Protozoa: Definition, Classification and Reproduction

Protozoa are the simplest animals, usually measuring only a few microns in size. The most primitive group of animal organisms is the protozoa. They are unicellular cells with no cell wall, motile organisms and form a very large, highly diverse group originating from several phylogenetic lines. Currently, there are roughly more than 50,000 different protozoa species. Some protozoa live as unwanted guests or parasites inside the bodies of other animals, causing serious diseases in their hosts such as malaria, kala-azar, sleeping sickness, dysentery, and so on. Read more about the definition of protozoa, classification, reproduction, importance and diseases caused by protozoa.

What are Protozoa?

Single-celled organisms are known as protozoa. They range in size and shape from an amoeba, which can change shape, to Paramecium, which has a fixed shape and sophisticated structure. They can be found in a range of damp conditions, such as freshwater, marine environments, and soil. The body of acellular protozoa is not separated into cells, and organelles are parts of protoplasm set aside for performing certain activities. Most protozoa are aerobic, mesophilic organisms that thrive at temperatures ranging from 16 to 25 degrees Celsius. Entamoeba, for example, is anaerobic.

Characteristics of Protozoa

1. Habitat- Protozoans exhibit mainly two forms of life;-living (aquatic, freshwater, seawater) and parasitic (ectoparasites or endoparasites). They are also commensal in habitat.

2. Size and Shape– They’re microscopic and can only be seen under a microscope. They are the most basic and rudimentary of all organisms. The body is made up of only one cell (without tissue and organs). Spherical, oval, elongated, or flattened body form variables are present. Body symmetry is either none or bilateral or radial or spherical. The body form is usually constant, varied in some, while changing with environment or age in many. The body is usually naked or bounded by a pellicle, but it can be covered in shells and has an internal skeleton in other species.

3. Cellular Structure–
a. Body organisation is simple, that is, with a protoplasmic grade of organisation. They have one or more nuclei that are monomorphic or dimorphic. They are solitary (existing alone/single) or colonial (individuals are alike and independent).
b. The protozoal cells are not enclosed by a cell wall; rather, their protoplasts are protected by a special thin and pliable layer which is commonly known as a pellicle or periplast.
c. The cytoplasm of many protozoal cells is separated into two layers: an outside layer called ectoplasm and an interior layer called endoplasm.
d. The consistency of the ectoplasm is often denser than that of the endoplasm. The protoplast consists of the ingredients typically present in eukaryotic cells, like the membrane-bound nucleus, Golgi bodies endoplasmic reticulum, ribosomes and mitochondria.
e. The single-cell body performs all the essential and vital activities, which characterise the animal body; hence only the subcellular physiological division of labour.
f. Another distinguishing trait of some protozoa, such as ciliates, is that they have two different types of nuclei: a polyploid macronucleus and a diploid micronucleus.

4. Locomotory organs are fingers like pseudopodia, whip-like flagella, hair-like cilia or none. Membrane-bound proteins play a role in how the body reacts to various environmental stimuli.

5. Nutrition may be holozoic (animal-like), holophytic (plant-like), saprozoic or parasitic. Euglena, for example, has chromatophores and does photosynthesis. These photosynthetic protozoa are commonly referred to as algae. The majority of protozoa are, however, heterotrophic or holozoic, i.e. they live by phagocytosis of other organisms.

6. Digestion takes place inside the food vacuoles, which is an intracellular process. Solid food particles collected by the protozoa are stored and digested in the food vacuoles. Enzymes are found in secretory vacuoles.

7. Respiration occurs by diffusion via the general body surface.

8. Excretion occurs primarily through the surface, although it can also occur through a brief opening in the ectoplasm or a permanent pore termed the cytopyge. In freshwater species, contractile vacuoles regulate osmoregulation and assist in the removal of excretory products.

9. Reproduction- Asexual and sexual reproduction are both possible for protozoa. However, sexual reproduction is less common and only happens in select taxa.
A. Asexual (binary or multiple fission, budding, sporulation)
a. Binary fission is used in amoebae to reproduce asexually. The pseudopodia are retracted into the protoplast before division. The nucleus then divides, and the protoplast is divided in the middle, resulting in the formation of two daughter amoebae. It occurs in flagellated protozoa as well, but it usually occurs along the cell’s long axis (longitudinal binary fission).
b. Budding is another way for protozoa to reproduce asexually. Mitotic daughter nuclei migrate into a cytoplasmic protrusion (bud) that is finally fissioned away from the mother cell.

B. Sexual (conjugation (hologamy), gamete formation (syngamy)
a. Syngamy: A motile microgamete unites with a non-motile macrogamete (anisogamy) to generate a diploid zygote in Sporozoa (Apicomplexa) similar to Plasmodium (malarial parasite).
b. Conjugation is another way of sexual reproduction found in ciliate protozoa such as Tetrahymena and Paramecium.
c. Autogamy: The male and female gametes give rise to the same unicellular fusion to form a diploid zygote. Thus, in autogamy, both gametes are produced by the same parent. It occurs in some ciliated protozoa like Paramecium.

10. The life cycle is often complicated with alternation of asexual and sexual phases (alternation of generation). Most protozoa have a life cycle that alternates between a latent cyst stage and a proliferating vegetative stage, such as trophozoites. Without water or nutrition, the cyst stage can survive in extreme environments. It can stay outside the host for prolonged periods of time before being transmitted. The trophozoite stage is contagious, and it is during this stage that they feed and multiply.

