolutely essential for growth in general 

 of the filamentous fungi. It is generally safe to assume that these 

 elements, which are required only in very minute quantities, are 

 normally present as impurities in sufficiently large amounts (in the 

 chemicals, in most samples of distilled water prepared by the usual 

 methods and glassware used for cultural purposes) despite the fact 

 that these elements are not purposely included in most media used 

 for culturing these organisms. 



In addition to the above-mentioned elements which must be fur- 

 nished the molds, some fungi require certain organic substances for 

 growth. Thus thiamin has been found essential or beneficial for the 

 growth of most molds. Robbins and Kavanagh ^-' ^^ have shown 

 that some fungi require the intact thiamin molecule; others may re- 



