[Dependence of nitrite assimilation in green algae on energy supplied by respiration and photosynthesis]

Regulation of assimilatory nitrate reduction at the level of nitrite in Chlorella fusca

Batch cultures of Chlorella fusca excreted nitrite into the medium if gassed with air (0.03% CO2), but they did not if supplied with air containing 5% CO2. After a change from high to low CO2 concentration in the gas stream, nitrite excretion started immediately. After an increase in CO2 concentration to 5%, nitrite uptake started within only 30 min. Changes of in-vitro activities of nitrate reductase, nitrite reductase and glutamine synthetase did not correspond to changes of nitrite concentration in the medium and therefore could not explain these observations. A nitrite-binding site, whose activity corresponded with both nitrite excretion and uptake, was detected at the chloroplast envelope. From these data an additional regulatory step in the assimilatory nitrate-reduction sequence is suggested. This includes an envelopeprotein fraction probably regulating the availability of nitrite within the chloroplast.

Nitrite reduction in leaves; Studies on isolated chloroplasts

Chloroplast preparations from spinach leaves containing a high percentage of intact chloroplasts were capable of light dependent nitrite reduction at rates around 9 μmol/mg chlorophyll/h for at least 50 min. This reduction was inhibited by DCMU but unaffected by uncouplers of photosynthetic phosphorylation. Nitrite reduction was not accompanied by a stoichiometric evolution of oxygen evolution. The disappearance of nitrite was accompanied by an approximately stoichiometric formation of reduced nitrogen.

[The nitrate- and nitrite-dependent O2-evolution in N 2 by Ankistrodesmus braunii]

O2-evolution in N2 by young cells of synchronous Ankistrodesmus braunii was measured manometrically in the presence and in the absence of nitrate or nitrite. Nitrate-starved cells produce O2 as a function of nitrate concentration with an optimum at 10 mM nitrate. The optimum rates are strongly dependent upon the pH of the medium culminating at pH 8.0.Nitrite excretion is initially slow and has its optimum at the same nitrate concentration as O2-evolution. It is slower under anaerobic conditions than in the presence of CO2 and slower at pH 5.6 than at pH 8.0. At pH 8.0 in N2 an accumulation of nitrite starts 40 to 100 min after the addition of nitrate to the algae.O2-evolution is faster with nitrite than with nitrate. No optimum curve is observed at pH 8.0 with various concentrations of nitrite in the medium; however, at pH 5.6 a distinct peak is found at 3 mM nitrite. This peak is found for a fast short-time reaction as well as for the following low rates of O2-evolution.The uncoupler carbonylcyanide-m-chlorophenylhydrazone (CCCP) equally inhibits nitrate- and nitrite-dependent O2-evolution and the incorporation of (32)P into cellular phosphate compounds. There is no indication of uncoupling in vivo of nitrite reduction which is completely independent of ATP in vitro. The inhibition of the nitrate-dependent O2-evolution by high concentrations of nitrate cannot be explained by an accumulation of products such as ammonia or nitrite.

The role of light in nitrite reduction; Studies with leaf discs

The reduction of nitrite by leaf discs has been studied. In short term experiments the reduction is markedly stimulated by light, but is not affected by the absence of oxygen or carbon dioxide from the gas phase. Carbon dioxide assimilation is more sensitive than nitrite reduction to 3-(3',4'-dichloro-)-1,1-dimethyl urea (DCMU) inhibition. Uncouplers such as carbonyl cyanide m-chlorophenyl-hydrazone (CCCP) do not inhibit nitrite reduction although dinitrophenol (DNP) has a small effect.Although nitrite stimulates oxygen evolution in the light in the absence of CO2 the stoichiometry of nitrite reduction to oxygen evolution is much less than would be predicted if nitrite is simply acting as a classical Hill reagent.