316 INORGANIC NUTRITION A N D M E T A B O L I S M 



is no absolute optimum concentration of an element for even one spe- 

 cies. Some of the complicating factors may be mentioned. First, the 

 need for inorganic nutrients is proportional to the carbohydrate sup- 

 ply (241). Second, the need for an element may be increased or de- 

 creased by the presence of a second element, the important nutritional 

 factor becoming then the ratio of the two (60, 129); in practice, this 

 means that equally good growth is observed over a range of total salt 

 concentration (43, 77, 128). Third, the nutrients and metabolites 

 which form chelate complexes with minerals are present in continu- 

 ally changing amounts; although a chelating agent may on the one 

 hand solubilize a mineral and so make it more available, on the other 

 hand the cell must compete with the chelate complex for the element. 

 Fourth, the pH of the medium — again a factor not usually constant 

 for the entire life cycle — affects the solubility of most inorganic salts 

 (62, 78, 88, 230), the state of oxidation of manganese (225), and the 

 stability of chelated complexes (25). Finally, changes in the conditions 

 of culture may change the apparent requirement for a mineral, as in the 

 case of the magnesium requirement of Aspergillus terreus (119, 120). 



The use of metabolically inert chelating agents, particularly ethylene- 

 diaminetetraacetic acid (EDTA, versene) solves some of the practical 

 problems of supplying a fungus with minerals; the compound has been 

 used successfully, at 2-5 ppm, for several fungi (194, 209a, 252) and for 

 Streptomyces nitrificans (213). However, this and similar agents may 

 prove to be toxic (63, 128, 243). Added citrate or malate, unless uti- 

 lized by the organism, can be used to keep minerals soluble (92). 



The fungi accumulate minerals against a concentration gradient; the 

 data are particularly clear for cobalt (11, 12), silver and cesium (144), 

 and potassium (54). These examples, and the accumulation of rubid- 

 ium by fleshy basidiomycetes (21), make it clear in addition that 

 accumulation of non-essential elements is common. The energy for 

 accumulation must come, of course, from respiration. The very strik- 

 ing effect of magnesium on the uptake of cobalt by Aspergillus niger 

 (3) and of copper by Sclerotinia fructicola (132) suggests that reactions 

 at a semi-permeable membrane are of importance in mineral nutrition. 

 In yeast, there is good evidence for carrier systems in the transport of 

 cations across the membrane (61). 



The functions of the minerals required by fungi deserve further 

 study. Phosphorus and sulfur occur in known essential compounds, 

 and their major role appears to be constitutive. The other essential 

 mineral elements are today more or less conventionally assigned a 

 "catalytic" function as activators or active centers of enzymes. Reasons 

 exist, of course, for this general belief: the function of molybdenum in 



