The effect of rapamycin on biodiesel-producing protist <i>Euglena gracilis</i>

Rapamycin induces autophagy with lipid remodeling in yeast and mammalian cells. To investigate the lipid biosynthesis of <i>Euglena gracilis</i>, rapamycin was supplemented in comparison with two model algae, <i>Chlamydomonas reinhardtii</i> and <i>Cyanidioschyzon merolae</i>. In <i>Euglena</i>, rapamycin induced the reduction of chlorophylls and the accumulation of neutral lipids without deterring its cell proliferation. Its lipidomic profile revealed that the fatty acid composition did not alter by supplementing rapamycin. In <i>Chlamydomonas</i>, however, rapamycin induced serious growth inhibition as reported elsewhere. With a lower concentration of rapamycin, the alga accumulated neutral lipids without reducing chlorophylls. In <i>Cyanidioschyzon</i>, rapamycin did not increase neutral lipids but reduced its chlorophyll content. We also tested fatty acid elongase inhibitors such as pyroxasulfone or flufenacet in <i>Euglena</i> with no significant change in its neutral lipid contents. In summary, controlled supplementation of rapamycin can increase the yield of neutral lipids while the scheme is not always applicable for other algal species. Lipid profile of <i>Euglena gracilis</i> under supplementation of rapamycin. With &lt;10 µM rapamycin, the neutral lipid contents increase without deterring cell growth.