Protozoa

Did you know that the term protozoa means "first Animals"? According to scientists, protozoa were probably the dominant form of life on earth 1.5 billion years ago. However, they remained undiscovered until the 1670s, when Anton van Leeuwenhoek was able to see the Bacteria and protozoa using a microscope! Read on if you want to learn more about protozoa!

Protozoa definition

Let's start by looking at the definition of protozoa.

Protozoa are also known as animal-like protists and are a part of the kingdom Protista. The kingdom Protista contains the following organisms:

• Protozoa (animal-like protists)

• Algae (plant-like protists)

• Slime molds (fungi-like protists)

Protozoa characteristics

Let's explore the different characteristics of Protozoa. Protozoa are microscopic, single-celled eukaryotic organisms. They lack a cell wall, but some of them have an elastic "shell" known as a pellicle. They are mostly naked, but some protozoa have a simple endoskeleton or exoskeleton.

They are mostly Heterotrophs, meaning that they cannot make their food, so they consume other Animals, Plants, or dead organic matter. However, some protozoans are autotrophic (make their own food). Protozoa can be motile or nonmotile, and protozoa locomotion depends on the presence of pseudopodia, cilia, or flagella.

Protozoa are found as free-living in different habitats such as freshwater and soils and also in symbiotic or parasitic relationships with other organisms. Protozoa can reproduce asexually or sexually.

Protozoa structure

Now that we know the basic definition of protozoa let's take a look at the structural characteristics of protozoa. There are four types of protozoa, and each of them has a different shape and Cell Structure.

Amoeboids

Let's start with amoeboids. Amoebas (Fig. 1) are protozoa that have the ability to change shape in order to move.

Amoeba locomotion occurs through pseudopods. Basically, amoebas extend their cytoplasm outward and use pseudopods to grip and pull themselves in a specific direction. Every time it wants to move, it changes shape!

Amoeba has two layers of cytoplasm. The ectoplasm is the outer layer, whereas the endoplasm is the inner cytoplasmic region. Its Cell Structure contains a nucleus, and two types of vacuoles:

• Food vacuoles contain engulfed food undergoing digestion.
• The contractile vacuole is responsible for regulating internal water pressure.

Amoebas also use their pseudopods to eat. They do this by surrounding a substance using its pseudopods and then dragging it into the food vacuole for digestion. This feeding mechanism is called Phagocytosis.

Flagellates

Another type of protozoa is flagellate protozoa. These protozoa use flagella as a means of locomotion. Some flagellates may have more than one flagellum.

Flagellate protozoa are divided into two types: zooflagellates and phytoflagellates. Zooflagellates are heterotrophic and use Phagocytosis (like amoebas) to obtain nutrients. Phytoflagellates are autotrophic, and they can obtain nutrients via Photosynthesis.

Flagellates also contain the following structures:

• An elastic layer outside the cell membrane called a pellicle.
• Chloroplast organelles containing chlorophyll (only present in zooflagellates).
• Nucleus
• An eye-sensitive pigmented spot called eye spot.

Fig. 2 shows the structure of euglena, a zooflagellate.

Ciliates

Next, we have ciliates (Fig. 3). Ciliates are protozoa that use cilia to move around.

Ciliates have two nuclei present: a macronucleus and a micronucleus. The macronucleus is responsible for handling mostly all cell functions, except for functions related to Reproduction. The micronucleus, on the other hand, contains the Genes that will be passed on to the offspring during reproduction.

Ciliated protozoa are able to ingest solid pieces of food through a small mouth structure called a cytostome.

Sporozoa

Sporozoa are nonmotile, meaning that they are not able to move by themselves (Fig. 4). But, as their name suggests, Sporozoa are capable of forming spore-like Cells.

Most Sporozoa are considered parasitic and make use of other organisms to move and to get nutrients. But how are they able to enter the host cell? Sporozoa enter the host cell via an apical complex. This apical complex is made up of an apical end (the tip of the Sporozoa) and rhoptries. Rhoptries are responsible for producing Enzymes that make it easier for the Sporozoa to invade host Cells.

Classification of Protozoa

Fig. 5 shows the Taxonomy classification of protozoa, based on the most accepted classification of protozoa, published in 1964 as "A Revised Classification of the Phylum Protozoa" by B.M. Honigberg. The phylum protozoa is divided into four subphyla: sarcomastigophora, sporozoa, cnidospora, cilosphora.

