ALIPHATIC ORGANIC ACIDS 137 



Citric Acid. The ability of fungi to accumulate citric acid was 

 first discovered by Wehmer (546, 547) in species of "Citromyces" 

 (Penicilliam). Later (155) it was found that higher yields are obtained 

 from selected strains of the black-spored Aspergilli, usually referred 

 to as Aspergillus niger. Certain of these strains are presumably used 

 for the commercial production of citric acid. Although the acid is 

 probably a metabolic intermediate of aerobic metabolism in all fungi 

 (Chapter 7), it appears in significant amounts only in a few forms 

 (Table 1), primarily species — or, more correctly, certain strains of 

 species — of Penicillium and Aspergillus. Most strains of A. niger have 

 the potentiality for the accumulation of citric, gluconic, and oxalic 

 acids; particular strains under a given set of conditions may form 

 predominantly one acid or a mixture of two or three (48, 58). Strain 

 differences contribute to the failure of different laboratories to agree 

 on the details of the citric acid process, but the basic physiological 

 events are presumably the same in all strains. Selection may increase 

 the yield of citrate (169, 307), and, correspondingly, a strain may 

 change its capacity as the result of mutation (218, 331). No adequate 

 genetic analysis has been made; probably the typical strain is a 

 heterocaryon with nuclei of different genetic types which segregate in 

 spore formation (28). 



The biosynthesis of citrate by Aspergillus niger was investigated 

 extensively during the period 1920 to 1940, largely by growth and 

 replacement culture experiments which by themselves do not dis- 

 criminate among the various possible metabolic pathways. Theories 

 based on this work, all obsolete or correct only by chance, are re- 

 viewed by Foster (200) and Walker (538). 



As discussed in Chapter 7, it now seems that fungi, like other 

 organisms, utilize the citric acid cycle in aerobic respiration; the 

 evidence, which is admittedly still incomplete, comes from enzymatic 

 studies and from isotope experiments with citrate formation in 

 Aspergillus niger. At least one other pathway of citrate (or isocitrate) 

 metabolism is known in fungi (405) and may be found in the future 

 to make a significant contribution to citrate synthesis. If we postulate 

 the ubiquity of the citric acid cycle in fungi, accumulation of citrate 

 is best regarded as an unusual modification, under genetic and en- 

 vironmental control, of a process which occurs in all cells under 

 aerobic conditions. 



Figure 1 is a simplified diagram of some of the known and probable 

 relationships of organic acids in Aspergillus niger and, with some ex- 

 ceptions, very likely in all fungi. The salient and physiologically 

 significant implications of the scheme may be summarized: 



