Lipids and lipid metabolism in the microaerobic free-living diplomonad Hexamita sp.

Hexamita sp. is a free-living heterotrophic flagellate ubiquitous in microaerobic environments. It is one of the deepest branching extant eukaryotes, lacking mitochondria and probably an energy-yielding oxidative metabolism. Hexamita is cultured in a medium supplemented with foetal calf serum, a ready supply of lipids essential for growth. This study has shown that Hexamita does not depend entirely on exogenous lipid supplies and is able to incorporate [1-14C] acetate, albeit poorly, into its fatty acids, phospholipids and sterols. The major polar lipids detected were phosphatidylethanolamine, phosphatidylglycerol, phosphatidylcholine, sphingomyelin and cerebrosides. Triacylglycerols were identified as the main storage lipids together with sterol esters. The major lipids identified in Hexamita reflect the lipids detected in the culture medium (with the exception of phosphatidylethanolamine); these lipids are believed to have been sequestered directly from the culture medium. It is possible that phosphatidylethanolamine was synthesised by Hexamita via phosphotransferase pathways. Incubation with radiolabelled palmitate, oleate or stearate showed that no further modifications (e.g. desaturation or elongation) had taken place.