Effect of Ammonium on Dark-CO2Fixation and on Cytosolic and Vacuolar pH Values inEremosphaera viridisde Bary (Chlorococcales)

At constant external [CO2], rates of dark-CO2 fixation of the unicellular green alga Eremosphaera viridis were drastically increased (up to 40-fold) by addition of ammonium (NH3+ NH4+) at external pH values (pH0) between 6.0 and 8.0. The cytosolic pH was monitored under identical conditions by micro-pH-electrode measurements, and cytosolic and vacuolar pH by the 31P-NMR technique. Addition of ammonium (5.0 mol m− pH0 7.0) caused a rapid and transient acidification of the cytosol during the first 4 min. Thereafter, the cytosolic pH remained constant at its original value. A rather constant cytosolic pH was also confirmed by 31P-NMR measurements, which, in addition, indicated a slow alkalization of the vacuole (about 0.5 units within 30 min after addition of ammonium). Since the dramatic stimulation of dark-CO2 fixation by ammonium is not mediated by an alkalization of the cytosol, nor by direct ammonium effects on phosphoenolpyruvate carboxylase (PEPC, E.C. 4.1.1.31), the role of vacuolar alkalization as a possible trigger for the stimulation of PEP-carboxylase is discussed.