Euglena is a genus of single-celled organisms that are found in fresh- and saltwater. Euglena are unique organisms because they can obtain their energy in multiple ways.
Like plants, Euglena can produce their own energy from sunlight, using photosynthesis. But they are also like animals, in that they can consume ‘food’, like amoebas and green algae, to generate energy. This means that Euglena are both autotrophic and heterotrophic.
While plants do also have both options available to them, due to the presence of both mitochondria and chloroplasts, this feature is quite unique in singled celled eukaryotes.
How can Euglena be both autotrophic and heterotrophic?
Euglena are autotrophic and heterotrophic, depending on the conditions they are in, meaning they are actually mixotrophic. But what do these words mean?
Autotrophs, or ‘primary producers,’ are organisms that have chloroplasts (containing chlorophyll) and produce their own ‘food’ or nutrients using energy from sunlight.
It is exactly the fact that Euglena can thrive on both sunshine and organic matter that has lead to the confusion with it being both plant and animal. I will not go further into that specific question here as I have already addressed it the article: Are Euglena plants or animals – or both?OutlifeExpert
Heterotrophs are organisms that ‘eats’ or consume other organisms, which they then convert into nutrients.
Since Euglena are single-celled organisms they don’t have mouths to eat as we do. Instead, they use a process called ‘phagocytosis’ where the cell uses its plasma membrane to engulf the food and bring it into the cell where the food is digested into the compounds later turned into energy by their mitochondria.
Euglena are capable of switching back and forth between autotrophic and heterotrophic behavior depending on whether there is sunlight available to photosynthesize nutrients.
When in complete darkness for longer periods of time, the Euglena slowly loses its chlorophyll, which is typically regained once the Euglena returns to the light.
However, some species of Euglena, like E. gracilis, cannot regain their chlorophyll once lost.
These Euglena are then forced to subsist entirely as heterotrophs. There are also some species of Euglena that don’t have chloroplasts at all-these are solely heterotrophs.
Where do Euglena find their food?
While Euglena are mixotrophic, their primary mode of obtaining nutrition is autotrophic, meaning they produce their own food using the energy from sunlight in the process of photosynthesis.
In the absence of light, or in species that don’t have chloroplasts, Euglena behave as heterotrophs. They can eat other microorganisms like green algae, paramecium, amoebas, rotifer, and other organic matter found in the water. They move through the water to try and find their way back to sunlight and to find microorganisms to eat.
What do Euglena need their energy for?
Euglena need their energy for general life processes, including reproduction and moving through the water. Euglena are able to move through water using several specialized structures.
The motility of Euglena is critical because it enables them to actively seek out light for photosynthesis and other food sources.
When do Euglena move?
The movement of Euglena is entirely for the purpose of finding or producing food (through sunlight). To produce their own food, Euglena must move to find sunlight and in the absence of sunlight, they must move to find prey to ‘eat’ through phagocytosis.
How do Euglena move towards light?
Euglena have two methods of movement that use several specialized cellular structures:
1) Flagellar movement and
2) Euglenoid Movement.
Euglena have a structure, called the eyespot, which is vital to providing a sense of direction for this movement. The eyespot is located on the anterior, or front, of the body and appears red under a microscope.
The eyespot is light-sensitive and helps the Euglena to detect light to be able to move towards it for photosynthesis.
1. Flagellar Movement
The euglena cell has a flagellum, which is a long whip-like structure similar to a tail, which they use to move like a motor or propeller.
The flagellum is located on the anterior, or the front end of the cell, and moves in a twirling motion to pull the Euglena through water. The length of the flagellum can differ between species of Euglena.
2. Euglenoid Movement or Metaboly
Euglena can also move themselves using slow wriggling movements. The cell changes its shape in a wave, starting at the anterior (front) of the cell and making its way to the posterior (end) of the cell. These cell deformations are most likely initiated due to close contact with other cells or boundaries and is a response to get away from a crowded environment where nutrients or sunlight is scarce.
Until recently, researchers thought that metaboly was a remnant functionality inherited from Euglenas ancestors that used these cell deformations to engulf their prey (phagocytosis).
The wave looks like a contraction and expansion passing through the length of the body, moving the body forward (see video below). This is accomplished because the Euglena has one more unique body structure, called the pellicle.
The pellicle lies under the plasma membrane and is composed of fibrous elastic proteins and microtubules. The pellicle is what gives the Euglena its flexibility and ability to contract and expand to facilitate euglenoid movement.
Euglenoid movement can vary between and within species of euglenoid. Movement can look like peristaltic waves (described above) or can appear as a gentle bend or twist.
Can all Euglena species move?
Most likely yes. Whereas the method and mode of movement varies between and within Euglena species, all Euglena are likely to be motile to some degree.
Euglena gracilis is the most studied of Euglena species and this species has been shown to move by both flagella and metaboly. As not all species within the Euglena genus are well studied, it is not 100% certain that all of these move in the same way, according to our current knowledge they all move.
While Euglena use their energy to generate movement, they also generate movement to obtain energy – utilizing either flagella or euglenoid movement to seek out sunlight or organic matter for consumption.
Euglena are fascinating microbes, first and foremost for the way they obtain their energy. This initially lead to the confusion in taxonomy that spurred the a entirely new kingdom of organisms – the protists!
If this all sounds confusing I do not blame you! See my recent post if you are still not sure what the differences between Euglena, animals, plants and bacteria are.
Euglena; an experimental organism for biochemical and biophysical studies. Appleton-Century-Crofts; 2nd edition (January 1, 1967).
Euglena: Biochemistry, Cell and Molecular Biology. Steven D. SchwartzbachShigeru Shigeoka. Springer publishing 2017.
The Biology Of Euglena, Volume III: Physiology. Academic Press 1982.