Euglena gracilis

A Microbial Biorealm page on the genus Euglena gracilis

Classification

Higher order taxa

Eukaryota; Protista; Pyrrophyta; Euglenoidea; Euglenales; Euglenaceae

Species

Euglena gracilis

There are some subspecies and laboratory strains.

Description and significance

Euglena gracilis is a popular flagellated laboratory microorganism found in freshwater environments.¹ A Euglena cell is represent of one of the simplest and earliest derived eukaryotic cells. Euglena gracilis have an exquisite spiral exoskeleton called a pellicle as well as many other novel cell structures such as photosensors and endosymbiotic chloroplasts. The taxonomy of this unique organism was debated for many years. This microorganism is now characterized as a eukaryotic protist but exhibits animal as well as plant like traits.² It is chimeric microorganism, functioning both as a photoautotroph and chemoheterotroph. Euglena gracilis can grow in the dark, when loosing it's chloroplasts by using it's heterotophic metabolic strategy.² Eventually, it regains it's chloroplasts once exposed to light.¹ Probably some of the most significant reasons it is so valued, today, is for it's secondary endosymbiotic acquiring and use of it's chloroplasts via phagocytosis and it's endosymbiotic gene transfer (EGT) as a mechanism for horizontal gene transfer (HGT). Wild strains are very resilient and can endure a number of environmental stresses including a known pollutant.⁴ It's comparatively greater photoautotrophic ability of carbon fixation and heterotrophic ability make it a model organism for experimentation for global warming and space station biosphere solutions.⁵

Genome structure

Their nuclear genome is not mapped. Currently, the Euglena gracilis nuclear genome is being mapped at the University of Montreal. It's being funded by Genome Canada and Genome Quebec. Eukaryote nuclear genomes are typical composed of linear DNA. Being chimeric, Euglena gracilis DNA shows similarities to protist, fungi, animals, plant, and prokaryote DNA.⁴ When creating a non-normalized cDNA library from the commercial UTEX 753 Euglena gracilis strain and testing the possible reading frames, 61% ESTs showed similarities to proteins with known function.⁴ Thirty nine percent of ETSs showed no similarity to any other known protein.⁴ Thirty six percent of ETS showed similarities to protist proteins, 21% to plant proteins, 2% to animal proteins, 1% to fungi proteins, and 1% to prokaryote proteins.⁴ This shows major indications of interkingdom gene fusion.

Having both mitochondria and chloroplasts, Euglena gracilis does have genomic material organized outside of it's nucleus. These organelles are known to have only circular DNA. The Euglena gracilis chloroplast genome is mapped and has circular DNA in which 35% is coding with 113 genes. The genes code for 66 proteins. The chloroplast genome is 143,171 nucleotides long. These chloroplast genes are theorized to actively exchange with the the Euglena gracilis nuclear genome via EGT.³ The symbiote is green algae.

Cell structure and Metabolism

The Euglena gracilis cell is approximately 50 micrometers long and 10 micrometers wide but dimensions varies between strains.¹ The Euglena gracilis have a nucleus, mitochondria, chloroplasts, golgi bodies, lysosomes, vacuoles, an endoplasmic reticulum, an eyespot, and a few other eukaryotic organelles.¹ An eyespot is used for phototaxis. At the anterior end of the cell there is an invagination that functions to phagosytose organic molecules. This invagination is also where a flagella protrudes from. A microscope can be used to notice it's single flagella protruding out from it's anterior end, positioned adjacent to a short flagella that doesn't protrude. Euglena gracilis has a total of two flagella.

