Unicellular Organisms – Discussing Bacteria, Protozoa, Fungi, Algae And Archaea

When it comes to the field of biology, classification is one of the main tools for a scientist or researcher when it comes to understanding the various forms of life that inhabit the natural.

This makes sense, of course. Animals that share key common characteristics can be used to describe a species, a genus, or even an entire order of life forms.

However, to those of us that are not experts in this field, it can sometimes feel like the decisions can feel arbitrary.

For example, the fact that seahorses that give birth are considered biologically male, despite the obvious characteristic of giving birth that makes them seem like they would otherwise be considered female.

Or the fact that Echidna and Platypuses are both considered mammals, even though they lay eggs instead of giving birth.

Both of these examples to most people might seem strange and make no sense.

However, to a scientist looking for other key factors that help classify organisms, one or two features being a little out of place does not negate the vast number of shared traits that they otherwise have.

But perhaps one of the strangest examples of classification creating strange groupings in the natural world is the classification of unicellular organisms.

Unicellular Organisms - Discussing Bacteria, Protozoa, Fungi, Algae And Archaea

On the one hand, the classification seems almost absurdly simple. The clue is in the name, after all!

But on the other hand, there is a huge amount of variety here too, from the minute salmonella bacteria, all the way up to examples like Valonia ventricosa, a species of algae that can easily grow to the size of a marble.

Do the similarities that these two examples and others have outweigh the vast number of differences they all share?

Well, that’s what we are here to discuss today. In this guide, we are going to explain what exactly a unicellular organism is, and what characteristics these forms of life have that allow them to be grouped in this way, as well as outline a few examples of classifications that you might not expect are excluded for being classified in this group.

We’ll also go a little more into detail on the different orders of organisms that are included in this group, and what their characteristics are.

Plenty of groups of life forms, despite all being in the same group, can have plenty of variation between them, and unicellular organisms are no different.

What Is A Unicellular Organism?

Before we get on to describing what separates them from the rest of Eath’s other lifeforms, we should probably give a brief description of what exactly a unicellular organism is in the first place.

Well, as we implied in the introduction, this group of celled organisms is as simple as their name implies.

They are organisms that consist of just one cell, and do not require the functions of other connected cells to survive necessarily.

This is the main factor that separates them from multicellular organisms, which do need multiple different cells to survive.

Unicellular organisms are some of the most simple, yet resilient, life forms on the planet.

Considering that they are amongst the oldest known types of life on the planet, with some estimates putting their first development as far back as 3.8 billion years ago, this probably isn’t too surprising!

Unicellular organisms would have made up the vast bulk of biomass on the planet for billions of years before the development of multi-celled life forms from them occurred.

Even to this day. Unicellular organisms still make up the vast bulk of biological processes in many life forms alive today.

Even many multicellular organisms rely on the processes of their simpler counterparts.

Organisms and bacteria that are found in the digestive tracks of many animals, including humans, rely on the processes of simple archaean organisms to function properly. 

Shared Characteristics Of Unicellular Organisms

So, we have described what unicellular organisms are, as well as a brief history lesson on their history in the story of life on Earth.

But what orders of life are included in this very broad definition? And are there any other characteristics that these organisms share?

The classification of organisms and life on Earth in this group is, perhaps appropriately, an ever-evolving field, as new orders and groups are introduced, and old groups that we believed were distinct are phased out in favor of more accurate ones.

However, there are currently five recognized groups that have examples of unicellular organisms in them. Those groups include:

  • Protozoa
  • Bacteria
  • Unicellular fungi
  • Unicellular algae
  • Archaea

Those of you with some prior knowledge when it comes to biology might have noted that this list includes quite a variety of different life forms.

For example, these five orders contain both organisms without a nucleus and other notable components or organelles, also known as prokaryotic cells, and those that do have these features, also known as eukaryotic organisms.

This is a major difference in how these life forms function and operate in the natural world, which just goes to show the sheer variety of life that can exist in a single group of life forms.

What Does And Doesn’t Make A Unicellular Organism

However, with such a broad category, it can be easy to confuse many other biological or chemical substances as unicellular organisms, either because of their microscopic size or because they are perceived as being biologically simple.

So, to help us better define a unicellular organism, these are some processes and characteristics that can help us better distinguish them from other microscopic objects and helps classify all these unicellular lifeforms

  • Growth Of The Organism – As living things, unicellular organisms, like any other life form, will continue to grow and develop as they go through their life cycle.
  • The Organization Of The Organism – Despite the massive variation of cell structures that is found within unicellular organisms, the fact that virtually all the groups included do have some type of structure is a main defining characteristic. The existence of some type of organization within a cell, whether it is the existence of a nucleus or nucleoid, that they are separated from their environment by the presence of a cell wall, and the internals being filled with some type of cytoplasm or similarly functioning agent or substance, is a key feature that makes up all living organisms, and unicellular life forms are no different. 
  • Reproductive Processes – This is another key feature that separates life from other natural processes. Whether it is through simple cell division, or a more complex method, an organism’s ability to make more of its kind is arguably the defining feature of life. In microbiological cases, this means that daughter cells will be genetic copies of their parent cell, containing all the same information that they originally had.
  • How The Organism Responds To Their Outside Environment – Another big characteristic of life is that it can react to the outside world in some capacity, whether that is changes in temperature or levels of light. These responses can be for several reasons and can help organisms to find more nutrients to help continue surviving, so they can continue to reproduce and propagate.

