Formation of Radioactive Citrulline during Photosynthetic C14O2-Fixation by Blue-Green Algae

Citrulline has been isolated and identified from extracts ofNostoc muscorum. All members of the Cyanophyceae hitherto investigatedshow a relatively large amount of the CO₂ fixed during photosynthesisin citrulline (ranging as high as 20 per cent. in Nostoc) whencompared to the trace amounts found in the Chlorophyceae. Nostocalso has the ability to fix C¹⁴ in citrulline during dark fixation,but at a rate slower than in light. As no free urea or arginine was found in Nostoc, it is likelythat citrulline is functioning in reactions other than thoseleading to arginine and urea synthesis. Other possible functions for citrulline are briefly discussed.