Effects of nitrogen limitation on the growth and composition of unicellular algae in continuous culture

Since fat accumulation takes place in many algae as a response to exhaustion of the nitrogen supply, it has been suggested that this may provide a means of enhancing the potential food value of algae. To test this possibility, chemostatic continuous cultures of Chlorella sorokiniana and Oocystis polymorpha were subjected to successive reductions in influent nitrogen. As cellular nitrogen content decreased from about 10 to 4%, oxygen evolution, carbon dioxide uptake, chlorophyll content, and tissue production were drastically reduced, but total lipid content was essentially unchanged. Caloric values and C, H, and N analyses suggested a moderate increase in carbohydrate content, but gas chromatographic analyses revealed no significant qualitative or quantitative changes in the fatty acid fraction. In batch-cultured cells, nitrogen could be reduced to 3% of dry weight, causing a concomitant increase in total fatty acids and pronounced changes in the composition of the fatty acid fraction. These results suggest that cellular nitrogen must fall to approximately 3% of dry weight before appreciable increases in lipid synthesis can occur. Cell nitrogen is then apparently completely bound in essential cell constituents, and carbon subsequently fixed is converted into lipid products. The findings indicate that nitrogen limitation may be useful in increasing the food quality of batch-cultured cells, but the technique has little value for continuous culture systems per se.