A MUTANT OF NANNOCHLOROPSIS DEFICIENT IN EICOSAPENTAENOIC ACID PRODUCTION

The most abundant fatty acids found in Nannochloropsis are 16:0, 16:1 (n-7) and 20:5 (n-3). By analysing the fatty acid composition of mutagenized cells by gas chromatography, we discovered a mutant strain of Nannochloropsis that is completely devoid of 5,8,11,14,17-cis-eicosapentaenoic acid [20:5 (n-3)]. An analysis of total fatty acids from whole cells showed that the loss of 20:5 (n-3) in mutant cells was accompanied by a concomitant increase in the relative amount of 20:4 (n-6). However, the increase in 20:4 (n-6) was not proportional to the loss of 20:5 (n-3) within individual lipid classes. The percentage of 20:4 (n-6) in monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol and phosphatidylglycerol (PG) in the mutant strain was lower than the wild type levels of 20:5 (n-3) in these lipids. Conversely, the percentage of 20:4 (n-6) in phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol in the mutant cells was higher than the wild type levels of 20:5 (n-3). Molecular species analysis of MGDG showed that, compared to the wild type strain, the mutant strain contained more 16:1/16:0 MGDG, and less of the MGDG species containing C-20 fatty acids. Relatively low levels of 20:4/20:4 MGDG were found in the mutant, although 20:5/20:5 MGDG was abundant in wild type cells. Relative membrane lipid levels were altered in the mutant compared to the wild type strain, particularly PG, which showed a fourfold decrease. The mutant cells grow more slowly than the wild type cells on both solid and in liquid media. Electron micrographs showed that the mutant strain had fewer thylakoid stacks per chloroplast, and fewer thylakoids per stack than wild type cells. We concluded that the mutation affects an extrachloroplastic (n-3) desaturase that is required for the synthesis of 20:5 (n-3), which is found in both chloroplast and extrachloroplast lipids.