Stoichiometry between photosynthetic nitrate reduction and alkalinisation by Ankistrodesmus braunii in vivo

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Abstract
The uptake of nitrate or nitrite in the light, the release of nitrite and ammonia, and the corresponding alkalinisation of the medium were measured in synchronous Ankistrodesmus braunii (Naeg.) Brunnth. The increase in the OH− concentration in the medium reflects a stoichiometric ratio between OH− and NO3 − of 1.3–1.8 in air, reaching almost 2.0 in CO2−free air or nitrogen. At low CO2 concentrations a large proportion of the nitrogen taken up as nitrate is released as ammonia, much less as nitrite. The stoichiometry of alkalinisation and NO3 − or NO2 − uptake can be quantitatively explained by assuming: 1) a counter-transport, at a ratio of 1:1, of OH− against NO3 − at the plasmalemma and of OH− against NO2 − at the chloroplast envelope, and 2) a co-transport of 1:1 of OH− and NH4 + to the medium through both membranes. The first OH− required is formed by proton consumption in nitrite reduction, the second OH− by proton consumption in the formation of NH4 + ions. Transport of K+, Na+ and Ca2+ is not or only scarcely involved. This proposed transport system could provide charge equilibrium between inside and outside the cells and could enable the cells to avoid nternal pH changes in nitrate and nitrite reduction.