- The biosynthetic pathways of eicosapentaenoic acid (20: 5n-3) in microalgae, in general, and in Porphyridium cruentum, in particular, are not known. Some of the putative intermediates along the suggested pathways could not be detected probably due to their low endogenous level. In order to increase the endogenous levels of the intermediates, we provided various fatty acids in the growth medium. Exogenously supplied fatty acids were indeed incorporated into algal lipids and were further metabolized along the n-6 and n-3 pathways. In the n-6 pathway, 18: 2 was desaturated to 18: 3n-6, elongated to 20: 3n-6, and subsequently desaturated to 20: 4n-6 and then to 20: 5n-3. In the n-3 pathway, 18: 2 was first desaturated to 18: 3n-3 which was then sequentially converted, apparently by the same enzymatic sequence of the n-6 pathway to 18: 4n-3, 20: 4n-3, and 20: 5n-3.