Euglena Central Metabolic Pathways and Their Subcellular Locations

Transcriptomic response analysis of ultraviolet mutagenesis combined with high carbon acclimation to promote photosynthetic carbon assimilation in Euglena gracilis

The potential of Euglena gracilis for carbon sequestration offers significant opportunities in the capture and utilization of carbon dioxide (CO2). In this study, a mutant LE-ZW of E. gracilis, capable of efficient growth and carbon sequestration, was obtained through ultraviolet mutagenesis combined with high carbon acclimation. Subsequently, the potential of LE-ZW for carbon assimilation was systematically analyzed. The results demonstrated that the cell density of the LE-ZW was 1.33 times that of the wild type and its carbon sequestration efficiency was 6.67 times that of the wild type when cultured at an optimal CO2 concentration of 5% until day 10. At this time, most key enzyme genes associated with the photosystem membrane protein complex, photosynthetic electron transport chain, antenna protein, and carbon fixation were up-regulated in mutant LE-ZW. Furthermore, after 10 days of culture under 10% CO2, the cell density and carbon sequestration efficiency of LE-ZW reached 1.10 times and 1.54 times of that under 5% CO2, respectively. Transcriptome analysis revealed significant up-regulation of key enzyme genes associated with carbon fixation, central carbon metabolism, and photosynthesis in LE-ZW under a 10% CO2 concentration. Physiological indices such as the amount of oxygen evolution, the values of Fv/Fm, the expression levels of photosynthetic protein genes and the enzyme activity of key enzymes related to photosynthetic carbon assimilation were corroborated by transcriptome data, elucidating that the mutant LE-ZW exhibited augmented photosynthetic carbon sequestration capacity and metabolic activity, thereby demonstrating robust adaptability to a high-carbon environment. This research contributes to a deeper understanding of the carbon assimilation mechanism in photosynthetic protists under elevated CO2 concentrations.

Metabolic responses of Euglena gracilis under photoheterotrophic and heterotrophic conditions

The protist Euglena gracilis has various trophic modes including heterotrophy and photoheterotrophy. To investigate how cultivation mode influences metabolic regulation, the chemical composition of cellular metabolites of Euglena gracilis grown under heterotrophic and photoheterotrophic conditions was monitored from the early exponential phase to the mid-stationary phase using two different techniques, i.e, nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectrometry (HRMS). The combined metabolomics approach allowed an in-depth understanding of the mechanism of photoheterotrophic and heterotrophic growth for biomolecule production. Heterotrophic conditions promoted the production of polar amino and oxygenated compounds such as proteins and polyphenol compounds, especially at the end of the exponential phase while photoheterotrophic cells enhanced the production of organoheterocyclic compounds, carbohydrates, and alkaloids.

CO2 gradient domestication improved high-concentration CO2 tolerance and photoautotrophic growth of Euglena gracilis

In order to improve CO₂ biofixation efficiency of microalgae cultivated with coal-chemical flue gas, CO₂ gradient domestication was employed to improve high-concentration CO₂ tolerance and photoautotrophic growth of acid-tolerant Euglena gracilis. The dried biomass yield of photoautotrophic growth of E.gracilis increased from 1.09 g/L (wild-type strain) by 21 % to 1.32 g/L with CO₂ gradient domestication to 15 % CO₂. The RuBisCO activity and biomass production of E.gracilis strain domesticated to 99 % CO₂ were 2.63 and 3.4 times higher, respectively, than those of wild-type strain. The chlorophyll a and b contents were 2.52 and 1.79 times higher, respectively, than those of wild-type strain. Superoxide dismutase and catalase activities of 99 % CO₂-domesticated strain increased to 1.24 and 6 times, which reduced peroxide damage under high carbon stress and resulted in lower apoptotic and necrotic rates of domesticated strain. Thus, this work provides valuable guidance for CO₂ fixation and adaptive evolution of E. gracilis in industrial flue gas.