These are the key details that help distinguish unicellular organisms from other simple biological processes. It is also why something that also has biological components isn’t considered part of this group.

Even though they can replicate themselves, and can adapt to their environment’s needs, they do not generate energy within themselves to carry out these other functions and lack the same stable structure that living cells usually possess.

So, we have clearly outlined both unicellular organisms, as well as demonstrated what separates them from other simple biological and chemical processes.

Now, we’re going to take a closer look at the different groups that make up this vast group of living things.

Whilst some have very clear differences, such as between yeast and algae, there are others where the distinctions are a little harder to distinguish



Probably the order of organisms that make up the bulk of unicellular life, bacteria are the most prolific, widespread, and well-known organisms of this group that you will find on the planet.

And they are most certainly prolific.

It is estimated by scientists that just one human body can contain as many as a hundred trillion bacteria cells, a figure which is estimated to outnumber the human cells in your body by a magnitude of approximately 9 to 1!

Along with archaea, bacteria make up the vast majority of organisms that are part of the large group that makes up prokaryotes.

These are single-celled organisms that lack many of the key features and organelles that can be found in more complex life forms, such as a nucleus and mitochondria.

This means that the DNA of these organisms isn’t separated from the rest of the cell’s content, being stored in a nucleoid, where its DNA loops around the center of the organism in its cytoplasm and behind the cell walls.

Outside this major feature, bacteria can have a wide range of other features that help them survive in their environment, such as flagella, or pili.

Uses And Functions Of Bacteria

Bacteria encompass a wide array of properties in the natural world, many of which can be either harmful to other lifeforms such as animals or humans, whilst others can be very beneficial and useful.

For example, species such as Staphylococcus and Salmonella can cause illnesses and infections that can be very serious and even fatal if not treated.

There are plenty of bacteria that have proven very helpful to us over the years though too. Take the bacteria actinomycetes, for example.

This bacterium is vital for the redistribution of nutrients and organic material in the natural especially for woods and forests across the globe, as it helps break down dead organic matter, which allows their components to return to the soil where they die.

Not only that, but they are also essential in the production of antibiotics, which are vital for treating other bacterial infections.

Bacteria are also vital for economic and industrial purposes. Many types of bacteria are used in the production of many kinds of foods, especially dairy products.

Species such as Lactobacillus are used in the making of yogurts, whereas others such as Streptococcus and Lactobacillus again are also used in making many types of European cheeses.

The Shape Of Bacteria

Bacteria come in a variety of shapes that can be separated into three broad groups.

They are found in more spherical forms, also known as Coccus bacteria, in long, rod-like shapes, such as with bacillus forms, as well as twisted, corkscrew-like shapes, such as those found within the spirilla group.

Coccus Bacteria

Coccus bacteria, as we mentioned before, usually appear as small spherical or oval-shaped microorganisms,

However, these types of bacteria can also attach themselves by their cell walls, creating unique patterns and arrangements, many of which are considered pathogens to humans.

Outside of single-celled arrangements, these include:


These are bacteria that have clustered together in pairs. A relatively well-known example of this type is gonococcus, which is known for causing the sexually transmitted disease known as gonorrhea.


This is a case where cocci bacteria have started forming chains of the same bacteria that are attached.

Pneumonia, the infection that causes inflammation of the air sacs in your lungs, is caused by several varieties of streptococci bacteria.


In examples of cocci bacteria from this group, you are likely to see that this variety tends to form large clusters that appear as almost grape-like constructs when viewed under a microscope, or even a square formation in some other examples, which are also known as tetrads.

Bacteria that are known to cause staph infections can be found in this group, although many different species that do not pose risks to humans or other animals are also found here.

Bacillus Bacteria

These bacteria are often long shapes, as we have already mentioned. Like cocci, they will often also cluster together into arrangements.


This is when a pair of these bacteria attach, usually at the end of another bacillus bacterium, although it is possible to find bacilli bacteria that are attached side-to-side, rather than end-toe-end (these are known as palisades).

Moraxella Bovis, an infectious disease that can affect the eyes of cattle, is classified as part of this group.


This arrangement can be seen when many bacilli bacteria join end-to-end, appearing like a chain when viewed.

There are several varieties o this arrangement, from the harmful S. moniliformis that causes rat-bite fever, to more beneficial species that can be used in the production of penicillin.


These bacteria, as their names suggest, tend to form long curved chains that corkscrew as they become longer and attach to other bacteria.

A singular bacterium from this group is known as a vibrio and looks more like a comma than it does a true spiral. However, once they start to attach to longer formations, the spiraling pattern becomes much clearer.

