The lipid metabolism in thraustochytrids

Abstract

Thraustochytrids are unicellular heterotrophic marine protists of the Stramenopile group, often considered as non-photosynthetic microalgae. They have been isolated from a wide range of habitats including deep sea, but are mostly present in waters rich in sediments and organic materials. They are abundant in mangrove forests where they are major colonizers, feeding on decaying leaves and initiating the mangrove food web. Discovered 80 years ago, they have recently attracted considerable attention due to their biotechnological potential. This interest arises from their fast growth, their specific lipid metabolism and the improvement of the genetic tools and transformation techniques. These organisms are particularly rich in ω3-docosahexaenoic acid (DHA), an 'essential' fatty acid poorly encountered in land plants and animals but required for human health. To produce their DHA they use a complex system different from the classical fatty acid synthase system. They are also a potential source of squalene and carotenoids. Here we review our current knowledge about the life cycle, ecophysiology, and metabolism of these organisms, with a particular focus on lipid dynamics. We describe the different pathways involved in lipid and fatty acid syntheses, emphasizing their specificity, and we report on the recent efforts aimed to engineer their lipid metabolism.

The Nutritional and Pharmacological Potential of New Australian Thraustochytrids Isolated from Mangrove Sediments

“…A type I fatty acid synthase pathway producing the C16 type of fatty acids is similar to the one found in plants and mammals and synthesizes a saturated version of C16 fatty acids, palmitic acid (C16:0), which is a well-known feedstock for biodiesel production. The second pathway involves the production of very-long-chain ω3and ω6-polyunsaturated fatty acids (VLC-PUFAs), such as eicosapentaenoic acid (EPA, C20:5), docosapentaenoic acid (DPA, C22:5), and docosahexaenoic acid (DHA, C22:6) [3]. The oil from Schizochytrium sp.…”

“…VLC-PUFAs are sensitive to oxidative stress and cellular responses, which can be triggered by day-night variations in salt concentrations in coastal waters of mangrove forests [3]. This may explain the evolutional development of the high level of antioxidant carotenoids in the cells of thraustochytrids.…”

The zoospores of the thraustochytrid Aurantiochytrium limacinum: Transcriptional reprogramming and lipid metabolism associated to their specific functions

“…limacinum were downregulated (Table S4). Desaturases and elongases are likely involved in the FAS pathway (for a review, see Morabito et al ., ), but FAS‐1 (e_gw1.21.366.1) was not differentially expressed. Likewise, subunits A and B of the PUFA synthase ( PUFA‐A fgenesh1_pg.14_#_251,· PUFA‐B estExt_fgenesh1_kg.C_140136) were not differentially expressed, and subunit C ( PUFA‐C estExt_fgenesh1_kg.C_190026) was slightly downregulated (Table S4).…”

“…Thraustochytrids are osmo‐saprotrophic protists belonging to the phylum Heterokonta, class Labyrinthulomycetes (Cavalier‐Smith et al ., ) and are among the first organisms that colonize and decompose a fallen mangrove leaf (Raghukumar et al ., ). The life cycle of thraustochytrids is characterized by vegetative reproduction and a motile stage, the zoospores (Honda et al ., ; Iwata et al ., ; Morabito et al ., ). The biflagellated zoospores released from sporangia are essential to colonize new environments and propagate the population.…”

“…Recently, thraustochytrids have attracted industrial interest for their ability to synthesize high levels of VLC‐PUFAs, in particular docosahexaenoic acid (DHA, 22:6), an essential compound for human health (Simopoulos, ). Indeed, thraustochytrids have the unique feature to synthesize FA via two enzymatic systems; the classical fatty acid synthase (FAS) involved in the synthesis of relatively short saturated FA chains (16–18 carbons) and the PUFA synthase involved in the synthesis of longer unsaturated chains, such as DHA (Morabito et al ., ). The biotechnological interest towards thraustochytrids has directed efforts to unravel their physiology and metabolism, with the main aim to enhance lipid production (Raghukumar, ; Taoka et al ., ; Aasen et al ., ).…”