Abstract
Deriving biofuels and other lipoid products from algae is a promising future technology directly addressing global issues of atmospheric CO2 balance. To better understand the metabolism of triglyceride synthesis in algae, we examined their metabolic origins in the model species, Coccomyxa subellipsoidea C169, using stable isotopic labeling. Labeling patterns arising from [U-13C]glucose, 13CO2, or D2O supplementation were analyzed by GC-MS and/or LC-MS over time courses during nitrogen starvation to address the roles of catabolic carbon recycling, acyl chain redistribution, and de novo fatty acid (FA) synthesis during the expansion of the lipid bodies. The metabolic origin of stress-induced triglyceride was found to be a continuous 8:2 ratio between de novo synthesized FA and acyl chain transfer from pre-stressed membrane lipids with little input from lipid remodeling. Membrane lipids were continually synthesized with associated acyl chain editing during nitrogen stress, in contrast to an overall decrease in total membrane lipid. The incorporation rates of de novo synthesized FA into lipid classes were measured over a time course of nitrogen starvation. The synthesis of triglycerides, phospholipids, and galactolipids followed a two-stage pattern where nitrogen starvation resulted in a 2.5-fold increase followed by a gradual decline. Acyl chain flux into membrane lipids was dominant in the first stage followed by triglycerides. These data indicate that the level of metabolic control that determines acyl chain flux between membrane lipids and triglycerides during nitrogen stress relies primarily on the Kennedy pathway and de novo FA synthesis with limited, defined input from acyl editing reactions.
Highlights
Oil production from algal feedstocks is a promising answer to the dwindling supply of extractable petroleum, one that is readily transferrable to the current transportation infrastructure and recycling of CO2 emissions
This approach facilitated study of acyl chain transfer from membrane lipids that could contribute to TG synthesis
The absolute quantities decreased during nitrogen starvation, 51 and 34% of the FA in the GL and PL pools, respectively, were synthesized during nitrogen stress. These results indicate that the influx of de novo fatty acids was comparable with the efflux of acyl chains from the PL and GL pools; there was discrepancy in the amount of PL synthesized between experiments
Summary
Oil production from algal feedstocks is a promising answer to the dwindling supply of extractable petroleum, one that is readily transferrable to the current transportation infrastructure and recycling of CO2 emissions. Oil accumulation is linearly correlated with, and may be caused by, a cessation or slowing of the cell cycle [2,3,4]. Acyl editing reactions include the Lands cycle [8], governing the removal of an unsaturated sn-2 position acyl moiety of phosphatidylcholine (PC) by hydrolysis via phospholipase A2 (PLA2) and replaced from the acyl-CoA pool by lyso-phosphatidylcholine acyltransferase (LPCAT). The reconversion of lyso-phosphatidylcholine generated by PDAT into a PC requires LPCAT as well, making it a central enzyme in acyl editing reactions. In contrast to acyl editing, lipid remodeling reactions alter membranes by removing and replacing the polar head group.
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