Electron microscopy may reveal structure of docosahexaenoic acid-rich oil within Schizochytrium sp.

Schizochytrium sp. is an algae-like microorganism utilized for commercial p roduction of docosahexaenoic acid (DHA)-rich oil and dried microalgae for u se as a source of DHA in foods, feeds, and nutritional supplements. Electro n microscopic analysis of whole cells of Schizochytrium sp. employing sampl e preparation by high-pressure freeze substitution suggests the presence of secondary and tertiary semicrystalline structures of triacylglycerols with in the oil bodies in Schizochytrium sp. A fine secondary structure consisti ng of alternating light- and dark-staining bands was observed inside the oi l bodies. Dark bands were 29 +/- 1 Angstrom in width, and light bands were 22 +/- 1 Angstrom in width. The tertiary (three-dimensional) structure may be a multilayered ribbon-like structure which appears coiled and interlaced within the oil body. In freeze-fracture photomicrographs, Schizochytrium o il bodies exhibited fracture planes with terraces averaging 52 +/- 7 Angstr om in height which could correspond to the combined width of two halves of two light bands and one dark band observed in the high-pressure freeze subs titution photomicrographs. The results suggest that triacylglycerols within Schizochytrium sp. oil bodies may be organized in a triple chain-length st ructure. High-pressure freeze substitution electron micrographs of two othe r highly unsaturated oil-producing species of microalgae, Thraustochytrium sp. and Isochrysis galbana, also revealed this fine structure, whereas micr oalgae containing a higher proportion of saturated oil did not. The results suggest that the staining pattern is not an artifact of preparation and th at the triple chain-length conformation of triacylglycerols in Schizochytri um sp. oil bodies may be caused by the unique fatty acid composition of the triacylglycerols.