Abstract
In recent years, the versatile phototrophic protist Euglena gracilis has emerged as an interesting candidate for application-driven research and commercialisation, as it is an excellent source of dietary protein, pro(vitamins), lipids, and the β-1,3-glucan paramylon only found in euglenoids. From these, paramylon is already marketed as an immunostimulatory agent in nutraceuticals. Bioproducts from E. gracilis can be produced under various cultivation conditions discussed in this review, and their yields are relatively high when compared with those achieved in microalgal systems. Future challenges include achieving the economy of large-scale cultivation. Recent insights into the complex metabolism of E. gracilis have highlighted unique metabolic pathways, which could provide new leads for product enhancement by genetic modification of the organism. Also, development of molecular tools for strain improvement are emerging rapidly, making E. gracilis a noteworthy challenger for microalgae such as Chlorella spp. and their products currently on the market.
Highlights
The unicellular phototrophic protist E. gracilis is ubiquitous in most freshwater biotopes
Relevant bioproducts synthesised by E. gracilis feature protein containing essential amino acids, pro(vitamins), lipids, and the β-1,3-glucan paramylon (Takeyama et al, 1997; Rodríguez-Zavala et al, 2010; Pollak et al, 2012)
Euglena gracilis can accumulate large amounts of the reserve polysaccharide paramylon, a β-1,3glucan, which can constitute over 80% (w/w) of the dry weight (DW, biomass dried to a constant weight without oxidation) (Barsanti et al, 2001; Sun et al, 2018)
Summary
The unicellular phototrophic protist E. gracilis is ubiquitous in most freshwater biotopes. Microalgae can accumulate large quantities of proteins intracellularly and represent an attractive alternative to more traditional sources of dietary protein like meat and fish (Henchion et al, 2017; Ritala et al, 2017) In this context, E. gracilis has been shown to produce protein containing all 20 proteinogenic amino acids (Isegawa et al, 1993; RodríguezZavala et al, 2010; Hasan et al, 2017). In an RNAi experiment, the latter enzyme has been shown to be essential for the ascorbatedependent growth of the organism These enzymes reportedly play a key role in E. gracilis ascorbate synthesis, a comparison between radiotracer studies indicated that the pathway flux is controlled by the epimerisation reaction catalysed by the uridine diphosphate D-glucuronate 4-epimerase (Shigeoka et al, 1979; Ishikawa et al, 2006; Ishikawa and Shigeoka, 2008). Maximum production titre or concentration in alternative source organism or product
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