SUMMARY: excreted more than 100 mmol citric acid l [2-9 mmol (g dry wt); 9 d] if an industrial filter dust (> 50 % ZnO) providing a high extracellular buffering capacity was present in the medium. A similar specific [2 mmol (g dry wt)], but lower absolute (26 mmol l), citric acid excretion occurred in the absence of an extracellular buffer and if amino acids or urea were used as nitrogen source. excreted no citric acid under conditions where produces citric acid (deficiency of trace elements, low pH and reduced biomass formation). Citric acid excretion by always paralleled biomass formation and occurred in a pH range between 4 and 7. This indicated that different imbalances of metabolism were responsible for citric acid excretion in and However, provided a high extracellular buffering capacity was present, the response of the system to different carbon and nitrogen sources was similar to the system. In contrast, the metals iron and copper had virtually no effect on citric acid excretion compared with Estimation of intracellular citric acid, as well as the effects of the uncoupler 2,4-dinitrophenol, and the H-ATPase inhibitor sodium orthovanadate, led to the conclusion that the buffer-stimulated citric acid efflux was dependent on metabolic energy and an energized plasma membrane, respectively. Despite similarities to the system, a different mechanism for buffer-stimulated citric acid excretion by seems probable.


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