What You Need To Know About Phylum Protozoa

The Protozoa are acknowledged as a sub-kingdom of the Protista kingdom, despite being classified in the kingdom Animalia in the traditional system.

More than 50,000 species have been identified, the majority of whom are free-living creatures; protozoa may be found in practically any environment.

What You Need To Know About Phylum Protozoa

These organisms are so intricate and unique that they have reasonably fascinated many researchers and biology aficionados.

Having been on this earth for a very long time, it is obvious that the research has been non-stop and only evolving from time to time.

Their structure, nutrition habits, and reproduction are all very interesting topics to discuss and learn about, so this article will take a deeper look into them.

We will also have a look at some sub-categories and examples of how protozoa can be both harmless and very dangerous for humans and which ones we should be aware of.

So, keep on reading this article to find out everything you need to know about phylum protozoa and their characteristics!

Phylum Protozoa: An Overview

Protozoa have existed since the Precambrian epoch, according to the fossil evidence in the shape of shells in sedimentary rocks.

The very first man to see protozoa was Anton van Leeuwenhoek, who was utilizing some microscopes he had built with basic lenses.

Between 1674 and 1716, he identified different parasitic organisms from animals, as well as Giardia lamblia from his own feces, in addition to the free-living protozoa.

These organisms live in or on almost all people at some point, and many adults are attacked by one or more types during our lives.

Some species have been reported to be commensals, meaning they are typically not dangerous, whilst others are pathogenic and cause illness.

Moreover, they are fundamentally single-celled eukaryotic organisms. This implies they are single-celled creatures with nuclei and a variety of other critical components contained inside the cytoplasm and protected by membranes.

They are also either free-living creatures or parasitic ones. This results in a varied array of unicellular creatures that vary in form and size.


The majority of parasitic protozoa in people are smaller than 50μm long. The smallest (mostly intracellular types) are 1 to 10μm in length, whereas Balantidium Coli can grow to be 150μm in length.

Since they are eukaryotic organisms, protozoa have a bigger cell structure than other organisms and more intricate anatomy than prokaryotes.

This implies that protozoa are developed with a cell membrane that surrounds the organelles, a membrane-bound DNA, nucleoli, ribosomes, Golgi apparatus, and numerous linear chromosomes with histone proteins, among other things.

It is worth mentioning that the organelles found in these cells differ from each other. There are also several organelles that are unique to protozoa, such as:

  • Paramecium trichocysts
  • Some bone structures
  • Vacuoles that contract

The nuclei of protozoa are vesicular in comparison to other ciliates. Because of that, the chromatic is distributed, leading to diffused-looking nuclei.

Nevertheless, this differs from one protozoon to the other. For example, in the case of the Phylum Apicomplexa, the vesicular nuclei included one or more nucleoli containing DNA, but trypanosome endosomes lacked DNA.

A protozoon also features locomotory characteristics such as pseudopodia, flagella, and cilia that allow it to move. The plasma membrane also surrounds these organelles.

Furthermore, the pellicle (the external layer of some organisms, such as Giardia) is hard enough to hold and keep a distinct form while still enabling the twisting and bending of the organism when in movement.

Classification Of Phylum Protozoa

Due to their diversity, protozoa pose various categorization challenges. They are classified as protista, and more than 50,000 species have been characterized as free-living.

Being characterized as such means that they belong to those organisms that can survive on their own, without depending on other organisms to do so, especially when it comes to food sources.

A free-living protozoon can be seen in almost every environment. It has been decided that it belongs in six different primary phyla on the basis of light and electron microscopy features, with the bulk of disease-inducing protozoa lying within the phyla Sacromastigophora and Apicomplexa.

