Atlant’s Column: Cyclops in a Kilt

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Welcome to the section on Micronaut.ch which explores, analyzes and explains fascinating microscopical structures.

Mosquito egg surface

It is a bacterium, a plant and an animal at the same time and almost as old as life itself. Euglena is so ingeniously built that researchers stilldo not fully understand how it works.

Text by science writer Atlant Bieri
Photography by Martin Oeggerli

These cucumber shaped creatures are among the oldest on earth. Biologically they belong to the so-called protists. They populated the world long before plants, animals or bacteria turned up. Euglena is so special because it combines the features of all three groups.

Like a bacterium, it consists only of a single cell. Reproduction is a simple matter of cell division. Males and females do not exist. But unlike bacteria, Euglena has no cell wall that gives it stability. It is only wrapped in a thin membrane, which should actually make it look like a shapeless water drop. But thanks to a design trick Euglena escapes this fate. Its membrane is not flat like cling film, but it is wrinkled like a kilt. Hence the banded structure that covers its entire body. Like this, the membrane is extremely stable and can take on any form.

Its relationship with the plants is revealed by the green chloroplasts, which are distributed evenly in its body. They are the natural solar cells with which sunlight can be converted into chemical energy. If it suddenly gets dark for a long time, most plants die. Euglena, however, then just switches into animal mode and goes hunting for bacteria in the water or nibbles at dead plant material. To get around it possesses a flagellum, which acts as a kind of outboard motor and makes its entire body spin around itself.

 

Euglena SkionThe King of Single-Celled Organisms

But what makes Euglena king of all single-celled organisms is its eye. It is located near the foot of the flagellum in a small body cavity. The eye comprises a crystal lens and the molecule flavin, which serves as a light sensor. It is only sensitive to blue light and a small portion of the UV spectrum. In addition, the sensor is not directed to the front, but in a right angle to the body axis to the left and to the right. On the left side there is a blind spot. It is an accumulation of fat droplets, which are enriched with the red pigment carotene. Light passing through it is filtered and the blue light and the ultraviolet light is removed. Hence, looking in this direction, the eye is totally blind.

While Euglena spins in the water, the seeing part of its eye is exposed to the glaring sunlight once per revolution. Euglena compares that state with the blind part of its eye and changes its direction until the difference between its blind eye and its seeing eye almost disappears.

This is only the case when it swims directly towards the sun. Now the seeing side of the sensor looks into the murky water, which comes very close to blindness. This way, Euglena always finds the brightest spot in the water in order to harvest enough sunlight with its chloroplasts.


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Continue reading Atlant’s Column: From Mouth to Mouth , August 2013, or go back to the archive