Conversion of membrane lipid acyl groups to triacylglycerol and formation of lipid bodies upon nitrogen starvation in biofuel green algae Chlorella UTEX29
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Algal lipids are ideal biofuel sources. Our objective was to determine the contributors to triacylglycerol (TAG) accumulation and lipid body formation in Chlorella UTEX29 under nitrogen (N) deprivation. A fivefold increase in intracellular lipids following N starvation for 24 h confirmed the oleaginous characteristics of UTEX29. Ultrastructural studies revealed increased number of lipid bodies and decreased starch granules in N-starved cells compared to N-replete cells. Lipid bodies were observed as early as 3 h after N removal and plastids collapsed after 48 h of stress. Moreover, the identification of intracellular pyrenoids and differences in the expected nutritional requirements for Chlorella protothecoides (as UTEX29 is currently classified) led us to conduct a phylogenetic study using 18S and actin cDNA sequences. This indicated UTEX29 to be more phylogenetically related to Chlorella vulgaris. To investigate the fate of different lipids after N starvation, radiolabeling using 14C-acetate was used. A significant decrease in 14C-galactolipids and phospholipids matched the increase in 14C-TAG starting at 3 h of N starvation, consistent with acyl groups from structural lipids as sources for TAG under N starvation. These results have important implications for the identification of key steps controlling oil accumulation in N-starved biofuel algae and demonstrate membrane recycling during lipid body formation.
KeywordsBiofuel Chlorella Galactolipid Radiolabeling Triacylglycerol Ultrastructure
We thank Dr. Byung-Ho Kang, Karen Kelley, and Kim Backer-Kelley (University of Florida, Interdisciplinary Center for Biotechnology Research, Electron Microscopy and Bio-Imaging lab) for help with transmission electron microscopy. The HPLC–MS-MS analyses of lipid fractions were done at Chemistry Department, University of Florida. EG thanks the Plant Molecular and Cellular Biology program, the College of Agriculture and Life Sciences and the Horticultural Sciences Department, University of Florida for a graduate research assistantship.
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