Depending on locomotive patterns, below are some of the sub-phyla and classes within this sub-phyla:

  • Plasmodroma – This is one of the sub-phyla whose locomotive structures might be flagella, pseudopodia, or none at all. This sub-phylum includes the classes Mastigophora (which employ one or more flagella for movement), Sarcodina (which employ pseudopodia for movement and food capture), and Sporozoa (which lack locomotive features).
  • Ciliophora – The Sub-phyla Ciliophora utilize cilia or grasping tentacles at particular phases of their life or throughout their entire existence. This sub-phylum includes the classes Ciliata (which uses cilia continuously) and Suctoria (which uses cilia when juvenile and tentacles when it becomes an adult).
  • Sarcomastigopohora – This group’s movement structure comprises pseudopodia or flagella. The nuclei in this case are also of one monomorphic, which means that they are of the same kind. Mastigophora, which belongs to this subphylum, is a flagellate and so uses flagella for movement.

This sub-phylum also includes the Phytomastogophoerea, which uses flagella in some circumstances. Order Chrysomonadida belongs to the Class Phytomastogophoerea and comprises species such as Chrys amoeba, synura, and ochromonas.

Of course, these are only a few examples from the categorization, as it is vast and has numerous creatures in general.

What You Need To Know About Phylum Protozoa

Classification Based On Mode Of Existence

There are around 21,000 kinds of protozoa that survive as free-living organisms in a range of environments, and another 11,000 types that thrive as parasitic microorganisms in hosts that are both vertebrate and invertebrates.

The free-living species may be found in a variety of environments, most notably soil and water.

Because they do not rely on other species for survival, these protozoa have little influence on our health as humans. Nevertheless, when injected into a human host, a few of the free-living may induce disease.

Some other organisms will also have an impact on human health as they produce toxins. A few of the free-living amoebae that can cause human sickness are:

  • Naegleria fowleri – These organisms are widespread worldwide and are mainly found in damp soil. They are responsible for acute primary amebic meningoencephalitis.
  • Acanthamoeba – Acanthamoebae are seen in soil and waters and can lead to chronic granulomatous amebic encephalopathy, amebic keratitis, granulomatous skin, and lung diseases.
  • Balamuthia mandrillaris – This species can lead to granulomatous amebic encephalitis, and also granulomatous skin and lung diseases.
  • Sappinia diploidea

Moreover, people whose immune systems can regulate but not remove a parasite infection develop into carriers and can contaminate other individuals.

In high-prevalence locations, well-tolerated diseases are frequently not handled to eliminate the parasites since elimination would reduce the person’s tolerance to the pathogen and increase the probability of contracting the disease again.

Many protozoan infections that are undetectable or moderate in healthy people can be fatal in immunocompromised people, particularly those with acquired immune deficiency syndrome (AIDS).

According to data, numerous healthy people have modest levels of Pneumocystis carinii in their lungs. Nevertheless, in immunocompromised people, like those with AIDS, this parasite frequently causes deadly pneumonia.

Toxoplasma gondii, a common protozoan parasite, often produces a modest first sickness followed by a long-term latent infection.

Toxoplasmic encephalitis, on the other hand, can be lethal in AIDS patients. Although Cryptosporidium was first identified in the nineteenth century, significant human infection was just recently discovered.

Another protozoan that can cause major difficulties in AIDS patients is Cryptosporidium. Before the development of AIDS, just a few cases of microsporidiosis in humans were documented. It has become a more prevalent infection among AIDS patients.

As more comprehensive investigations of AIDS patients are conducted, it is conceivable that further rare or odd protozoan infections may be identified.

Acanthamoeba types are amebas that live in soil and water. Cyst stages can float in the air. Contact lens wearers have been known to have serious eye-threatening cornea ulceration caused by Acanthamoeba types.

The parasites are most likely transferred by contaminated lens-cleaning liquid.

Naegleria amebae, which live in bodies of freshwater resources, are responsible for the vast majority of instances of the typically deadly illness primary amebic meningoencephalitis.

The amebae are believed to enter the body by water sprayed over the upper nasal tract during a swim or a dip.

Based on laboratory investigations of Acanthamoeba infections in cell cultures and animals, human infections of this sort were anticipated before they were discovered and published.

