Acid‐base regulation during nitrate assimilation in Hydrodictyon africanum

The acid‐base balance during NO 3 − 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 No 3 − as N‐source and C02 as C‐source; the excess OH − (0.75 OH − per NO 3 − 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 ψ K + during NO 3 − 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 + −NO 3 − symport which causes the entry of more H + than is consumed in NO 3 − 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 O 2 evolution in the light) has commenced, suggesting a mechanistic relation of H + and NO 3 − influxes. This is consistent with the interpretation suggested above. Determinations of cytoplasmic pH showed no significant effect of NO 3 − 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.