Docosahexaenoic acid accumulation in thraustochytrids: search for the rationale |
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Authors: | Ruchi Jain Seshagiri Raghukumar Kari Sambaiah Yasuyuki Kumon Toro Nakahara |
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Institution: | (1) 313, Vainguinnim Valley, Dona Paula, Goa, 403 004, India;(2) Central Food Technology Research Institute, Mysore, 500 083, India;(3) National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8566, Japan |
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Abstract: | Thraustochytrid protists characteristically accumulate high amounts of lipids, much of which is comprised of the polyunsaturated
fatty acid, docosahexaenoic acid (DHA). Since DHA is important in human health, thraustochytrids have attracted much attention
from the point of view of biotechnology. However, the biological rationale of DHA production in the storage lipids of these
organisms is not clear. We carried out three experiments to study this. (1) The fate of lipids in thraustochytrids during
development was studied by staining lipids of vegetative cells with the fluorescent vital stain for lipids, namely Nile blue,
following the growth of the cells. The fluorescent lipid bodies decreased in abundance in freshly formed motile limaciform
amoeboid cells and subsequently disappeared when they moved away. When vegetative cells produced vast extensions of plasma
membrane, the ectoplasmic net elements (EN), the fluorescent lipids appeared to be transported to them. (2) Changes in lipids
and DHA during starvation were examined in cells with enhanced lipid and DHA contents and those in which they were not enhanced.
Cells in which lipids and DHA were enhanced by refrigerating them for 48 h survived starvation for a longer period. Compared
to cells that had not been refrigerated, total fatty acids, as well as DHA were marginally higher in pre-refrigerated cells,
while palmitic acid levels were lower. Starvation of cells resulted in a gradual decrease of absolute concentrations and percentage
levels of DHA after 8 days, while percentage of palmitic acid levels increased. (3) The relationship between DHA and specific
gravity of cells was studied by comparing cells pre-refrigerated as above with those which were not pre-refrigerated. Refrigerated
cells with increased lipids and DHA showed higher specific gravities than non-refrigerated cells. We suggest the following
roles for storage lipids and DHA in thraustochytrids: (1) lipids serve as energy sources during movement of cells and for
production of EN; (2) DHA in storage lipids become distributed in the plasma membrane when EN are formed; (3) DHA is preferably
utilized as a fatty acid energy reserve during starvation and (4) an unknown mechanism results in negative buoyancy of cells
when total lipids and DHA are marginally enhanced. DHA in storage lipids might be crucial to the survival of thraustochytrid
cells. |
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