The WHO targeted six illnesses for enhanced training and research due to a lack of effective vaccinations, a scarcity of dependable treatments, and other issues, including vector control challenges.

Malaria, trypanosomiasis, and leishmaniasis were all protozoan illnesses. Despite a new understanding of these illnesses, most of the issues with treatment remain.

Parasitic Protozoa

Parasitic protozoa are those that rely on their host to survive. As a result, they survive within the host and might potentially create health concerns.

Among the parasites are the following:

  • Entamoeba histolytica – Entamoeba histolytica are amoebae that reside in the human alimentary tract. They are mostly benign and feed on bacteria and debris that may be found in the colon. Even though most parasites are innocuous, this parasite can infiltrate the intestinal wall or the rectum, causing ulcerations and even hemorrhage, as well as discomfort, puking, and diarrhea, among other problems.
  • Trypanosomes – These are flagellates that dwell in the bloodstream. These parasites’ several species cause ailments such as Chaga’s disease, sleeping sickness, leishmaniasis.
  • Mastigophora (Giardia) – This is a flagellate that is typically found in the patient’s small intestine. Giardia generally attaches to the intestinal wall, producing inflammation, diarrhea, and abdominal discomfort, among other complications.
  • Sporozoa, like the Plasmodium – The plasmodium types are parasites that live in the bloodstream of humans and feed on the cytoplasm of red blood cells. While they keep growing within the cells, they force them to explode, releasing many more pathogens into the bloodstream.

Life Cycle Of Parasitic Protozoa

The life cycle phases of parasitic species might occur intercellularly, intracellularly, or in the lumens of specific organs.

Due to the general variability, describing a unique or universal life cycle pattern is impossible. So, in this section, we’ll look at three of the most prevalent broad patterns displayed by this category of protozoa.

First Pattern

This motif is frequent in the Apicomplexa phyla and entails a change between the asexual and sexual breeding phases.

The process begins with asexual procreation phases, followed by schizogony cycles (including mitosis and cytokinesis) in the host tissues, which leads to a growth in population.

After this phase, certain members of the population begin gametogony (a sexual procedure) in order to create gametes. These gametes then asexually combine and split to generate sporozoites, a practice called sporogeny.

The sporozoites are then able to infect a new host, and the cycle repeats itself again. It really is worth emphasizing at this point that the shift into a new host is accomplished via cysts, which are difficult to form under harsh circumstances.

The cysts can withstand exogenous variables and hold the sporozoites.

What You Need To Know About Phylum Protozoa

When sporozoites enter a new host, they begin the reproductive cycle all over again. Some organisms (Apicomplexa) in this division need two hosts to fulfill their life cycle.

This comprises a vertebrates’ host in which the parasite undergoes schizogony and gametogony, as well as an invertebrate host in which the gametes join and sporiogony occur in the tissues.

Second Pattern

The second pattern, which is shared by most flagellates, is asexual reproduction. Several structural modifications happen during the lifecycle of these organisms. They all, nonetheless, multiply by binary fission.

Some organisms in this category will end this cycle in a vertebrate host by transmitting from one host to the other via cysts, which can withstand harsh circumstances effectively.

As a result, certain organisms in this category, like those in the Apicomplexa phylum, will also need two hosts to fulfill their life cycle.

Third Pattern

This pattern is most prevalent among amoebas, because it includes asexual reproduction. In contrast to the other patterns, in this case, only one host is needed to complete the reproductive cycle. Trophozoites in the host are found in the lumens of guts and proliferate by binary fission.

In some circumstances, the trophozoites inside the cyst may be recruited to encyst while they execute nuclear division. The cycle will continue after the cyst is absorbed by another host.

Life Cycle Of Free-Living Protozoa

For this category, the life span mostly consists of organism development and expansion, succeeded by binary fission (or alternative types of asexual reproduction).

