Unicellular Organisms

Barring some, experts generally agree that planet Earth is more than 4.5 billion years old. It is also understood that the earliest life-forms originated about 3.5 billion years ago, according to the Royal Society of Sciences. Some of these earliest living things were unicellular organisms.

For a long time, it has been obvious that unicellular beings gradually gave way to multicellular organisms that we, human beings, and other plants and animals are. It is also the basis of the ‘RNA World Hypothesis’, an explanation put forth by many leading scientists who are trying to explain the origins of life. 

Before you move on to other, more complicated subjects in biology, you must understand and define unicellular organisms.

What is a Unicellular Organism?

All life forms consisting of a single cell where all complicated processes including respiration, digestion, excretion and reproduction successfully occur are examples of unicellular organisms.

Most such organisms are microscopic, for apparent reasons. All bacteria, flagellates and other microorganisms are single-celled. 

For advanced students: Not all single-celled organisms are microscopic. There are exceptions. Bubble algae (Valonia ventricosa), also called ‘Sailor’s Eyeballs’, are green single-celled algae with diameters between 1-4 centimetres. That’s between 0.39 to 1.57 inches!

Types of Unicellular Beings

There are two types of unicellular organisms: prokaryotes and eukaryotes. Of these, scientists estimate that prokaryotic life forms came first.

Prokaryotes

Prokaryotes are unicellular microorganisms which lack common cell organelles including mitochondria, Golgi bodies or even a proper nucleus. Instead, these beings have a DNA-containing nucleoid. This nucleoid lacks a membrane and is irregular in shape.

The average size of most prokaryotes is between 0.1 and 5.0 µm. Locomotion is usually achieved via a flagellum, a tail-like extension which acts as a rudder. 

These organisms are some of the hardiest and most resilient of all life forms on earth. They have been found in undersea hot-water springs to the frozen Tundra. 

For this reason, these unicellular examples are also called extremophiles. Prokaryotes are ubiquitous. They also form plankton, a vital source of food for whales.

Fact of the day: A special prokaryote, Thermus aquaticus, was discovered in a hot steam vent at Yellowstone Park in the USA. T. Aquaticus was later discovered to contain TAQ polymerase, a unique enzyme capable of processing huge chunks of DNA in mere hours. This discovery gave rise to modern-day genetic testing & forensic science.

Examples of prokaryotes are bacteria, including Archaeabacteria and Eubacteria. 

Eukaryotes:

These are slightly larger in size than prokaryotes. Eukaryotes have membrane-bound organelles which prokaryotes lack. Also unlike prokaryotes, these single-celled organisms are capable of reproducing through mitosis and meiosis.

Experts estimate the first eukaryotes arose about 2 billion years ago.

Eukaryotes were the first unicellular organisms which later formed multicellular beings. All eukaryotes have a well-defined DNA and mitochondria, which produces energy and powers the cells.

If you are asked to name two unicellular organisms which are eukaryotes, protozoa and unicellular algae will come first. 

Of these, unicellular algae include dinoflagellates, chlorophyta or green algae and diatoms. Diatoms alone have over 100,000 species. 

Other eukaryotes include single-celled fungi like yeasts. Some yeasts are useful to mankind, like Saccharomyces cerevisiae which is used in fermentation processes and for making bread. Others, like Candida and its variants, cause fungal skin and mouth infections like thrush or candidiasis.

Task for advanced students: Research about the ‘Red Tide’ and find out how dinoflagellates are related to this natural phenomenon.

A list of unicellular organisms

For your quick reference, here are some common examples:

1. Phytoplankton

2. Brewer’s Yeast

3. Escherichia Coli (a disease-causing bacteria)

4. Amoeba

5. Chlamydomonas, a photosynthetic alga capable of detecting light!

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