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

Summary

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.