The unique structure of the Euglena pellicle, the endoskeleton, and flagella allows the Euglena genus to to move via swimming, contracting, crawling, and, or gliding.² A spiraling exoskeleton gives some of the swimming species of Euglena a helix swimming path. Some euglena species exhibit all forms of movement while others exhibit a few or one of the forms of movement.² The Euglena gracilis species, having a relativity thin and flexible pellicle, is known to use amoeboid, the crawling type, movement but also swims.² The eye spot does function to direct this swimming movement toward light but also has a role in chloroplast development.²

Euglena gracilis is capable of both photoautotrophy and chemoheterotrophy. As a chemoheterotroph, it engulfs organic molecules using aerobic respiration to metabolize them.² Euglena gracilis can use ethanol, acetate, glucose, succinic acid, lactate, glycolate, peptone, as well as other organic molecules as carbon and energy sources.² Euglena gracilis absolutely needs vitamins B1 and B12 for growth.² It also functions as a phototroph using it's engulfed green algae. As an autotroph, it's known to utilize carbon dioxide for it's carbon source. It does participate in carbon fixation producing oxygen as a by-product. Euglena gracilis require H, C, N, O, Mg, P, S, Cl, K, Ca, Mn, Co, Zn, and some other elements that it might use at very low levels.² As a phototroph, Euglena gracilis's chloroplasts have both photosystem II and I, containing chlorophylls a & b and carotene.²

Ecology

Euglena gracilis is very common and inhabits fresh water environments.² They are present in high numbers in poluted waters.⁸ Other Euglena species can be found in soil, freshwater environments, or some marine environments.⁴ Euglena gracilis can exist in marsh lands as thick mats.⁸ It produces oxygen at a high rate, can significantly reduce carbon dioxide, and breaks down organic matter.⁶

Euglena gracilis is a host to it's engulfed endosymbionte, green algae.³ The Euglena gets energy from the green algae while the endosymbiotic green algae gets the ability to movement toward light. "Euglena gracilis" are also known to engulf and consume microorganisms besides green algae.⁸

Pathology

E. gracilis is not a known pathogen. Its sister class, kinetoplastea, consist of a number of animal pathogens. One pathogen, Trypanosoma brucei causes African sleeping sickness. It's a parasite transmitted by the bite of a fly. Its symptoms include personality change, irritability, weight loss, loss of concentration, progressive confusion, slurred speech, seizures, difficulty walking, difficulty talking, sleeping for long periods during the day, and insomnia at night.⁷ Interestingly, kinetoplastea are thought to have had an endosymbiote where there was gene transfer via cryptic EGT.³

Current Research

• Euglena gracilis provides a perfect example of secondary endosymbiosis and the genetic exchange between endosymbiote and host, EGT.⁹ One hint to Euglena having this ability to take up foreign DNA lies in the work of dos Santos Ferreira et al.'s (2007) work.⁴ They isolated 1000 Euglena gracilis cDNA and showed, through expressed sequence tag (EST) analysis, that 39% unknown, 36% protist, 21% plantae, 2% animal, 1 % fungi, and 1% prokaryote proteins are encoded.⁴ The fact that the endosymbiotic green algae has lost its nuclear material could mean that Euglena have incorporated much of the green algal genome into it's own.⁹ Euglena actively phagosytose green algae. Another analysis by Ahmadinejad et al. (2007), has shown similar results but pertaining to more general groups of heterotroph, photoautotroph, kinetoplastida, and Euglena gracilis specific ESTs.⁹ These researchers have concluded that a phylogenetic tree is not the appropriate means to identify phylogeny for the Euglena gracilis. There are just too many similarities with numerous taxonomic groups. When an organelle is present that was once an endosymbiote, we can understand how there could be EGT. If there is no organelle but a phylogenetic similarity, we must look to genetic clues. Henze et al. (1995) has concluded that there is evidence supporting cryptic endosymbiosis within Euglena gracilis.³

• Euglena gracilis have photoreceptors. Their photoreceptors are used for phototaxis but also for chloroplast development.² It is essential for the processing of information. Its molecular structure and arrangement is essential in understanding how this organism processes information. Barsanti et al. (2008) explains that the Euglena gracilis photoreceptor has some similarities to photoreceptors of complex vision systems.¹⁰ The Euglena gracilis photoreceptor is highly organized and uniform.¹⁰ According to these researchers, the Euglena gracilis photoreceptor proteins could be very similar to proteorhodopsine.¹⁰

