[Polarographic measurement of nitrite reduction by chlorella in monochromatic light]

Studies on nitric oxide (NO) formation by the green alga Scenedesmus obliquus and the diazotrophic cyanobacterium Anabaena doliolum

This study provides preliminary evidence that NO production could be a general attribute of algae. Anabaena doliolum was found to be a better NO producer than Scenedesmus and Synechoccocus. Experiments conducted with inhibitors of photosynthesis (DCMU), ATP synthesis (DCCD), and the uncoupler (2,4-DNP) and its analog arsenate clearly revealed that inhibition of nitrite assimilation through the blockage of nitrite reductase (NiR) is primarily responsible for NO emission. A linear relationship between nitrite concentration in the culture medium and NO in the exhaust gas supports the view that accumulation of nitrite is responsible for NO formation. A failure of Scenedesmus, grown in the medium substituted with W for Mo, to produce either NO/NO-2 in light or a 'light-off' peak, and a resumption of these activities upon the addition of Mo proved beyond doubt that a functional nitrate reductase (NR) is necessary for the production of nitrite and NO by algae grown on nitrate as the nitrogen source. Moreover, the appearance of a NO peak immediately after nitrite supplementation under dark conditions in W-substituted cultures with or without glucose ruled out an enzymatic role of NR in NO emission.

On the respiratory enhancement in Chlorella pyrenoidosa by blue light

Previous authours have suggested that the Type I respiratory enhancement in Chlorella was the result of an increased supply of a respiratory substrate or intermediate, or a change in activity of a respiratory enzyme. Our studies with respiratory inhibitors show that the Type I effect is not a general respiratory enhancement, as would be expected from an increase in available substrate, but rather is specifically associated with the tricarboxylic acid cycle and the electron transport chain. The feedback controls on these two processes are such that changes in activities of the component enzymes or in concentrations of carbohydrate intermediates would not be expected to affect the overall respiration rate: an ATP demand is needed to explain the results. A stimulation of chloroplast RNA and protein synthesis by blue light may be the basic mechanism.

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 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.

The effect of nitrate and nitrite on oxygen evolution and carbon-dioxide assimilation and the reduction of nitrate and nitrite by intact chloroplasts

Intact chloroplasts isolated from spinach reduced NO3 (-) and NO2 (-) in the light without the addition of either co-factors or added enzymes. The maximum rate observed, however, for the reduction of NO3 (-) was approximately 3μMoles hr(-1) mg(-1) (chlorophyll) and for NO2 (-) 6 μMoles hr(-1) mg(-1) (chlorophyll). These rates were consistent with the enzyme content of whole chloroplasts, but much lower than those found in whole leaf extracts.The addition of both NO3 (-) and NO2 (-) in low concentrations resulted in transient increases in both O2 evolution and CO2 fixation. The increases in oxygen evolution were not consistent in amount and bore no relation to the amount of substrate reduced. Similar transients were observed in a number of experiments when NaCl or NH4Cl were added.The addition of NO2 (-) at concentrations of 10(-4) M and above resulted in marked inhibition of both O2 evolution and CO2 fixation. NO2 (-) appears to inhibit by blocking the reduction of NADP. NO3 (-) at similar concentrations had no such effect.An increase in the soluble amino nitrogen content of the chloroplasts was observed when NO3 (-) or NO2 (-) was reduced. There was, however, no increase in the incorporation of (14)C from (14)CO2 into amino acids under these conditions. Even with the addition of ammonia the amount of (14)C incorporated into the amino acids was not changed from less than 5% of the total (14)C fixed. We conclude that while intact chloroplasts do have the ability to reduce both NO3 (-) and NO2 (-) at low rates, they do not synthesize appreciable amounts of amino acid directly, and this fact must be considered when formulating any pathways for nitrogen metabolism during photosynthesis.