Buffer-stimulated citrate efflux in Penicillium simplicissimum: an alternative charge balancing ion flow in case of reduced proton backflow?

Organic acids excreted by filamentous fungi may be used to win metals from industrial secondary raw materials. For a future commercial use a high production rate of organic acids is necessary. The conditions under which the commercially used fungus Aspergillus niger excretes high amounts of citric acid can not be maintained in metal leaching processes. However, Penicillium simplicissimum showed an enhanced citric acid efflux in the presence of an industrial filter dust containing 50% zinc oxide. Because Good buffers of high molarity were able to mimic the effect of zinc oxide, the high buffering capacity of zinc oxide and not an effect of the zinc ions was held responsible for the enhanced citric acid efflux. The presence of ammonium and trace elements reduced this buffer-stimulated citric acid efflux, whereas the plant hormone auxine canceled this reduction. This citric acid efflux was influenced by a depolarization of the membrane: the freely permeable compound tetraphenylphosphoniumbromide decreased the citric acid efflux, without decreasing intracellular citric acid or consumption of glucose and oxygen. Vanadate, an inhibitor of the plasma membrane H(+)-ATPase also reduced the buffer-stimulated citric acid efllux. The role of the efflux of citrate anions as an alternative charge balancing ion flow in case of impaired backflow of extruded protons because of a high extracellular buffering capacity is discussed.