Further Studies on the Acid Metabolism of Aspergillus niger

Abstract

Some recent works on the formation of oxalic acid by various fungi are critically considered. The present work deals with the role of oxalic acid in the metabolism of Aspergillus niger . When glucose solutions were supplied to preformed mats of the fungus oxalic acid accumulated, attaining an equilibrium level which was not exceeded despite the presence of a considerable concentration of glucose. When the glucose supplies were depleted the oxalic acid concentration fell steeply to a low level. The concentration of oxalic acid was dependent on the glucose concentration. In three separate series of experiments it was shown that the oxalic acid concentration diminished with increasing glucose concentration. Similar results were obtained when the cultures were reared from spores on culture solutions with the normal amounts of nutrient salts but different glucose concentrations. In all cases the CO ₂ output increased with the glucose concentration. When cultures were supplied with glucose+oxalic acid, the concentration of the latter fell steeply to the equilibrium level attained on glucose only. In a culture receiving glucose+oxalic acid, with the oxalic acid concentration somewhat below the normal equilibrium concentration, the formation of oxalic acid from the glucose ceased as soon as the equilibrium level had been attained. When 1 per cent. oxalic acid only was supplied to the fungus the concentration gradually diminished to a low level. When 3 per cent. oxalic acid was supplied the rate of acid utilization soon fell to low value. In several experiments it was shown that the rate of CO ₂ output was higher from cultures supplied with glucose+excess oxalic acid than from cultures supplied with glucose only. The rate of oxalic acid carbon loss was always below that of the CO ₂ carbon output both in cultures supplied with oxalic acid only and in cultures receiving glucose+oxalic acid. The cultures were incapable of utilizing neutral sodium oxalate and the presence of this substance had no effecft on the of CO ₂ output. The results indicate that the utilization of oxalic acid is associated with the liberation of at least an equivalent amount of CO ₂ . It is suggested that the utilization of oxalic acid is promoted by the presence of glucose, thus accounting for the lower oxalic acid concentrations and higher rates of CO ₂ output of cultures with higher glucose concentrations.