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Microbiology 152 (2006), 887-893; DOI  10.1099/mic.0.28454-0
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Microbiology 152 (2006), 887-893; DOI  10.1099/mic.0.28454-0
© 2006 Society for General Microbiology

Thermodynamic boundary conditions suggest that a passive transport step suffices for citrate excretion in Aspergillus and Penicillium

Wolfgang Burgstaller

Institute of Microbiology, Technikerstrasse 25, A-6020 Innsbruck, Austria

Correspondence
Wolfgang Burgstaller
wolfgang.burgstaller{at}uibk.ac.at

Excretion of organic acids, e.g. citrate, by anamorphic fungi is a frequent phenomenon in natural habitats and in laboratory cultures. In biotechnological processes for citrate production with Aspergillus niger extracellular citrate concentrations up to 1 mol l–1 are achieved. Intracellular citrate concentrations are in the millimolar range. Therefore the question arises whether citrate excretion depends on active transport. In this article thermodynamic calculations are presented for citrate excretion by A. niger at an extracellular pH of 3 and by Penicillium simplicissimum at an extracellular pH of 7. From the results of these calculations it is concluded that in both cases a passive transport step suffices for citrate excretion.

Abbreviations: {Delta}{Psi}, electrical potential difference between the inside and the outside of the cell (membrane potential at the plasma membrane); pKa, negative decadic logarithm of the acid dissociation constant; pKc, negative decadic logarithm of the complex formation constant; pHe, extracellular pH; pHi, intracellular pH; pHcyt, cytoplasmic pH; CCCP, carbonyl cyanide 3-chlorophenylhydrazone; 2,4-DNP, 2,4-dinitrophenol






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