Biogenesis and function of lipid droplets in microalgae

In single-celled photosynthetic microalgae, oil-storage organelles, known as lipid droplets (LDs), sequester carbon and energy-rich neutral lipids, mainly triacylglycerols (TAG), under adverse conditions, such as nutrient deficiency, excessive irradiance, and other environmental stresses. Furthermore, in some species, LDs accumulate high-value hydrophobic molecules. Such species play an important role in biotechnology. One prominent example is the carotenogenic microalga Hamaetococcus pluvialis, which accumulates the valuable keto-carotenoid astaxanthin in LDs, along with TAG, to over 4% of its biomass under nitrogen starvation. The major LD protein of H. pluvialis was associated with LDs when ectopically expressed in the evolutionary distinct diatom Phaeodactylum tricornutum, suggesting that mechanisms exist which enable the association of the green-algal protein with LDs in the diatom. Our current study aims to characterize the native lipid droplet proteome of P. tricornutum and identify targets for the manipulation of TAG accumulation. Insights from the proteome analysis attribute a pleothra of functions to these organelles beyond oil storage, including autophagy. In the unique arachidonic-acid producing microalga Lobosphaera incisa, LD biogenesis and degradation are accompanied by their close interactions and fusion with vacuoles in a microphagy-like manner, implying a dual function of lipophagy in LD metabolism. A set of putative orthologs of the core ATG-related proteins was identified. A hallmark of autophagy, ATG8, was upregulated at the transcript and protein levels under nitrogen deprivation. Our findings are consistent in demonstrating the pivotal and dynamic role of autophagy in orchestrating LD metabolism in microalgal cells’ responses to nutrient deprivation.