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The genus Ulkenia is characterized by the naked protoplast stage within its life cycle. However, the 18S rRNA gene tree clearly shows that this genus is not a natural taxon, because our own isolates and reported strains separately form four well-supported monophyletic groups. These four groups are clearly distinguishable by their profi les of polyunsaturated fatty acids and carotenoid pigments and cell and colony morphology, e.g., persistence of sporangial wall, manner of the cell cleavage at the zoospore formation, and development of the ectoplasmic nets. Therefore, the four groups are assigned to four genera including three new genera, i.e., Ulkenia sensu stricto, Botryochytrium, Parietichytrium, and Sicyoidochytrium gen. nov.

We show here that a new labyrinthulid strain, L72, isolated from a fallen leaf in the Seto Inland Sea of Japan, produced only docohexaenoic acid (DHA) among all the long-chain polyunsaturated fatty acids (LCPUFAs). The main fatty acid composition was 16:0 (28.9%), 18:0 (7.2%), 18:1 (5.7%), 18:2 (10.4%), and DHA (45.9%) without any other LCPUFA. The lipid content of the strain was 27.4%. The cells had many lipid bodies, which were densely located in all of the cells. On phylogenetic analysis using the 18S rDNA sequence, the strain was located in the labyrinthulids group, forming a monophyletic group with Labyrinthula sp. (strain s) and Labyrinthuila sp. (strain L59). We further tested the culture optimization of strain L72 to evaluate the ability of the strain to produce DHA. The optimum salt concentration and the temperature of the strain were 100% of artificial seawater and 20 degrees C. Strain L72 could grow well on soybean oil (SBO) or soybean lecithin (SBL) as the carbon source. When 20 g/l of SBL was added to the medium, DHA production reached the maximum amount at 0.67 g/l for 14 d. The two important facts, that the strain can use SBL as the main nutrient and contains only DHA among the LCPUFAs, will be of great advantage for industry.

A labyrinthulid strain, L59, was isolated from a leaf floating on seawater collected at the coastal area of Hokkaido Prefecture, Japan. Strain L59 contained only n-6 docosapentaenoic acid ( n-6 DPA) among all the long-chain polyunsaturated fatty acids. The proportion of n-6 DPA in the total fatty acids was 48.1% and the total fatty acids content in the cell dry weight was 26.6%. Many oil bodies were observed in the cell, mostly in the vicinity of cell membranes. The strain had spindle-shaped cell bodies and all cells were surrounded by ectoplasmic net elements. It was also clearly classified in the labyrinthulid group by phylogenetic analysis. In the optimum culture condition, using soybean oil and peptone as carbon and nitrogen sources, 0.53 g of n-6 DPA/l was produced at 20 degrees C in 7 days.