Author(s): N S Bolan, J Elliott, P E H Gregg
Acidification observed in cultures of Penicillium cyclopium was mostly due to the extrusion of protons into the medium. In the absence of NH,+ , protons were extruded together with citrate. The citric acid thus produced underwent a dynamic equilibrium of continuous efflux and uptake, which was shifted towards the latter at decreasing glucose concentrations. In cultures supplied with NH,+ and glucose a stoichiometric coupling of H+ excretion with NH,+ uptake was observed. If the protons excreted were not absorbed by external buffers (e.g. tartrate ions), the pH of the medium dropped to below 2, thereby exceeding the capacity of the cells to maintain their internal pH. The decrease of intracellular pH was accompanied by the cessation of NH,+ uptake, H+ extrusion and growth. In the presence of tartrate ions the NH,+/H+ exchange proceeded until the glucose concentration of the medium dropped below 25 mM. Here, the external pH was kept above 3, without seriously affecting the intracellular pH. After termination of H+ extrusion an appreciable amount of NH,+ was taken up together with hydrogentartrate anions, thereby causing a further acidification of the medium, which was finally reversed due to the uptake of tartaric acid in the late phase of growth.