The Major Classification and Characteristics of Protozoa

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Protozoa: Classification and Characteristics

Protozoa are single-celled organisms without cell walls. They are believed to be a part of the microbial world as they are unicellular and microscopic. There is a great deal to know about their classification, characteristics and more.

First animals?

Single Celled Organisms

The term protozoa implies ‘first animals’. As the primary hunters of the microbial world, protozoa help in continuing the equilibrium of bacterial, algal and other microbial life forms.

Protozoa also means ‘little animal’. They are named so because many species act like small animals. They search for and collect other microbes as food. Previously, protozoa were specified as unicellular protists possessing animal-like characteristics such as the capability to move in water. Protists are a class of eukaryotic microorganisms which are a part of the kingdom Protista.

The term ‘protozoan’ has become debatable. Modern science has shown that protozoans refer to a very complex group of organisms that do not form a clade or monophylum. This has led scientists to give up the term protozoa. Hence, the sub-kingdom Protozoa is not used today. Currently, protozoa are defined as single-celled, heterotrophic, or colonial eukaryotes possessing non-filamentous structures.

Characteristics of Protozoa

Protozoa do not have a cell wall and therefore can have a variety of shapes. Nevertheless, some of the protozoans have a pliant layer, a pellicle, or a stiff shell outside the cell membrane.

Protozoa vary in size and shape. Their sizes range from 10 to 55 micrometers, but they can be as large as 1 mm. The largest protozoa are called xenophyophores, which can measure up to 20 centimeters in diameter.

Protozoa prefer living in moist and aquatic habitats. Their cysts can be found in the bleakest parts of the ecosphere.

Protozoa are found drifting in the oceans, seas, and freshwater. They are at the base of food chains.

The life cycle of protozoa changes between proliferative stages and dormant cysts.

When in the cystic stage, protozoa can live in utmost temperatures or harsh chemicals, or without nutrients, water, or oxygen for a long time. Being a cyst enables parasitic species to dwell on the host externally. This lets them transmit from one host to another. In the form of trophozoites, protozoa feed actively. The transition of a trophozoite to a cyst is called encystation and the transition back to a trophozoite is called excystation.

The mode of nutrition of protozoa is heterotrophic, and most species obtain food by phagocytosis. Phagocytosis is the process where the cell changes shape by sending out pseudopodia to make contact with food particles.

Protozoa take food into the cell at a point called the cytostome. The food is ingested by them and lysosomal enzymes digest the food. There are also certain types of protozoa that take in food by their cell membranes. Some others such as the amoeba, surround food and absorb it. Others have mouth pores into which they pull in food.

Protozoans digest their food in spaces called vacuoles. Contractile vacuoles that are found in protozoa thriving in freshwater, excrete water that penetrates into the cells by osmosis. While chewing down the food, protozoans produce and release nitrogen.

Protozoa species move on their own by one of the three types of locomotor organelles such as flagella, cilia, or pseudopodia.

Protozoa reproduce by the method of binary fission or multiple fission. Some of the members reproduce by asexual mode, some by sexual means, and some by both.


The protozoa group comprises more than 65,000 species. All the protozoan species belong to the kingdom Protista. Many kinds of protozoa are symbionts. Some of the protozoan species are parasites and some are predators of bacteria and algae. Some examples of protozoans are dinoflagellates, amoebas, paramecia, and plasmodium.

Based on the mode of locomotion, protozoa have been divided into four types.


Amoeba Proteus


An amoeboid (ameba or amoeba) is a type of cell or organism that is capable of changing its shape, mainly by extending and retracting pseudopods. They are normally found in the soil and in aquatic habitats. They move by using pseudopods. They typically ingest their food by phagocytosis. They extend their pseudopods to engulf a prey. They do not possess a mouth or cytostome.

There is no specific place on the cell where phagocytosis takes place. The food sources of amoebae differ. Some of them feed on bacteria and other protists. Some others feed on dead organic material. Some also feed by absorbing dissolved nutrients through vesicles. The examples of amoeboids are Amoeba proteus, Chaos carolinense (the giant amoeba), Naegleria fowleri (the brain-eating amoeba), Entamoeba histolytica (the intestinal parasite of commensals and humans), and Dictyostelium discoideum (the multicellular social amoeba).