• Wild strains of Euglena gracilis are known to be a very resilient and versatile organism. This microbe can withstand acidic growing conditions, as well as some pollutants that are known to be extremely harmful to microorganisms (ie. chromium), and slightly intensified light. In fact, Euglena gracilis do best in the laboratory at a 3.5 pH.⁶ Hayashi et al. (2004) noticed that Euglena gracilis are also able to withstand ionizing radiation via a photoprotective mechanism.⁵ Being chimeric is partly what enables Euglena gracilis to be so resilient. When it's photoautotrophic capabilities are inhibited it's heterotrophic abilities can take over. Euglena gracilis also have the protective ability of a number of systems throughout the eukaryotic domain.⁴ The work of dos Santos Ferreira et al. shows that SOD genes become active when in the presence of chromium.⁴ Chromium creates reactive oxygen species. SOD produces one of the main natural antioxidant enzymes. Its indicated that this ability is part of what gives Euglena gracilis such an a resistance to chromium polution.⁴

• Being such a resilient and useful organism, some researchers have proposed that Euglena gracilis could be used as a possible solution to global warming. The wild type Euglena gracilis Z strain can grow in conditions of 40% carbon dioxide, has a photosynthetic ability 60-fold that of rice plants, and converts carbon dioxide to oxygen two times the efficiency of Chlorella.⁶ Chae et al. (2006) produced a laboratory-scale photo-bioreactor and then a pilot-scale photo-bioreactor. They found that the pilot-scale photo-bioreactor was an improvement.⁶ Since Euglena gracilis have a high protein content the carbon that was fixed could be used as animal food.⁶ It's been thought to use Euglena in space stations to produce food and oxygen.⁵

References

1. Buetow, D.E., ed. Bouck, G.B., Kempner, E.S., Bovee, E.C., Leedale, G.F., Colombetti, G., Lefort-Tran, M., Diehn, B., Lenci, F., Dubertret, G., Schiff, J.A., et al. 1982. The Biology of Euglena. Academic Press, New York.

2. Wolken, J.J. 1961. Euglena: An Experimental Organism for Biochemical and Biophysical Studies. Quinn and Boden Company, Rahway (NJ).

3. Henze, K., Badr, A., Wettern, M., Cerff, R., and Martin, W. 1995. A nuclear gene of eubacterial origin in Euglena gracilis reflects cryptic endosymbioses during protist evolution. Proceedings for the National Academy of Science, v. 92, p. 9122-9126.

5. Hayashi, H., Narumi, I., Wada, S., Kikuchi, M., Furuta, M., Uehara, K., and Watanabe, H. 2004. Light dependency of resistance to ionizing radiation in Euglena gracilis. Journal of Plant Physiology, v. 161, p. 1101-1106.

4. dos Santos Ferreira, V., Roccetta, I., Conforti, V., Bench, S., Feldman, R., and Levin, MJ. 2007. Gene expression patterns in Euglena gracilis: Insights into the cellular response to environmental stress. Gene, v. 389, p. 136-145.

6. Chae, S.R., Hwang, E.J., and Shin, H.S. 2006. Single cell protein production of Euglena gracilis and carbon dioxide fixation in an innovative photo-bioreactor. Bioresource Technology, v 97, p. 322-329.

7. Despommier, D.D., R. W., Hotez, P. J., and Knirsch, C. A. 2005. Parasitic Diseases. Apple Trees Productions, New York.

8. Pennak, R. W. 1979. Fresh-water Invertebrates of the United States. John Wiley and Sons, New York.

9. Ahmadinejad, N., Dagan, T., and Martin, W. 2007. Genome history in the symbiotic hybrid Euglena gracilis. Gene. v. 402, p. 35-39.

10. Barsanti, L., Coltelli, P., Evangelista, V., Passarelli, V., Frassanito, AM., Vesentini, N., and Gualtieri, P. 2008. Low-resolution characterization of the 3D structure of the Euglena gracilis photoreceptor. Biochemical Biophysical Research Comunications, v. 375, p. 471-476.

Edited by student of Andrew J Garcia at University of Massachusetts Dartmouth.

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