CHARACTERIZATION OF LIPIDS PRODUCED BY PARACHLORELLA KESSLERI, CULTURED UNDER NITROGEN STARVING CONDITIONS
Résumé
This work is developed in the context of biofuel production from microalgae. Nitrogen starving Parachlorella kessleri can accumulate lipids up to 45% (w/w) of the dry matter (Montalescot, 2015). Mechanical methods, like bead milling (BM) or high pressure cell disruption (HPCD) can be used (Cohen, 1999; Montalescot, 2015; Shene, 2015) to release the oil from the cell, before fractionation by membrane processes. For a better understanding of their behavior during fractionation, a detailed profile of the neutral and polar lipids is required. Indeed, the surface properties of lipid droplets stabilized by the polar compounds deeply impact the triacylglycerides (TAG) recovery. The lipid composition of other strains was published but mainly for cultures at the end of the stationary growth phase (Chen et al., 2007; Bigogno et al., 2002; Alonso et al., 1998). It has not been studied with such details for disrupted P. kessleri cultured under nitrogen starving conditions. In this work, non-disrupted and disrupted cells samples were characterized, as well as the supernatant of the latter obtained by centrifugation. Lipids were extracted with a modified Bligh and Dyer method, using CHCl 3 /MeOH (2:1, v/v). Total fatty acids (FA) were analyzed by GC-FID. The lipids classes separation (neutral and polar lipids) was performed by solid-phase extraction and analyzed by RP-HPLC and by HPTLC. The total FA profiles were similar for samples from the same batch, including BM and HPCD: C18:2n-6c (22-35%), C18:1n-9c (20-27%), C18:3n-3 (21-22%) and C16:0 (14-20%). The neutral lipids fraction (31-85% TAG) was up to 9 times higher than the polar one. The HPLC chromatograms present a highly complex profile. Concerning the polar fraction, several polar compounds already reported in the literature were identified: phosphatidylcholine PC (27-31%), phosphatidylethanolamine + phosphatidylglycerol + sulfoquinovosyldiacylglycerol PE+PG+SQDG (25-34%) and digalactosyldiacylglycerol DGDG (9-17%). The disruption method had an impact on the lipid class proportions. Then the formulation of a synthetic lipid mixture was deduced from analytical results to reproduce interfacial properties. This study is a first step to understand how the interfacial phenomena influence the lipids fractionation by membrane processes after disruption in a wet pathway.