Citrulline has been isolated and identified from extracts of Nostoc muscorum . All members of the Cyanophyceae hitherto investigated show a relatively large amount of the CO 2 fixed during photosynthesis in citrulline (ranging as high as 20 per cent. in Nostoc ) when compared to the trace amounts found in the Chlorophyceae . Nostoc also has the ability to fix C 14 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 likely that citrulline is functioning in reactions other than those leading to arginine and urea synthesis. Other possible functions for citrulline are briefly discussed.
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Linko et al. (1957) studied this question.