Acid-base regulation during nitrate assimilation in Hydrodictyon africanum

The acid-base balance during NO3− assimilation in Hydrodictyon africanum has been investigated during growth from (1) an analysis of the elemental composition of the cells, (2) the alkalinity of the ash and (3) the net H+ changes in the medium during growth. These investigations agree in showing that some 0.25 excess organic negative charges are generated per N assimilation from No3− as N-source and C02 as C-source; the excess OH− (0.75 OH− per NO3− assimilated) appears in the medium. Approximately half of the excess organic negative charge is attributable to cell wall uronates; the remainder is intracellular. All of the excess OH− appearing in the medium must have crossed the plasmalemma (as net downhill H+ influx or OH− efflux). Previous work has shown that the value of ψco is more negative than ψK+ during NO3− assimilation, suggesting that the active electrogenic H+ extrusion pump is still operative despite the net downhill H+ influx. The interpretation of this in terms of H+−NO3− symport which causes the entry of more H+ than is consumed in NO3− metabolism, with extrusion of the excess H+via the active, electrogenic H+ pump, was tested by measuring short-term H+ influx upon addition of NO−3. A net H+ influx occurs before NOa assimilation (as indicated by additional O2 evolution in the light) has commenced, suggesting a mechanistic relation of H+ and NO3− influxes. This is consistent with the interpretation suggested above. Determinations of cytoplasmic pH showed no significant effect of NO3− assimilation, suggesting that cytoplasmic pH changes sufficient to change the ‘pH-regulating’ H+ fluxes are smaller than the errors in the determination of cytoplasmic pH.