A. Ashford et al., Electron microscopy may reveal structure of docosahexaenoic acid-rich oil within Schizochytrium sp., LIPIDS, 35(12), 2000, pp. 1377-1386
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.