Euglena Protozoa


Flagellates are organisms which have one or more whip-like organelles called flagella. They may be solitary, colonial, free-living or parasitic. Parasitic forms live in the intestine or bloodstream of the host. An example of a parasitic flagellate is Trypanosoma, which has an interesting life cycle as it uses two hosts; humans and tsetse fly. Many other flagellates like dinoflagellates live as plankton in the oceans and freshwater. Some flagellates are autotrophic while others are heterotrophs.

Flagellates are divided into two classes:

Phytomastigophorea: The Phytomastigophorea includes protozoans that contain chlorophyll. They can produce their food photosynthetically, like plants. Examples include Euglena and Dinoflagellates. Euglena is regarded as both an alga and a protozoan.

Zoomastigophorea: It is the phylum commonly called zooflagellates. Zooflagellates include protozoans which are colorless. They ingest organic substances by osmotrophy (uptake of dissolved organic compounds through plasma membrane) or phagotrophy (engulfing prey in food vacuoles). They may be free-living, symbiotic, commensal, or parasitic. Examples include hypermastigids, holomastigotoides, and trichomonads.


Structure of Paramecium


The ciliates are a group of protozoans which possesses hair-like organelles called cilia. Cilia are used in swimming, crawling, attachment, feeding, and sensation. Most ciliates are heterotrophs. They eat organisms such as bacteria and algae. They sweep the food by their modified oral cilia into their oral groove (mouth). The food is moved with the help of cilia through the mouth pore into the gullet, which forms food vacuoles.

Some ciliates do not have a mouth and they feed by absorption (osmotrophy), and some others are predatory and feed on other protozoa, especially ciliates. Some ciliates also parasitize animals. Examples of ciliates include free-living forms like Paramecium caudatum, Stentor polymorpha, Vorticella campanula, and parasitic forms like Balantidium coli.

There are three types of ciliated protozoa. They are free-swimming ciliates, crawling ciliates, and stalked ciliates. All of them use cilia for locomotion and capturing food. Examples of free-swimming ciliates include Litonotus and Paramecium. Examples of crawling ciliates are Aspidisca and Euplotes.


Structure of Plasmodium


Sporozoans are non-motile, unicellular protists, usually parasites. These protozoans are also called intracellular parasites. An example is Plasmodium vivax, that causes malaria in humans. The earlier stage sporozoan forms show some movement. They do not possess locomotor organelles in their later stage.

Four main groups of sporozoa (based on spore structure) include:

Apicomplexa: The Apicomplexa, also called Apicomplexia, are a large phylum of parasitic protists. They are spore-forming unicellular parasites. Most of them have a unique organelle that is made up of a type of plastid called an apicoplast, and an apical complex structure. The organelle is used by the organism for penetrating into a host cell. Flagella or pseudopods are found only in certain gamete stages. This group includes organisms like coccidia, gregarines, piroplasms, haemogregarines, and plasmodium. All organisms of this phylum have an infectious stage, the sporozoite. All the species of this group, except Nephromyces, a symbiont in marine animals, are endoparasites of animals.

Microsporidia: The microsporidia constitute a group of spore-forming unicellular parasites. They were at a time known to be protists, but are now known to be fungi. They have a polar tube or polar filament in the spore with which they infiltrate host cells. Microsporidia do not have mitochondria, and instead possess mitosomes. They also do not have flagella. Most organisms in this group infect animals and insects and a few infect humans. Microsporidia can also infect hosts which are themselves parasites.

Ascetosporea: They are a group of protists that are parasites of animals, especially marine invertebrates. Two groups which come under this are the haplosporids and paramyxids. Haplosporid spores have a single nucleus and an opening at one end, covered with an internal diaphragm. After emerging, it develops within the cells of its host, usually a marine invertebrate. However, some infect other groups or freshwater species. Paramyxids grow within the digestive system of marine invertebrates, and produce multicellular spores.

