Understanding the lipid production mechanism in Euglena gracilis with a fast-response AIEgen bioprobe, DPAS
Lipid bodies are lipid-rich organelles that can regulate the storage of neutral lipids as energy sources in organisms. Visualisation of lipid droplets is an effective approach to understand lipid dynamics in microalgae. This study explores the required environmental conditions to yield lipid in a microalgal species Euglena gracilis as the biofunctional component using the lipid-specific aggregation-induced emission fluorogen, DPAS (C20H16N2O)), and compares it to the commercial lipid staining probe BODIPY for visualising lipid production in vivo. Five treatments are investigated for lipid production: (1) modified Cramer-Myers medium (MCM), (2) MCM without nitrogen (-), (3) MCM without nitrogen (-) and calcium (-), (4) MCM without nitrogen (-) and calcium (-), but with glucose (+), and (5) MCM without nitrogen (-) and calcium (-), but with glucose (+). Illumination was continually at 70 mmol photons per m-2 s-1 in all treatments except no light in treatment 5. Distinctive lipid droplets are labelled by DPAS and detected with confocal microscopy and flow cytometry to clarify the understanding of lipid enrichment mechanism in various conditions. Treatment 1 indicates low lipid production in E. gracilis in an autotrophic condition. DPAS benefits from a very low background signal, and therefore is more sensitive than BODIPY for semiquantitative in vivo fluorescence measurements. Co-staining in the presence of BODIPY and chlorophyll also indicates that DPAS is suitable for multicolour imaging with red and green fluorophores. The present study demonstrates that DPAS is a highly effective biocompatible and photostable fluorophore for rapid and sensitive visualisation of lipid droplets. This novel staining method could be used to screen microalgae that have a potential to produce lipid droplets as a health supplement for humans.