Protozoa subphylum Sarcomastigophora

In the subphylum Sarcomastigophora, we have protozoans that contain pseudopodia or flagella as their locomotor organelle. They do not form spores and have a single nucleus. Inside this phylum, there are three superclasses: Mastigophora, Opalinata, and Sarcodina.

Flagellates are commonly found in the superclass Mastigosphora. They are mostly free-living, but some can be parasitic. They can be autotrophs and/or Heterotrophs. In the superclass Mastigophora, there are two classes: phytomastigophorea, and zoomastigophorea. Protozoa of the class phytomastigophorea contain chlorophyll-bearing chromatophores, a vesicular nucleus, and usually one or two flagella. In the class zoomastigophorea, no chlorophyll or chromatophores are present. They are mostly parasitic

In the superclass opalinata, we have protozoans with cilia-like organelles and two or more monomorphic nuclei present. They are all parasitic and tend to infect frogs and toads.

The third superclass in the subphylum Sarcomastigophora is called sarcodina. These protozoans contain pseudopodia, meaning that the amoeboid form is predominant. In this superclass, there are three classes: rhizopodea, actinopodea, and piroplasmea.

Protozoa subphylum Sporozoa

Protozoa in the sporozoa subphylum lack locomotory organelles, contain spores and are mostly endoparasites. This subphylum is divided into three classes: telosporea, toxoplasmea, and haplosporea.

Protozoa in the class telosporea lack pseudopodia, but are able to move by gliding or body flexion. They contain spores and are able to reproduce both sexually and asexually. Now, protozoa in the class toxoplasmea contain no spores, flagella, or pseudopodia. Also, they can only reproduce asexually. In the class haplosporea, we have protozoa that contain spores, and pseudopodia may also be present.

Protozoa subphylum Cnidospora

Protozoa in the subphylum cnidospora, we have protozoa that contain spores and are all parasitic. In the class myxosporidea, the spores are large, and they usually infect fish (fish parasites). On the other hand, those in the class microsporidea have small spores and are intracellular parasites of arthropods and vertebrates.

Protozoa subphylum Ciliophora

Lastly, we have the subphylum ciliophora. In this subphylum, protozoa have simple ciliary organelles for locomotion, two nuclei (macronucleus and micronucleus), and heterotrophic nutrition.

Parasitism in Protozoa

To finish off, let's talk about Parasitism in protozoa. Pathogenic protozoans are found as amoebas, flagellates, ciliates, and sporozoa. They can cause different infections, such as intestinal, urogenital, and Blood and tissue infections (Fig. 6).

Intestinal protozoal Infections

The three most important protozoal parasites that cause intestinal infections are Entamoeba histolytica (amoeba), Giardia lamblia (flagellate), and Cryptosporidium parvum (sporozoan).

• A host gets infected with E. histolytica by the ingestion of protozoal cysts from contaminated food or water. These cysts form trophozoites in the small intestinal, eventually causing amoebic liver abscesses, and amoebiasis.

• The parasitic protozoa G. lamblia is responsible for the diarrheal disease giardiasis. This infection results from drinking contaminated water. In the United States, giardiasis is the most frequently diagnosed parasitic intestinal disease.

• C. parvum is a parasitic sporozoan that infects the lower small intestine.

Urogenital tract protozoal infections

When it comes to urogenital tract infections related to parasitic protozoa, the most common is the flagellate Trichomonas vaginalis. This parasite infects the vagina, vulva, and cervix of females. However, although uncommon, they can infect the urethra, prostate, and seminal vesicles of males. The most common form of trichomoniasis (a sexually transmitted disease) in humans is caused by T. vaginalis.

Blood and tissue protozoal infections

In this category, we have different known protozoal species, such as Plasmodium falciparum and toxoplasma gondii.

• P. falciparum is the most lethal and drug-resistant causative agent of Malaria.

• T. gondii is a sporozoan parasite that mostly infects cats but is capable of infecting nearly all warm-blooded animals. Infection can occur from cat feces, raw or undercooked meat, in utero from an infected mother, or from Blood transfusions.

Now, I hope that you feel more confident in your understanding of protozoa!