Studies on the Formation of Oxalic Acid by Aspergillus Niger

Although Aspergillus niger, strain 72-4, has been studied previously because of the high yields of citric acid it gives when fermenting sugars, it also produces oxalic acid under the proper conditions. The present work is a study of the conditions under which oxalic acid is formed, the substrates that are converted to it, and the mechanism of its formation from these substrates. Oxalic acid is never found in normal growth fermentations with strain 72-4 until after autolysis has begun. However, in replacement fermentations in which preformed, washed mycelium is resuspended in nitrogen-free medium, oxalic acid is often formed together with citric acid. It was originally suggested (1) that oxalic acid is made by hydrolytic splitting of oxalacetate. Later work (2, 3), although consistent with this theory, gave data more in line with the results of the present work, which indicate that oxalacetate is oxidatively split to 2 molecules of oxalic acid. Direct oxidation of acetate to oxalate was ruled out by the work of Bomstein and Johnson (3) and also by the present work. The present study also shows that, at neutral pH, a great variety of sugars and related compounds can be fermented to oxalic acid by fluoride and fluoroacetate-insensitive pathways. Compounds part of or related to the Krebs cycle still appear to form oxalic acid by oxidative splitting of oxalacetate.