Metabolic network reconstruction of Euglena gracilis: Current state, challenges, and applications

A metabolic model, representing all biochemical reactions in a cell, is a prerequisite for several approaches in systems biology used to explore the metabolic phenotype of an organism. Despite the use of Euglena in diverse industrial applications and as a biological model, there is limited understanding of its metabolic network capacity. The unavailability of the completed genome data and the highly complex evolution of Euglena are significant obstacles to the reconstruction and analysis of its genome-scale metabolic model. In this mini-review, we discuss the current state and challenges of metabolic network reconstruction in Euglena gracilis. We have collated and present the available relevant data for the metabolic network reconstruction of E. gracilis, which could be used to improve the quality of the metabolic model of E. gracilis. Furthermore, we deliver the potential applications of the model in metabolic engineering. Altogether, it is supposed that this mini-review would facilitate the investigation of metabolic networks in Euglena and further lay out a direction for model-assisted metabolic engineering.

Using chemometric models to predict the biosorption of low levels of dysprosium by Euglena gracilis

The critical rare earth element dysprosium (Dy) is integral for sustainable technologies. What is concerning is that Dy is in imminent short supply and no current replacements yet exist, coupled with increasing environmental Dy levels influenced by anthropogenic activities. This study applies chemometric methods such as response surface methodology and artificial neural networks to predict low Dy removal levels using the biosorbent Euglena gracilis. A three-factor Box-Behnken experimental design was conducted with initial concentration (1 to 100 µg L^-1), contact time (30 to 180 min), and pH (3 to 8) as the three independent variables, and percentage removal and sorption capacity (q) as dependent variables. Using Dy percentage removal as response, for the worst and best conditions ranged from 0 to 92% respectively, with an average removal of 66 ± 4%. Using sorption capacity (q) as a different response variable, q varied from 0 to 93 µg/g with 27 ± 4 µg/g capacity as average. Maximum removal was 92% (q = 93 µg/g) was at pH 3, a contact time of 105 min and at a concentration of 100 µg/L. Using sorption capacity as the response variable for ANOVA, pH and metal concentrations were statistically significant factors, with lower pH and higher metal concentration having improved Dy removal, with a desirability near 1. Statistical tests such as analysis of variance, lack-of-fit, and coefficient of determination (R²) confirmed model validity. A 3-10-1 ANN network array was used to model experimental responses (q). RSM and ANN effectively modeled Dy biosorption. E. gracilis proved to be a cheap and effective biosorbent for Dy biosorption and has the potential to remediate acid mine drainage areas exhibiting low Dy concentrations.

Evolution and function of carbohydrate reserve biosynthesis in parasitic protists

Nearly all eukaryotic cells synthesize carbohydrate reserves, such as glycogen, starch, or low-molecular-weight oligosaccharides. However, a number of parasitic protists have lost this capacity while others have lost, and subsequently evolved, entirely new pathways. Recent studies suggest that retention, loss, or acquisition of these pathways in different protists is intimately linked to their lifestyle. In particular, parasites with carbohydrate reserves often establish long-lived chronic infections and/or produce environmental cysts, whereas loss of these pathways is associated with parasites that have highly proliferative and metabolically active life-cycle stages. The evolution of mannogen biosynthesis in Leishmania and related parasites indicates that these pathways have played a role in defining the host range and niches occupied by some protists.

A strategic approach to apply bacterial substances for increasing metabolite productions of Euglena gracilis in the bioreactor

Bacterial extracellular polymeric substances (EPS) are promising materials that have a role in enhancing growth, metabolite production, and harvesting efficiency. However, the validity of the EPS effectiveness in scale-up cultivation of microalgae is still unknown. Therefore, in order to verify whether the bacterial metabolites work in the scale-up fermentation of microalgae, we conducted a bioreactor fermentation following the addition of bacterial EPS derived from the marine bacterium, Pseudoalteromonas sp., to Euglena gracilis. Various culture strategies (i.e., batch, glucose fed-batch, and glucose and EPS fed-batch) were conducted to maximize metabolite production of E. gracilis in scale-up cultivation. Consequently, biomass and paramylon concentrations in the continuous glucose and EPS-treated culture were enhanced by 3.0-fold and 4.2-fold (36.1 ± 1.4 g L^-1 and 25.6 ± 0.1 g L^-1), respectively, compared to the non-treated control (12.0 ± 0.3 g L^-1 and 6.1 ± 0.1 g L^-1). Also, the supplementation led to the enhanced concentrations of α-tocopherols and total fatty acids by 3.7-fold and 2.8-fold, respectively. The harvesting efficiency was enhanced in EPS-supplemented cultivation due to the flocculation of E. gracilis. To the best of our knowledge, this is the first study that verifies the effect of bacterial EPS in scale-up cultivation of microalgae. Also, our results showed the highest paramylon productivity than any other previous reports. The results obtained in this study showed that the scale-up cultivation of E. gracilis using bacterial EPS has the potential to be used as a platform to guide further increases in scale and in the industrial environment. KEY POINTS: Effect of EPS on Euglena gracilis fermentation was tested in bioreactor scale. EPS supplement was effective for the paramylon, α-tocopherol, and lipid production. EPS supplement induced the flocculation of E. gracilis.

