Euglena gracilis: A Closer Look

by Tina Soltani

Take a deep breath. As you inhale, oxygen enters into your lungs and carbon dioxide is then exhaled. Organisms like us require this oxygen for our everyday function. Most of us are generally aware that oxygen comes from trees and plants. But what many people are probably not aware of is that many microbes are also responsible for the oxygen we breathe. Through photosynthesis, phytoplanktons, such as diatoms and dinoflagellates, and euglenoids, like Euglena gracilis, contribute to the oxygen supply found on Earth. E. gracilis’s ability to photosynthesize allows it to consume carbon dioxide and release oxygen, which we then breathe. With carbon dioxide levels rising every year and contributing to global warming, microbes, such as E. gracilis, are essential in combating this issue. A species as versatile as this deserves a more in-depth analysis. Now take another breath and read on. 

Euglena gracilis in its free-living flagellated state (8)    

Euglena gracilis are free-living flagellated protists and contain chloroplasts; they are not known pathogens. They are part of one of the most primitive eukaryotic groups, the euglenoids. They are primarily found in freshwater habitats, but they can also inhabit marine and soil environments. E. gracilis tends to favor more alkaline environments; neutral or acidic environments are not conducive for nutrition or reproduction, although they can withstand such conditions (1). An interesting fact about E. gracilis: it is both chemoheterotrophic and photoautotrophic, meaning that it can not only consume organic molecules around it but it can produce its own food source as well. As a chemoheterotroph, E. gracilis is able to utilize a number of organic molecules such as ethanol, lactate, glucose, lactose and malate as carbon sources. As a photoautotroph, it utilizes carbon dioxide as its carbon source when undergoing photosynthesis (2,3). One of the most fascinating attributes of E. gracilis is its reaction to light beyond food production. E. gracilis is well known for its ability to lose chloroplasts when grown under constant darkness conditions, but then regain this ability when exposed to light again. When grown in the dark, they lose their chlorophylls and the chloroplasts regress to form proplastids; upon exposure to light they can re-differentiate chloroplasts (3).

There is no known sexual reproduction in E. gracilis. They usually reproduce by longitudinal binary fission in either a free-swimming or encysted state. If conditions are favorable, reproduction usually occurs in the free-swimming state, whereas if the environment is too hot or lacks water or the proper nutrients, E. gracilis resorts to an encysted state and remains that way until conditions become favorable again (1). The genome of E. gracilis has not been fully mapped yet. There has been difficulty conclusively classifying E. gracilis; they share similarities to protists, fungi animals and plants. Santos Ferreira et al were able to determine using expressed sequence tags (ESTs) of E. gracilis cDNAs that about 61% of these ESTs had similarities with proteins of known function. A breakdown of the known 61% of ESTs revealed that these proteins have similarities to different group classifications – 36% protist, 21% plant, 2% animal, 1% fungi and 1% prokaryote (4). E. gracilis is thought to have an endosymbiotic relationship with green algae, possibly due to the consumption of a eukaryotic alga. E. gracilis is well suited for the study of endosymbiosis and endosymbiotic gene transfer (EGT) because its plastid was acquired by secondary endosymbiosis, but no remains of the endosymbiotic nucleus is present (5). Endosymbiosis (or primary endosymbiosis) is when an organism lives inside another organism or its cells. Secondary endosymbiosis, then, occurs when an organism engulfs another organism that has underdone primary endosymbiosis. This loss of nucleus from the algae suggests that E. gracilis has transferred key genetic material from these algae, and a separate nucleus within E. gracilis is no longer required. The occurrence of EGT makes it difficult to map out a phylogenetic tree, and is certainly the case for E. gracilis. Phylogenetic trees are designed to reflect scenarios of vertical evolution with descent from a single common ancestor; E. gracilis has two (5).

Current levels of atmospheric CO2 levels as

of October 2012 (9)

E. gracilis is also a key component in biological carbon dioxide fixation. Because of its high tolerance of carbon dioxide, E. gracilis is one of the many favorable microalgae considered for projects centered on carbon dioxide sequestration (6). Chae et al used E. gracilis as a model by which to study the effects of elevated carbon dioxide and how it affects the ability to photosynthesize. They found that E. gracilis could withstand an environment containing up to 40% carbon dioxide. E. gracilis can also convert carbon dioxide to oxygen at a much more rapid rate than some other photosynthetic microbes (2,7). This suggests E. gracilis as one of the possible solutions to global warming attributed by the increase in carbon dioxide emissions.

Experiments by Santos Ferreira et al and Chae et al are just the beginning in understanding the complexity of E. gracilis and how, despite extensive research, there is still much unknown regarding Euglena species. Further research in their genomic sequence can give scientists further understanding in how their genetic makeup has evolved over time and why it shares so many similarities to other types of organisms found in other kingdoms. It can also highlight at what point in time E. gracilis began its endosymbiotic relationship with green algae. Additional investigations in its ability to withstand such high amounts of carbon dioxide and still be able to function relatively well can offer a better understanding of a possible evolutionary adaptation to climate change. Because current carbon dioxide levels are progressively increasing, photosynthetic organisms like Euglena are crucial for carbon fixation. No matter what the future holds, thanks to Euglena gracilis, we can all breathe a little easier.

References:

1. Tannreuther, George W., 1922. Nutrition and Reproduction in Euglena. Zoological Library, University of Missouri. Mit 52 Textabbildungen, 367-383.

2. Euglena and Euglena gracilis. <http://en.wikipedia.org/wiki/Euglena> and <http://microbewiki.kenyon.edu/index.php/Euglena_gracilis>

3. Regnault, Annie, Francoise Piton and Regis Calvayrac, 1990. Growth, Proteins and Chlorophyll in Euglena Adapted to Various C/N Balances. Phytochemistry, Vol 29(12): 3711-3715.

4. dos Santos Ferreira, Veronica, Iara Rocchetta, Visitacion Conforti, Shellie Bench, Robert Feldman and Mariano J. Levin, 2007. Gene expression patterns in Euglena gracilis: Insights into the cellular response to environmental stress. Gene 389:136-145.

5. Ahmadinejad, Nahal, Tal Dagan and William Martin, 2007. Genome history in the symbiotic hybrid Euglena gracilis. Gene 402:35-39.

6. Ono, Eiichi and Joel L. Cuello. Selection of optimal microalgae species for CO2 sequestration. <http://www.netl.doe.gov/publications/proceedings/03/carbon-seq/PDFs/158.pdf>

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

8. Figure 1 http://www.britannica.com/EBchecked/media/5411/Euglena-gracilis

9. Figure 2 http://www.co2now.org