Some colonies can grow large enough to be seen with a strong magnifying glass, as opposed to a traditional microscope.

Campylobacter jejuni, one of the leading bacterial causes of food poisoning, can be found in this group.

The Nutrition Of Bacteria

Bacteria, being a wide and varied group of organisms, have a variety of methods to obtain the nutrients they need to survive.

There are two primary methods through which bacteria sustain themselves: autotrophic, and heterotrophic.

Autotrophic Bacteria

The bacteria in this group will generate the food and nutrients they need to survive by sourcing them from inorganic materials and substances, including minerals, as well as from various forms of energy and radiation

The most common types of bacteria in this subgroup are either photosynthetic or chemosynthetic bacteria.

Photosynthetic Bacteria

Thanks to a pigment that can photosynthesize within their cells, these bacteria can generate carbohydrates for themselves when they are exposed to sunlight, the same process that many plant cells use to create energy for themselves.

Chemosynthetic Bacteria

Although similar in that they can produce their energy, chemosynthetic bacteria will digest and create nutrients and energy from completely inorganic substances.

This makes them separate from sunlight when it comes to their food, something that is somewhat rarer in the natural world. 

Heterotrophic Bacteria

These bacteria, unlike their self-sustaining counterpart, need to obtain their energy from other organic materials.

There are a few different methods that these bacteria use to obtain the nutrients that they need.


These bacteria are the types that you will find are involved in the process of decomposition and fermentation. They break down organic matter that has already died or is currently decaying.

These are the bacteria that will eat dead leaves, animals, and other soft parts of organic tissues.


These types of bacteria, as their name implies, will attach themselves to still living organic tissue, and obtain their energy by consuming the tissues from their hosts while they are still alive.

These bacteria can range from harmless, to very serious health issues for their hosts.


These bacteria coexist with their hosts, as both parties can have mutual benefits for the other, and do not cause harm to either.



Unlike bacteria, protozoa are no prokaryotes and have more in common with animal cells than they do with many other single-celled organisms.

They are what are known as eukaryotes, which means that, unlike bacteria and archaea, these unicellular organisms have more complex cell structures, such as having a nucleus and having many of the organelles and features that we have come to expect from many life forms in the world, such as mitochondria and Golgi apparatus.

They are also characterized as being generally more active than bacteria, as they have shown that they are capable of acts such as independent motor functions, and even predation of other organisms.

This is why they are sometimes known as ‘one-celled animals’, as they have shown a surprisingly wide range of behavior, especially for single-celled organisms.

The Shape And Movement Of Protozoa

Protozoa have a massive amount of variety when it comes to the shape of their cells. More often, protozoa are classified by their locomotion, or their methods and ability to move around.

This can range from the whip-like flagella appendages that groups such as Mastiogophora have, to the thin hair-like structures that protozoa found in the Ciliophora group.

One group, the Sporozoa cannot move on their own and require the use of a host body to help them move around, making them parasitic.


Although we tend to think of fungi as complex organisms, being mushrooms and toadstools, readers who bake may be aware that yeast, a vital component in bread making, is also a fungus, being a unicellular one at that.

They are part of the eukaryotic domain of organisms, like protozoa and algae.

Of the million and a half fungi species that have been identified, around 800 unicellular yeast species have been found, with wildly varying properties between them.

Many will know that yeast is used in the fermentation of many alcohols and bread, as they convert carbohydrates into carbon dioxide and other materials.

However, some yeasts can also behave as pathogens, causing a lot of harm to infected bodies.


Algae, or alga, are another eukaryotic organism like fungi and protozoa. However, unlike the other two, algae are known for their ability to photosynthesize and generate energy.

The unicellular organism we mentioned in the introduction, Valonia ventricosa, is a part of this group, making algae some of the largest unicellular organisms on the planet.

Whilst we often think of algae as living mostly sedentary, many algae species have flagella, similar bacteria, and are able to move around under their own locomotion.

Many algae also contain chlorophyll, like plants, but not all species will have this property. 


Archaea, along with bacteria, are some of the oldest living organisms that can be found on the planet.

Developing at a similar time as bacteria, and are quite likely being evolutionarily linked, archaea are also part of the domain of organisms like bacteria, being a part of the prokaryotes group along with their bacterial cousins.

This means that archaean also lack much of the complex structures that many other organisms have, such as a nucleus.

The subgroups of this organism are even harder to distinguish than other groups but are still possible.


These archaean are usually found in incredibly salty waters and other liquids, or found in the bowels and intestines of animals.


This group is rare, but can be found absorbing nutrients around extremely hot or acidic vents deep underwater, making them incredibly hardy, and are also considered extremophiles, organisms that can survive in extreme conditions that are unsuitable for most other creatures.

Final Thoughts

So, there you have it! There is plenty more information on unicellular organisms out there, and researchers have spent their entire lives studying just one part of this group.

But hopefully, you know now a little more about these simple, yet resilient, forms of life.

Jennifer Dawkins