- Treating the red alga, Porphyridium cruentum, with the herbicide, SAN 9785, an inhibitor of chloroplastic ω3-desaturation, resulted in a decrease in the proportion of eicosapentaenoic acid (EPA), especially in monogalactosyldiacylglycerols (MGDG). However, when the eustigmatophyte, Monodus subterraneus, was treated with this herbicide, the proportion of EPA in MGDG surprisingly increased from 54 to 81%. Molecular species analysis of MGDG, showed that the prokaryotic molecular species 20:5/16:0 was almost eliminated, while the eukaryotic 20:5/20:5 dominated. These findings indicate that EPA biosynthesis in M. subterraneous is different from that in P. cruentum. We suggest that in M. subterraneus there are at least two pathways involving ω3-desaturase activity which lead to the production of EPA. The chloroplastic pathway, which is inhibited by SAN 9785, and the cytoplasmic pathway, which presumably uses phospholipid-bound fatty acids and is not affected by this inhibitor. The product of the latter pathway is exported through the chloroplast membrane resulting in the production of 20:5/20:5 MGDG. We further suggest that, by analogy with higher plants, microalgae can be categorized according to their respective EPA biosynthetic pathways.