Synthesis of Organic Nitrogen and Chlorophyll by Nitzschia Closterium

When cells of Chlorella are grown in a medium containing a limiting quantity of nitrate or ammonia as nitrogen source, photosynthesis and cell division continue after all the available nitrogen in the medium has been abstracted by the growing plants. The cells become nitrogen-deficient. When kept in darkness with added nitrate these nitrogen-deficient cells synthesize a considerable quantity of organic nitrogen (Ketchum, 1939) and use more oxygen in respiration than when kept in darkness without added nitrogen (Myers & Cramer, 1948, p. 106). Growing in darkness with glucose as a carbon source, they use more oxygen when supplied with nitrate than when supplied with ammonium, and even more carbon dioxide is evolved (Cramer & Myers, 1948, fig. I), indicating a greater break-down of carbohydrate to supply the necessary energy for synthesis of organic nitrogen compounds from nitrate than from ammonium. These observations of nitrogen metabolism in Chlorella have suggested that diatoms, and phytoplankton generally, may become nitrogen-deficient when the nitrogen source is reduced to very low concentrations, such as occur in the sea during summer, and in consequence are able to absorb and build up organic nitrogen during the night (Harvey, 1945, p. 133). In order to obtain more information concerning nitrogen metabolism in phytoplankton the following experiments were made. The marine diatom Nitzschia closterium var. minutissima, free from bacteria, was inoculated into autoclaved sea water of 26 %0 salinity enriched with phosphate, iron, manganese and a limiting quantity of nitrate or nitrite or ammonia. The cultures were aerated and illuminated continuously by fluorescent lamps. Under these conditions, after a short lag period depending upon the physiological state of the inoculum, exponential growth proceeds, slowing after the nitrogen source in solution has been absorbed by the cells and stopping some 2 days later, when the cells attain a 'stationary state', being fully deficient in nitrogen. When these' stationary' deficient cells were stored in darkness for periods up to 120 hr., there was no significant change in cell numbers, optical density (turbidity) of the culture, content of cellular nitrogen or of chlorophyll. On the other hand, when illumination was continued a slow decrease in chlorophyll occurred.