94 PHYSIOLOGY OF THE FUNGI 



nine, and cystine. It was postulated that sulfate sulfur was first reduced 

 to sulfite and then to sulfoxylate, which was assumed to dimerize to 

 thiosulfate. The next reaction was believed to be between serine and 

 thiosulfate to form cysteine S-sulfonate, which is then converted to 

 cysteine. Cysteine on being oxidized forms cystine. 



Two vitamins, thiamine and biotin, contain sulfur. The role of these 

 compounds will be considered in Chap. 9. In addition to the sulfur-con- 

 taining amino acids and vitamins there is evidence that other types of 

 organic sulfur compounds are formed by fungi. Raistrick and Vincent 

 (1948) found that many strains and species of Aspergillus and Penicillium 

 converted essentially all of the sulfate sulfur into organic sulfur com- 

 pounds, but not all of these compounds were found in the fungus proteins. 

 Penicillium chrysogenum excretes into the medium various unidentified 

 organic sulfur compounds (Plumlee and Pollard, 1949). The function of 

 these compounds is unknown. 



The reactions whereby a fungus transforms a single source of sulfur into 

 these various compounds are obscure. When sulfate or other sources 

 containing oxidized sulfur are utilized, it is necessary for the fungus to 

 reduce the sulfur to its lowest valence. Schizophyllum commune has been 

 shown to reduce sulfate to methyl mercaptan, CH3SH (Birkinshaw et al., 

 1942). This substance contributes to the characteristic odor of this 

 fungus. 



PHOSPHORUS 



Raulin (1869) found phosphorus to be an essential element for Aspergil- 

 lus niger. Omission of phosphate from his synthetic medium reduced the 

 yield approximately 50 per cent. Phosphorus is essential for all forms of 

 life. Phosphorus may be classified as a structural element in the sense 

 that definite compounds containing this element have been isolated from 

 fungi. Phosphorus compounds play an important role in the functions of 

 chemical transformations and energy transfer. 



Sources of phosphorus. Apparently phosphorus is utilized only when 

 it is in the form of phosphate. This element is taken up as phosphate and 

 functions in this form, mainly in the form of phosphate esters. It will be 

 recalled that there are several different phosphates. The formulas for the 

 potassium salts are K3PO4, potassium orthophosphate ; KPO3, potassium 

 metaphosphate ; and K4P2O7, potassium pyrophosphate. More complex 

 phosphates than pyrophosphate occur. Orthophosphoric acid may be 

 neutralized in three steps to produce the following types of salts: KH2PO4, 

 monopotassium orthophosphate; K2HPO4, dipotassium orthophosphate; 

 and K3PO4, tripotassium orthophosphate. All these salts furnish utiliz- 

 able phosphate, but the effects of these three salts on the acidity of the 

 medium are quite different. In addition to inorganic phosphates, the 



