VOL. 12 (1953) PHOTOCHEMICAL REDUCTION OF NITRATE 7I 



since the studies of Myers and Cramer^" provide evidence that this may be true also 

 for Chlorella. 



The cultures were aerated with a continuous stream of 5% COg in air. Some of the 

 cultures were grown in the intense red light from a neon grid, others in white light of 

 35-40 f.c. In order to obtain constant rates of photosynthesis at a given light intensity 

 it was found desirable to place the cells at low light intensity [ca 20 f.c.) for the last 

 10-12 hours of growth. Cells were harvested after 3-4 days' incubation. 



Most experiments were carried out with cells suspended in ilf/ioo-M/15 KH2PO4; 

 nitrate, when present, was added in a concentration of 0.1%. Measurements of Oo evolu- 

 tion and CO 2 assimilation were performed by two different procedures with concordant 

 results. The methods employed were: A. The two- vessel method of Warburg, based on 

 the different solubihties of Og and CO 2, and requiring simultaneous readings of pressure 

 changes in two flask-manometer combinations, identical except for the relative volumes 

 of the liquid and gas phases. The gas phase in these experiments was 5-10% CO2 in air. 

 B. An alternative two-vessel method, in which the gas phase consisted of 5% CO^ in 

 N2. CO 2 assimilation was determined directly in one vessel which contained a CrClg 

 solution ("Oxsorbent") in its side arm, while O2 production was calculated from the 

 concurrently recorded pressure changes produced by a duplicate suspension in the 

 absence of Oxsorbent. 



It was observed that absorption of O^ by the CrClg solution lags behind its liberation 

 into the gas phase. Hence a small amount of O 2 is present as long as the suspensions are 

 illuminated. By measuring the pressure decrease immediately after darkening it was 

 estabhshed that, under the conditions ordinarily used, approximately 4-8 ix\ O2 remained 

 unabsorbed during the period of illumination. The inaccuracy resulting from this 

 phenomenon was eliminated by taking into account only the pressure changes recorded 

 after a situation of equilibrium had been reached, which required about 15 minutes. 



Comparison of the rate of O2 production by the cells in the acidic suspensions 

 simultaneously with that in M/io carbonate-bicarbonate buffer at pH 9.5 generally 

 showed the former to be higher, indicating lack of CO2 saturation under the conditions 

 of low (0.2%) CO2 concentration, as also reported by Warburg et al}^~^^. The reduction 

 of nitrate by cell suspensions in KH2PO4 solution did not lead to the accumulation of 

 nitrite, even in experiments with CO2-N2 mixtures as the gas phase; this is in agreement 

 with the results described by Kessler--. 



Illumination of the suspensions was achieved by means of a bank of 50-100 Watt 

 Mazda bulbs, placed below a glass window in the bottom of the thermostat. Care was 

 taken to ensure a homogeneous light field at the place where the vessels were suspended 

 in the water bath. Variations in light intensity were produced by changing the capacity 

 of the light bulbs, or the distance between the lights and the reaction flasks, or by at- 

 taching calibrated neutral filters to the bottoms of the reaction vessels. In order to obtain 

 clear-cut results, especially at low light intensities, it was found desirable to use suspen- 

 sions sufficiently thin (less than 2.5 ^d cells per ml suspension) to avoid mutual shading 

 of the cells. 



Originally it was the intention to study in detail the relation between the assimila- 

 tory rates in the presence and absence of nitrate as a function of light intensity. The 

 results of numerous experiments showed, however, that the photosynthetic characteris- 

 tics of the cell suspensions did not remain constant during the course of a run involving 

 determinations at several different light intensities. Generally, the photosynthetic rates 



References p. 73174. 



