1. Nitrate reduction and assimilation have been studied in Chlorella pyrenoidosa under growth conditions by observing effects on the CO2/O2 gas exchange quotient.

2. During assimilation of glucose in the dark, nitrate reduction is noted as an increase in the R.Q. to about 1.6 caused by an increased rate of carbon dioxide production.

3. During photosynthesis at low light intensity nitrate reduction is evidenced by a reduction in the CO2O2 quotient to about 0.7 caused by a decreased rate of carbon dioxide uptake.

4. Chlorella will assimilate nitrogen from either nitrate or ammonia. When both sources are supplied, only ammonia is utilized and no nitrate reduction occurs. It is inferred that under the usual conditions of growth nitrate is reduced only at a rate required for subsequent cellular syntheses. The effect of nitrate reduction on the CO2O2 quotient therefore provides a measure of the relative rate of nitrogen assimilation.

5. Over-all photosynthetic metabolism may be described from elementary analysis of the cells since excretory products are negligible. The gas exchange predicted in this way is in good agreement with the observed CO2/O2 quotients.

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