A drop of intracellular pH stimulates citric acid accumulation by some strains of Aspergillus niger.

By comparing kinetic parameters of plasma membrane proton pumps from two Aspergillus niger strains, significant differences in specific activities were observed. In low citric acid producing A158 strain the H+ -ATPase activity was about four-fold higher than in a high yielding A60 strain. Previously pH homeostasis was reported in A158 strain while in A60 strain spontaneous drop of intracellular pH was observed. During the growth in the medium with ammonium ions more rapid drop of extracellular pH was recorded with A158 strain and not so fast proton accumulation in the medium with A60 strain, indicating that proton pumps from later strain perhaps can not extrude all the protons that are released in the cytosol after the assimilation of ammonium ions. Vanadium ions were found to be potent inhibitors of both H+ -ATPases. By adding sodium vanadate in millimolar concentrations to the chemically defined medium that induces citric acid accumulation by A. niger, reduced pHi and increased rate of acid production was observed in A158 strain while in A60 strain intracellular pH decreased below 6.5 and concomitantly citric acid overflow was suppressed. The presented results suggest that one of the mechanisms stimulating citric acid accumulation by A. niger could be also a slight cytoplasmic acidification.