A Uniquely Complex Mitochondrial Proteome from Euglena gracilis

Euglena gracilis is a metabolically flexible, photosynthetic, and adaptable free-living protist of considerable environmental importance and biotechnological value. By label-free liquid chromatography tandem mass spectrometry, a total of 1,786 proteins were identified from the E. gracilis purified mitochondria, representing one of the largest mitochondrial proteomes so far described. Despite this apparent complexity, protein machinery responsible for the extensive RNA editing, splicing, and processing in the sister clades diplonemids and kinetoplastids is absent. This strongly suggests that the complex mechanisms of mitochondrial gene expression in diplonemids and kinetoplastids occurred late in euglenozoan evolution, arising independently. By contrast, the alternative oxidase pathway and numerous ribosomal subunits presumed to be specific for parasitic trypanosomes are present in E. gracilis. We investigated the evolution of unexplored protein families, including import complexes, cristae formation proteins, and translation termination factors, as well as canonical and unique metabolic pathways. We additionally compare this mitoproteome with the transcriptome of Eutreptiella gymnastica, illuminating conserved features of Euglenida mitochondria as well as those exclusive to E. gracilis. This is the first mitochondrial proteome of a free-living protist from the Excavata and one of few available for protists as a whole. This study alters our views of the evolution of the mitochondrion and indicates early emergence of complexity within euglenozoan mitochondria, independent of parasitism.

Molecular characterization of water extractable Euglena gracilis cellular material composition using asymmetrical flow field-flow fractionation and high-resolution mass spectrometry

Asymmetrical flow field-flow fractionation (AF4) and high-resolution Orbitrap mass spectrometry (HRMS) were used to separate and characterize cellular fractions of the dark- and light-grown Euglena gracilis cellular material. Biological replicates analyzed by HRMS shared 21-73% of commonly detected m/z values. Greater variability in shared features was found in light-grown cellular fractions (p < 0.05), likely due to small variations in growth stage. Significant differences in molecular composition were observed between AF4 cellular fractions, with dark cell fractions showing a propensity towards carbohydrate-like and tannin-like compounds, and higher double-bond equivalent (DBE) and modified aromatic index (AI_mod) were associated with light-grown cell fractions. Fractionation and high-resolution mass spectrometry aided characterization demonstrated the power of the AF4 to selectively cater to certain compounds/cellular entities with distinct compositional classes and double-bond equivalents and aromaticity index characteristics. Graphical abstract.

Metabolic quirks and the colourful history of the Euglena gracilis secondary plastid

Euglena spp. are phototrophic flagellates with considerable ecological presence and impact. Euglena gracilis harbours secondary green plastids, but an incompletely characterised proteome precludes accurate understanding of both plastid function and evolutionary history. Using subcellular fractionation, an improved sequence database and MS we determined the composition, evolutionary relationships and hence predicted functions of the E. gracilis plastid proteome. We confidently identified 1345 distinct plastid protein groups and found that at least 100 proteins represent horizontal acquisitions from organisms other than green algae or prokaryotes. Metabolic reconstruction confirmed previously studied/predicted enzymes/pathways and provided evidence for multiple unusual features, including uncoupling of carotenoid and phytol metabolism, a limited role in amino acid metabolism, and dual sets of the SUF pathway for FeS cluster assembly, one of which was acquired by lateral gene transfer from Chlamydiae. Plastid paralogues of trafficking-associated proteins potentially mediating fusion of transport vesicles with the outermost plastid membrane were identified, together with derlin-related proteins, potential translocases across the middle membrane, and an extremely simplified TIC complex. The Euglena plastid, as the product of many genomes, combines novel and conserved features of metabolism and transport.