PHOTOCHEMICAL NITRATE REDUCTION 253 



TABLE I. Reduction of Nitrite (moles X 10 ~«) by 25 mm.' Ankislrodesmus Cells 



in 1 Hour at pH 4.5. NaN02 (5.5 X 10"* Moles per Liter) Added to Algae in 



N-free Culture Medium. Concentration of Nitrite Measured Colorimetrically. 



Experimental Conditions Otherwise as Indicated in Fig. 1 



These findings indicate the important role of photochemical proc- 

 esses in the reduction of nitrite. It should be stressed, however, 

 that there are some marked differences between the reduction of 

 nitrite and that of an ordinary Hill reagent. As shown in Fig. 1, the 

 rate of nitrite reduction decreases faster than that of quinone reduc- 

 tion. Moreover, a second addition of quinone immediately results 

 again in a strong evolution of oxygen, whereas after a second addi- 

 tion of nitrite only a small increase of oxygen production is ob- 

 served. The rate of nitrite reduction is apparently determined by the 

 supply of carbon compounds necessary for the further assimilation 

 of the products of its reduction. In addition, isolated chloroplasts of 

 higher plants, which readily reduce quinone and other Hill reagents, 

 are unable to reduce nitrite (6,13), just as they do not reduce nitrate 

 (17) unless nitrate reductase and triphosphopyridine nucleotide are 

 added (6). 



From these results we conclude that the mechanisms of nitrate 

 and nitrite reduction by algae in the light are essentially different. 

 The first step of nitrate reduction, nitrate -»- nitrite, Avhich goes on 

 rapidly only in the presence of CO2 or of glucose, seems to be in- 

 directly accelerated by light via the photosynthetic formation of 

 carbon compounds. The further reduction of nitrite, however, 

 seems to be more closely connected to photochemical processes. 

 Recently Vanecko and Frear (18) have obtained results with higher 

 plants that point in the same direction. 



Addendum 



Recent experiments have shown that the reduction of nitrite in 

 the light without CO2 saturates at low light intensity (approximately 

 750 lux). Furthermore, this reaction is less sensitive toward diiii 

 trophenol (DNP) than is the aerobic reduction of nitrite in the dai k 



