I 



70 C. B. VAN NIEL, M. B. ALLEN, B. E. WRIGHT VOL. 12 (1953) 



O2 production and light intensity should persist over a longer range when two simul- 

 taneously reducible substrates are present 

 than when either one alone is available. 

 Ideally, one might expect to observe an 

 additive effect. 



Less can be said with any degree of 

 certainty about the quotient CO 2/O 2 during 

 photosynthesis at different light intensi- 

 ties. This ratio will depend upon the 

 relative efficacy of the enzyme systems 

 involved in the reduction of the different 

 hydrogen acceptors, competing for the 

 hydrogen from E'H. It should, however, 



be clear that at a sufficientlv high light 



Fig. I. Relation between rate of photosynthesis . . •"j-ii: 



(R) and Hght intensity (I) in the presence and intensity the rate of reduction of each of 



absence of nitrate. Curve i: Nitrate absent; the components will become independent 



O production or CO assimilation. Curve 2; ^^ ^^^ presence of others. 

 Nitrate present; O^ production. Curve 3 : Nitrate ^ 



present; COg assimilation. 



APPLICATION TO THE PROBLEM OF NITRATE REDUCTION 



There are some reports in the literature which clearly show that photosynthesizing 

 algae can reduce nitrate and CO2 simultaneously, and that, even in experiments of 

 short duration, the nitrate reduction may attain considerable magnitude. Particularly 

 the studies of Cramer and Myers^^'^'' have contributed much in this direction, having 

 demonstrated that the quotient CO2/O2, measured at low intensities of illumination, is 

 very much smaller in the presence (0.6-0.8) than the absence of nitrate (0.94-1.0). It 

 was established that under these conditions the rate of O2 production was identical in 

 the two cases^^. This is in agreement with the above considerations. 



Furthermore, the fact that the CO2/O2 ratio in the presence of nitrate increased 

 from 0.7 at low, to 0.86 at high light intensity^'^ indicates that the rate of nitrate reduction 

 was relatively greater in the former case. But measurements of the absolute rate of Og 

 evolution at high light intensity were not reported. From the point of view of an inter- 

 pretation of the mechanism underlying the phenomenon of photochemical O2 production 

 in the presence of nitrate it is, however, especially this value that is of great significance. 

 As the analysis of the hypothetical situation in which two different acceptors compete 

 for the hydrogen from E'H has shown, one might predict that in strong light the rate of 

 O2 evolution in the presence of both nitrate and CO2 should exceed that measured in the 

 presence of CO2 alone. Such a result would effectively eliminate the possibility that Og 

 liberation in nitrate solutions is the result of CO 2 production from cell material and 

 nitrate, since measurements could be conducted under conditions of COg saturation. An 

 extension of the observations of Cramer and Myers therefore seemed desirable. 



A pure culture of Chlorclla pyrenoidosa was used for the experiments. The algae were 

 grown in Gaffron's medium or in modified Knop's solution, as described by Myers^^. 

 In both cases nitrate was the only nitrogen source, since it is well known that many 

 micro-organisms, potentially capable of reducing nitrate, fail to do so when they have 

 been harvested from cultures in media containing reduced nitrogen compounds, and 



References p. 73I74. 



