92 PHYSIOLOGY OF THE FUNGI 



used has a great effect on the results obtained. Nickerson and Carroll 

 (1943) have indicated that the culture history of the cells used influences 

 the amount of aerobic respiration. Some of their data for Zygosaccharo- 

 myces acidifaciens are shown in Table 16. 



SULFUR 



Not all compounds which contain an essential element are equally 

 useful. In fact, some compounds are useless because the essential ele- 

 ment is unavailable. Among the factors which may affect availability is 

 the state of oxidation of the essential element. This is particularly true 

 of sulfur, phosphorus, and nitrogen. Among the organic compounds, 

 structure is enormously important. The situation is further complicated 

 in that not all fungi utilize the same compounds. Many examples of thi, 

 will be cited in connection with nitrogen and carbon nutrition. Atten- 

 tion must be given the sources of the essential elements as well as the uses 

 fungi make of them. 



Sources of sulfur. This element is present in many types of com- 

 pounds, both inorganic and organic. The state of oxidation of sulfur, as 

 well as the specific structure of organic sulfur compounds, affects utiliza- 

 tion. Sulfate sulfur, SO4"", is the most common source of sulfur used in 

 media. Some fungi, however, require specific organic sources of sulfur. 

 Steinberg (1936, 1941) has made an exhaustive study of sulfur sources for 

 Aspergillus niger and reached the general conclusions that inorganic sulfur 

 compounds containing oxidized sulfur are utilized, while sulfide and 

 disulfide sulfur are not utiHzed. Of the organic compounds containing 

 sulfur, the alkyl thioalcohols, sulfides, and disulfides are not used. 

 Alkyl sulfonates and sulfinates are excellent sources of sulfur. Steinberg 

 is of the opinion that oxidized sulfur is reduced to suKoxylate before it 

 enters the normal metabolic channels. An exception to the nonutiliza- 

 tion of reduced sulfur was noted for compounds which occur as normal 

 metabolites, such as cysteine, cystine, methionine, and homocystine. 

 These are assumed to enter normal metabolic channels without pre- 

 liminary modification. An exception to this statement was noted with 

 thiamine (thiazole sulfur), but the enormous (physiologically) amounts 

 used may have upset the metabolic activities of the fungus. 



In spite of the general utility of sulfate sulfur in fungus nutrition, many 

 fungi either utilize organic sulfur contained in natural metabolites to bet- 

 ter advantage or require these compounds as a source of sulfur. Leonian 

 and Lilly (1938) reported that the addition of cystine to a synthetic 

 medium was necessary for the grovv^th of Saprolegnia mixta, Achlya con- 

 spicua, Isoachlya monilijera, and Aphanomyces camptostylus. Since other 

 naturally occurring sulfur-containing amino acids were not tested, it 

 should not be concluded that these species are deficient for cystine. 



