POTASSIUM 303 



made limiting (71, 72) and, of course, if the external phosphate con- 

 centration is lowered (33, 70). When conidia appear, mycelial phos- 

 phorus enters the spores (10). Autolysis is marked by the appearance 

 of phosphate, primarily inorganic, in the medium (33, 42, 143, 217); 

 most of this appears to be derived from mycelial organic phosphorus, 

 inasmuch as the inorganic mycelial phosphorus remains relatively 

 constant during autolysis. Most of our information is from studies on 

 Aspergillus niger; the data of Figure 1 are typical. 



Combined forms of phosphate, organic and inorganic, are hydro- 

 lyzed by fungal enzymes, the phosphatases, the occurrence of which is 

 summarized in Table 1. It may be that the acid and alkaline phos- 

 phatases of fungi are identical with phosphomonoesterases I and II 

 of other organisms (204), but the specificity is very much in doubt. 

 Indeed, there is no certainty in most of the examples of Table 1 that 

 separate specific enzymes are involved. 



Probably most or all fungi and actinomycetes form a phosphatase 

 active at low pH against organic phosphates, although particular prep- 

 arations have been reported to lack this activity (34, 206, 207). 



2. POTASSIUM 



Potassium is required by fungi; at the usual levels of carbohydrate, 

 concentrations of 0.001-0.004 M are adequate (100, 187, 238). Sur- 

 prisingly little information is available on the physiology of potassium 

 in fungi; the most striking effect of potassium deficiency in Aspergillus 

 niger is an increase in oxalic acid accumulation (29, 146, 196). A 

 very low supply of the element is accompanied by poor sugar utiliza- 

 tion (195), and it seems likely from work with organisms other than 

 fungi that potassium has an essential function in carbohydrate metabo- 

 lism (154, 176, 202). Certain enzymes, however, are more active in 

 preparations from potassium-deficient cells (85). 



Sodium can partially replace potassium in the nutrition of Asper- 

 gillus niger (73, 238), but the degree of replacement is small. Under 

 some conditions, rubidium and cesium have slight effects (238), under 

 others, none (149). In certain of the bacteria, rubidium and other 

 alkali metals have a much more pronounced action, either as replace- 

 ments for or competitors of potassium (55, 124, 130, 202, 247). It 

 seems likely that studies of this problem in fungi other than Asper- 

 gillus niger would be rewarding. Rubidium can replace potassium 

 in certain cell-free enzyme systems (126). 



Figure 2 illustrates the typical growth response to the four major 

 elements — potassium, magnesium, sulfur, and phosphorus. 