Sexual reproduction happens only under challenging circumstances for the free-living (undesirable temperature, limited food sources, etc). However, these parameters frequently differ between species.

There is a stage of DNA synthesis, chromosome reproduction, and cell expansion during the development and division stages of free-living protozoa.

The cycle’s stages are as follows:

  • The first division stage
  • The division phase has ended, and DNA synthesis has begun.
  • Synthesis of DNA
  • End of DNA synthesis and the start of the next division

Classification Based On Nutrition

Nutritionally, there are three major groups into which protozoa are split and these are:

  • Photo-autotrophs
  • Photo-heterotrophs
  • Chemoheterotrophs


Autotrophs, such as certain flagellates, use chlorophyll to create carbs from water and carbon dioxide. The sun’s radiant energy is harnessed for these purposes.

Several photoautotrophic flagellates, such as organisms of the Euglenida, Cryptomonadida, and Volvocida, merge autotrophy with heterotrophy.

As a result, they are frequently referred to as acetate flagellates. Acetates, simple fatty acids, and alcohols are among their carbon sources. These flagellates are photosynthetic organisms in the light and heterotrophs in the dark.


This group includes the overwhelming bulk of free-living protozoa. As a result, they must rely on a diverse diet.

Some graze on bacteria (microbivores), while others live on algae and are classified as herbivores. Carnivores consume both trophic levels, that is the herbivores and the microbivores.

These free-living organisms also divide into two types morphologically. These include those with a mouth/cytostome as well as those without a mouth or a clear site of entrance for food.

For example, unlike certain flagellates and many ciliates (except for the apostomatida), the Sarcodina does not have a mouth.

What You Need To Know About Phylum Protozoa


These organisms demand energy and organic carbon sources.

Under The Microscope

Protozoa, as already stated, are extremely varied. Consequently, they are recognized from one another by their distinct structural traits, modes of movement, and spore generation.

Different forms of protozoa can be distinguished under a light microscope.

Collection Of Samples

Protozoa may be found in practically every environment. While free-living organisms may be found in water and many wet settings, parasitic species occur in most metazoans (developed animals).

It would be beneficial for academics to utilize free-living protozoa, which may be collected from environments such as muck, ponds, and transitory bodies. It is important to note that they are quite delicate. As a result, they must be handled with caution.

It is also critical to exercise caution since even free-living protozoa might become parasitic. Protozoa can also be cultivated to boost their numbers for research. Among the mediums utilized are split pea (for Eglena), distilled water with wheat grains (for chilomonas), and hay (for peranema).

Observation Under The Microscope

Some of the prerequisites for microscopy are as follows:

  • A magnifying glass
  • Microscope slieds
  • Clips for microscopes
  • Distilled water
  • Dropper

Wet Mounting Technique

The wet mounting technique involves just placing the samples or specimens in a bit of water and observing them under a microscope.

If the samples came from a lake, the following procedure might be used:

  • Shake the containers lightly (to distribute the protozoa in the water)
  • Take a drop of lake water from the containers with a dropper.
  • Protect a microscope slide with a cover slip after placing a drop of the specimen in the middle (always make sure that the microscope slide and slip are clean so as not to introduce any new unwanted microorganisms.
  • To examine the sample, place it on the microscope stage.

In some circumstances, staining may be utilized to improve contrast and clarity. Among the stains employed here are:

  • Brown Bismarck
  • Cresyl Blue Shine
  • Blue Bromothymol
  • Powdered Carmine
  • Blue Methylene

The Bottom Line

Phylum Protozoa are exciting organisms to observe and research, With many sub-categories and several distinct as well as universal patterns and structures, it is a species that has a lot to offer to both the science itself and us humans.

However, we should not neglect its potential harm as many protozoa are infectious and people, especially the ones who are immunocompromised, can suffer greatly in case they get infected by one of the parasitic protozoa.

Jennifer Dawkins

Leave a Comment

Your email address will not be published. Required fields are marked *