Lipid Droplets in Unicellular Photosynthetic Stramenopiles

Abstract

The Heterokonta or Stramenopile phylum comprises clades of unicellular photosynthetic species, which are promising for a broad range of biotechnological applications, based on their capacity to capture atmospheric CO2 via photosynthesis and produce biomolecules of interest. These molecules include triacylglycerol (TAG) loaded inside specific cytosolic bodies, called the lipid droplets (LDs). Understanding TAG production and LD biogenesis and function in photosynthetic stramenopiles is therefore essential, and is mostly based on the study of a few emerging models, such as the pennate diatom Phaeodactylum tricornutum and eustigmatophytes, such as Nannochloropsis and Microchloropsis species. The biogenesis of cytosolic LD usually occurs at the level of the endoplasmic reticulum. However, stramenopile cells contain a complex plastid deriving from a secondary endosymbiosis, limited by four membranes, the outermost one being connected to the endomembrane system. Recent cell imaging and proteomic studies suggest that at least some cytosolic LDs might be associated to the surface of the complex plastid, via still uncharacterized contact sites. The carbon length and number of double bonds of the acyl groups contained in the TAG molecules depend on their origin. De novo synthesis produces long-chain saturated or monounsaturated fatty acids (SFA, MUFA), whereas subsequent maturation processes lead to very long-chain polyunsaturated FA (VLC-PUFA). TAG composition in SFA, MUFA, and VLC-PUFA reflects therefore the metabolic context that gave rise to the formation of the LD, either via an early partitioning of carbon following FA de novo synthesis and/or a recycling of FA from membrane lipids, e.g., plastid galactolipids or endomembrane phosphor- or betaine lipids. In this review, we address the relationship between cytosolic LDs and the complex membrane compartmentalization within stramenopile cells, the metabolic routes leading to TAG accumulation, and the physiological conditions that trigger LD production, in response to various environmental factors.

Active reconfiguration of cytoplasmic lipid droplets governs migration of nutrient-limited phytoplankton

“…Energy-rich starch and lipid bodies maintain phytoplankton fitness under nutrient limitation (26)(27)(28). Persistent limitation transforms heavier, short-lived starch bodies into lighter cytoplasmic lipid droplets (LDs), retaining key metabolic processes including amelioration of physiological stress (41)(42)(43). LDs are formed as nutrient levels dip, supplementing cells with energy-rich units, which are used during nutrient-depleted conditions, or for specific functional requirements, for instance, to alleviate high phospholipid demands during formation of membranes (42).…”

“…Over prolonged durations of nutrient limitation, heavier, short-lived starch bodies transform into lighter cytoplasmic lipid droplets (LDs). Though LD synthesis and maintenance come at the cost of immediate growth and division, LDs facilitate key metabolic processes during nutrient limitation, including crucially, amelioration of physiological stress (36)(37)(38). Larger storage capacity is potentially beneficial when nutrients are scarce (39,40), however migration can offset competitive advantages therefrom by enhancing encounters and expanding the available resource pool (35).…”

Lipid Constituents of Diatoms (Halamphora) as Components for Production of Lipid Nanoparticles

“…In addition to being food for aquatic grazers, diatomaceous lipids can have numerous applications as they can be easily converted into biodiesel [8] and many more substances, see the summary in [9]. The ability to store lipid droplets is unique to diatoms and a few other Stramenopiles (e.g., [10]).…”

Circulating Mitochondrial DNA and Inter-Organelle Contact Sites in Aging and Associated Conditions

“…Though for long time considered just an inert cytoplasmatic inclusion of fat, in recent years lipid droplets (LDs) have begun to be considered as full-fledged organelles with several functions [ 136 ]. Every eukaryotic cell contains LDs and their main function is the storage of fatty acids in the form of neutral lipids, mostly triacylglycerols and sterol esters, and hydrolysis [ 137 ]. To carry out this function, they possess a unique structure encompassing a core of neutral lipids surrounded by a phospholipid monolayer [ 138 ].…”