Ultrastructural characterization of lipid droplet biogenesis and microlipophagy in the pennate diatom Encyonema silesiacum

Abstract

Lipid droplets of diatoms are dynamic organelles essential for energy and lipid metabolism within the cell; however, their intracellular life cycle remains largely unexplored at the ultrastructural level. In this study, we investigated the dynamics of lipid droplets in the pennate diatom, Encyonema silesiacum, using fluorescence and transmission electron microscopy. The size and number of lipid droplets were assessed using BDP 505/515 staining. After five days of cultivation, lipid droplets had a diameter of 0.6±0.1 μm, while with an increase in the cultivation time to 25 days, the diameter decreased to 0.8±0.2 μm. In other words, with increasing culture age, the size increased, indicating stress-induced accumulation of TAGs. We suggest that lipid droplets undergo complete vacuolar engulfment, followed by morphological degradation. Analysis of literature data suggests that complete lipid droplet uptake by vacuoles represents a widespread and conserved microlipophagic pathway in diatoms. These results contribute to our understanding of the lipid metabolism diversity in diatoms.