Myxosporidia: The Myxosporea are a class of microscopic parasites, belonging to the Myxozoa (group of parasitic animals of aquatic environment). They have a life cycle which comprises vegetative forms in two hosts, an aquatic invertebrate, usually an annelid, and an ectothermic vertebrate, usually a fish.

As a phylum, protozoa are divided into three subphyla.

Subphylum Sarcomastigophora

The subphylum Sarcomastigophora belongs to the kingdom Protista and includes many unicellular or colonial, autotrophic, or heterotrophic organisms. It is divided into three superclasses, the Mastigophora, the Sarcodina and the Opalinata.

Superclass Mastigophora: This group of protozoa is also flagellates. They move with the help of flagella. They feed on bacteria, algae, and other protozoa.

Superclass Sarcodina: This group includes amoeba, heliozoa, radiozoa, and foraminifera. Amoeba have pseudopods that are used for locomotion and feeding. In amoeba, the flagellas are lobe-like protrusions that extend from the cell membrane. In heliozoa, radiozoa, and foraminifera, the pseudopods are like needles jutting out from the cells.

Superclass Opalinata: The opalines are a small group of protists, which belong to the family Opalinidae. The microscopic organisms of this group are opalescent (having or emitting an iridescence like that of an opal) in appearance when they come under full sunlight. Most opalines live as endocommensals (a commensal living within the body of its host) in the large intestine and cloaca of frogs and toads. They are sometimes found in fish, reptiles, molluscs, and insects.

Subphylum Sporozoa

Sporozoa include organisms that are also called sporozoans or intracellular parasites. In the early stages, they show some movement. They do not possess locomotor organelles in their later stage. All forms of sporozoa are parasites. They include plasmodium, the malarial parasite.

Subphylum Ciliophora

This group of organisms is of ciliates. Their locomotion is with the help of cilia. The cilia enable them to move quickly, stop suddenly, and turn sharply while following their prey. The types include free-living forms like paramecium and parasitic forms like balantidium coli. Many ciliates eat bacteria, fungi, and other protozoa.

Based on the mode of nutrition, protozoa are divided into the following two types.

Free-living protozoa

Euglena Protozoa


The free-living protozoa are those which do not infect or live on hosts for their survival. They may produce their food photosynthetically, or eat bacteria, yeast and algae. Example: Euglena

Parasitic protozoa

They depend on their hosts for survival. They take in fluids from the body of their hosts. Example: Plasmodium

Based on the mode of respiration, protozoa are classified into two groups.

Aerobic Protozoa

Amoeba Proteus

Amoeba Proteus

Most species of free-living protozoa are aerobic. They cannot live without oxygen. Aerobic protozoa are tiny and so are capable of getting oxygen from the liquid medium by diffusion. Example: Amoeba proteus

Anaerobic Protozoa

Giardia Protozoa


They can survive in the absence of oxygen and are not commonly found amidst eukaryotic organisms. Normally, anaerobic eukaryotes are either parasites or symbionts of multicellular organisms that have originated from aerobic ancestors. Examples: Giardia and Trichomonads.

Some Facts

A wide number of protozoans do not cause any harm, but there are a few that cause diseases in humans.

Trypanosoma brucei causes the African sleeping sickness. Giardia causes diarrhea. They are flagellates.

Another protozoan is Trichomonas vaginalis, a sexually transmitted flagellate that can induce urogenital symptoms in infected women.

Amoebiasis is a gastrointestinal disease caused by Entamoeba histolytica. It also causes dysentery.

Plasmodium is the cause of malaria in humans.

Ciliates feed on bacteria and are often an indicator of good-quality sludge and generally seen in young to medium age sludge. They are important because they eat the bacteria in the sludge and help to clarify the effluent.

Protozoa possess varying characteristics. Scientists consider that animals developed from protozoan ancestors. Modern studies are helping us understand the evolutionary relationship between protozoa and complex multicellular organisms.

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