76 PHYSIOLOGY OF THE FUNGI 



The iron concentration of the medium has been shown to affect the 

 amount of pigmentation of Torulopsis pulchcrrima (Roberts, 1946). 



Zinc. This element is essential for Aspergillus niger (Raulin, 1869; 

 Steinberg, 1919). Foster (1939) lists Trycliophytoninterdigitale, Rhizopus 

 nigricans, and Saccharomyces cerevisiae as recjuiring zinc, and Roberg 

 (1928) found zinc to be essential for A. fumigatus and A. oryzae. Blank 

 (1941) reported the amount of growth oi Phymatotrichum omnivorum to be 

 increased by the addition of zinc to a medium treated with calcium car- 

 bonate, and Perlman (1948) noted that the sclerotia of Sclerotium 

 delphinii are more highly pigmented in the presence of added zinc. 



Zinc ions activate (and inhibit) various enzymes such as enolase and 

 dipeptidase. Zinc is contained in carbonic anhydrase, an enzyme which 

 catalyzes the decomposition of carbonic acid to carbon dioxide and water. 

 In addition to these specific uses the zinc concentration has a decided 

 effect on a number of physiological or biochemical processes in fungi. 

 Foster and Waksman (1939) found that the production of fumaric acid 

 from glucose by Rhizopus nigricans varied according to the amount of zinc 

 added to the medium. Fumaric acid was produced most efficiently when 

 the concentration of zinc was low (1.2 mg. per liter). Higher concentra- 

 tions of zinc resulted in increased growth and decreased production of 

 fumaric acid. From these results it appears that zinc plays a role in the 

 utilization of glucose, the completeness of oxidation and assimilation 

 being favored by relatively high concentrations of zinc. A somewhat 

 similar effect of zinc on the production of lactic acid by Rhizopus sp. has 

 been noted (Waksman and Foster, 1938). Zinc was found to cause 

 increased growth and a decrease in the production of lactic acid, while the 

 effect of iron is to increase the yield of lactic acid. For a further dis- 

 cussion of the mechanism of zinc in fungus metabolism, see Foster (1949). 



Copper. This element is essential for animals, green plants, and fungi. 

 From the work of Steinberg (1936) it appears that 0.04 mg. of added 

 copper per liter of purified medium is sufficient for the maximum growth 

 of Aspergillus niger. Under these conditions omission of copper decreased 

 the yield only from 984.8 to 774.3 mg. It is probable that purification of 

 the medium by the calcium carbonate treatment is not very satisfactory 

 for this element. The ^veight of metal needed to obtain maximum growth 

 with A. niger is much less for copper than for iron or zinc. The experi- 

 mental difficulties increase as the amount of a micro element needed 

 becomes less. Apparently it is very difficult to prepare a copper-free 

 medium. Roberg (1931) made use of Bortel's method of adding a trace 

 of ammonium sulfide to convert heavy metals to sulfides and adsorbing 

 these impurities with charcoal. This treatment is very efficient in remov- 

 ing iron and zinc but somewhat less satisfactory for removing copper. 

 The essential nature of copper for A. flavus and Rhizopus nigricans was 



