iron 307 



gilhis terreus in surface culture requires much more magnesium than 

 it does in shaken culture (119, 120). 



The enzymes activated by magnesium in in vitro systems are often 

 also activated by manganese or other divalent ions, but it is believed 

 that magnesium is the physiologically active metal (115, 126). None 

 of the physiological effects of magnesium deficiency has been conclu- 

 sively related to enzyme malfunction, but it is reasonable to suppose 

 that, for example, enhancement of riboflavin formation by magnesium 

 deficiency in Aspergillus niger (114, 210) reflects an impairment of the 

 normal oxidative system. In bacteria, magnesium accelerates the up- 

 take of phosphate, possibly by reason of the role of the metal in 

 transphosphorylations (3). 



The absorption of magnesium is somewhat slower at neutral than 

 at acid reaction, resulting in a slightly higher optimum at neutrality 

 (236). Magnesium is precipitated and rendered virtually unavailable 

 at high pH in the presence of ammonium ion (62). 



As suggested above, the apparent magnesium requirement is prob- 

 ably affected by other ions. This is deduced from the very great ac- 

 tivity of magnesium in antagonizing the toxicity to fungi of such 

 metals as aluminum, copper, and mercury (120, 122, 136); similar re- 

 sults with other ions have been reported in studies on bacteria (3, 130). 

 The most attractive theory explaining this antagonism is that of Mac- 

 Leod and Snell (130), that other metals compete with magnesium for 

 enzyme surfaces and that the resulting metal-enzyme complex is inactive 

 or less active than the natural magnesium complex. Competition, how- 

 ever, need not be limited to enzymatic reactions. Marsh (136) showed 

 clearly that magnesium prevents the absorption of copper by conidia 

 of Sclerotinia fructicola; this finding suggests a competition of the two 

 elements at the cell membrane. 



Beryllium is said to replace magnesium partially and under special 

 circumstances (238). 



5. IRON 



Fungi and actinomycetes require approximately 0.1-0.3 ppm of iron, 

 more in concentrated media (36, 37, 100, 169, 241). Aspergillus niger 

 utilizes either ferrous or ferric iron (168). Iron is lost from solution 

 at neutral or alkaline pH; inclusion in the medium of a chelating 

 agent, e.g., citrate, reduces the loss (62, 88). 



The most obvious metabolic role of iron is in the formation of iron- 

 containing metabolites. Those known in the fungi include catalase 

 and the cytochromes (Chapter 7), coprogen and the ferrichromes 