11. Encystment commonly occurs to resist unfavourable conditions of food, temperature, and moisture and also helps in dispersal.

12. The single-celled individual is not differentiated into somatoplasm and germplasm; therefore, exempt from a natural death is the price paid for the body.

13. Examples: Euglena, Amoeba, Plasmodium, Paramecium, Podophyra, etc.

Classification of Protozoa

1. Protozoa are a highly diverse group of unicellular eukaryotic organisms having a wide range of sizes, morphological characteristics and physiological properties.

2. In 1980, an expert committee of the Society of Protozoologists proposed a protozoa classification scheme.

3. Protozoa were classified as a sub-kingdom of the kingdom Protista, and protozoa were divided into six phyla in the proposed classification.

4. Sarcomastigophora, Labryinthomorpha, Apicomplexa, Microspora, Myxozoa, and Ciliophora are among the species. This classification, however, is not phylogenetic.

5. Based on studies made till now, a classification system based on evolutionary history has divided the subkingdom of Protozoa also into six phyla. These have been recognised as:
a. Archaezoa
b. Microsporidia
c. Rhizopoda
d. Apicomplexa
e. Ciliophora
f. Euglenozoa

There are four classes of protozoa in the phylum protozoa. The primary criterion for classification is the mode of locomotion.

Class I:
Rhizopoda (Rhiza= root; podus = foot). Protozoa have peculiar temporary organelles for locomotion, called pseudopodia or false feet—examples—Amoeba proteus; Entamoeba histolytica (causing dysentery).

Class II:
Mastigophora (Mastix =whip; phoros=bearer) or Flagellata (flagellum = whip). Protozoa that move by the lashing of whip-like organelles called flagella. Example—Euglena Viridis; Trypanosoma Gambiense(causing African sleeping sickness); Leishmania donovani (causing kala-azar).

Class III:
Sporozoa (Spora —seed; animal). Parasitic protozoa possess no locomotor organelles and reproduce using spores. Example—Monocystis gregarina; Plasmodium vivax (causing malaria).

Class IV:
Ciliophora (Cilium— eyelash). Protozoa that move by hair-like cilia. Example—Paramoecium caudatum (the slipper- animalcule); Vorticella campanula (the bell-animalcule).

Importance of Protozoans

1. Protozoans have a critical function in soil fertility. They manage bacterial populations and keep them in a state of physiological youth, or active growth phase, by feeding on soil bacteria. This accelerates the decomposition of dead organic substances by microbes.

2. Protozoans also excrete nitrogen and phosphorus as products of their metabolism, in the form of ammonium and orthophosphate, and studies have shown that the presence of protozoans in soils promotes plant growth.

3. In both activated sludge or slow percolating filter plants, protozoans play a significant role in wastewater treatment.

4. Protozoans provide shelter, carbon, and vital phytonutrients in exchange.

5. Protozoan grazing can have a significant impact on phytoplankton. These planktonic protozoans, like soil protozoans, expel a lot of nitrogen and phosphate.

6. Protozoans play a critical role in the phytoplankton’s recycling of key nutrients (nitrogen and phosphorus).

Protozoan Disease

Summary

Single-celled organisms are known as protozoa. They range in size and shape from an amoeba, which can change shape, to Paramecium, which has a fixed shape and sophisticated structure. They can be found in a range of damp conditions, such as freshwater, marine environments, and soil. Protozoa are known for their ability to move on their own, which is a trait shared by the majority of species. They cannot frequently photosynthesise, even though the Euglena genus is known for both motility and photosynthesis (and is therefore considered both an alga and a protozoan).

Protozoans play a critical role in the phytoplankton’s recycling of key nutrients (nitrogen and phosphorus). Protozoa that dwell in a human’s intestine are often transmitted to another human via a faecal-oral pathway (for example, contaminated food or water or person-to-person contact). African sleeping sickness, amoebic dysentery, and malaria are among the most common and deadly human diseases caused by protozoan infections. Malaria and African sleeping sickness are two protozoan-bourne illnesses.

Frequently Asked Questions (FAQs) on Protozoa

Q.1. What are protozoa?
Ans: Single-celled organisms are known as protozoa. They range in size and shape from an amoeba, which can change shape, to Paramecium, which has a fixed shape and sophisticated structure. They can be found in a range of damp conditions, such as freshwater, marine environments, and soil.

Q.2. What are the unique characteristics of protozoa?
Ans: Protozoa are known for their ability to move on their own, which is a trait shared by the majority of species. They cannot frequently photosynthesise, even though the Euglena genus is known for both motility and photosynthesis (and is therefore considered both an alga and a protozoan).

Q.3. Which disease is caused by protozoa?
Ans: African Sleeping Sickness, amoebic dysentery, and malaria are among the most common and deadly human diseases caused by protozoan infections. Malaria and African sleeping sickness are two protozoan-borne illnesses.

Q.4. Where do protozoa live?
Ans: Protozoa can be found in a range of moist environments, including freshwater, seawater, and soil.

Q.5. How are protozoa transmitted?
Ans: Protozoa that dwell in a human’s intestine are often transmitted to another human via a faecal-oral pathway (for example, contaminated food or water or person-to-person contact).

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