Protozoa – Concept, types, characteristics and examples

We explain what protozoa are, how they originated and their characteristics. In addition, its classification, reproduction and examples.

Most protozoa can be seen under a microscope.

What are protozoa?

It is called protozoa or protozoa to a set of microorganisms found in humid or aquatic environments, and that could be considered as microscopic animals. However, in some biological classification systems they form a kingdom of their own called Protozoa; and in other cases they are part of the Protista Kingdom, since they are considered the first evolutionary step of eukaryotic beings, prior to the existence of the animals, plants, fungi and algae that we know.

Traditionally, however, protozoa they are considered primitive single-celled animals: hence its name, a union of the Greek words protos, “first”, and zoo, “animal”. This is because they are heterotrophic (they must consume organic matter) and are endowed with voluntary movement. There is currently scientific debate regarding its correct classification in the different branches of the tree of life.

Most protozoa can be seen under a microscope, since its size ranges between 10 and 50 micrometers, and around 300,000 species of them are known, along the various rungs of the microscopic food chain: herbivores, decomposers, predators and parasites. Many of them are capable of infecting and making humans sick.

Origin of protozoa

Protozoa are thought to be around 1.630 million years old on our planet, from their initial appearance in the Mesoproterozoic period. Its origin coincides with the emergence of the first eukaryotic cells, that is, with a defined cell nucleus, and with the subsequent inauguration of a wide category of living beings.

Various theories try to explain this passage from the simple and primitive world of prokaryotes to that of eukaryotes, and one of the most accepted has to do with a process of endosymbiosis between two prokaryotic organisms. Those first eukaryotic organisms were, precisely, the first protozoa in history.

Characteristics of protozoa

Protozoa - biology
Protozoa are unicellular organisms endowed with their own mobility.

Protozoa are an extremely diverse group, whose fundamental characteristics are:

  • Microscopic size and varied shape. Most protozoa are between 10 and 50 microns in size, but some species can grow to a millimeter or more. Their shapes, on the other hand, oscillate between amorphous (like amoeba) or elongated and oval (like paramecium).
  • They are single-celled organisms. Your entire body is a single cell, endowed with various organelles and structures, which fulfill nutritional and mobile functions, etc.
  • They have their own mobility. And they move through flagella, cilia or the elongation of their cytoplasms, as if they were “fingers.”

Classification of protozoa

Flagellate protozoa have “tails” that aid in their movement.

The traditional classification of protozoa distinguishes between the following types:

  • Rhizopods. They are characterized by their displacement by pseudopods, that is, the formation of protrusions of their cytoplasm and the plasma membrane, projecting them towards where they want to advance. These projections also serve to capture food and introduce it to the cytoplasm (phagocytosis), either by predating other organisms or assimilating waste organic matter.
  • Flagellated. Cells endowed with one or more flagella, which is the name of the “tails” with which they propel themselves forward in the environment.
  • Ciliates. Its plasma membrane is surrounded by cilia, that is, by smaller and more numerous filaments than flagella, which also serve to mobilize itself.
  • Sporozoans. Parasitic protozoa without much mobility, which have a phase of multiple division known as sporulation: a type of asexual reproduction that consists of producing spores or endospores, resistant structures that generate a new identical individual.

Reproduction of protozoa

Binary division consists of a cell dividing in two.

Protozoa can reproduce sexually and asexually, depending on environmental conditions and their life cycles. They tend to do so abundantly, which is key to their biological and evolutionary success. Its main methods of reproduction are:

  • Binary (asexual) division. A process of cellular fission after mitosis (genetic replication), which consists of a cell dividing in two and generating new individuals identical to it and to each other.
  • Budding (asexual). A protozoan generates an identical copy of itself, within a resistant structure that remains with its parent and can even survive it during difficult periods. Eventually, that structure (gem) is reactivated and brings back to life a specimen identical to the parent.
  • Sporulation (asexual). The original protozoan fragments into a set of spores or endospores, which support environmental changes and then give rise to entire individuals.
  • Cell fusion (sexual). Protozoa generate gametes or microgametes in their interior, which allow them to unite and form a zygote, mixing their genetic materials and obtaining in return a new individual of greater genetic variety, original. This process can be total or partial, and is usually carried out in periods of abundance of resources.

Diseases caused by protozoa

Amebiasis is an intestinal infection caused by pathogenic amoebae.

Some species of protozoa are harmful to humans and have adapted to parasitize the body, causing diseases such as:

  • Malaria. Also called “malaria”, it is responsible for a genus of protozoa called plasmodium. Its symptoms are high fever, chills, sweating, headache, as well as nausea, cough, bloody stools, muscle aches, jaundice, and worsening with shock, kidney or liver damage and death.
  • Amoebiasis This is a common intestinal infection, caused by the presence of pathogenic amoebae (there are free-living and non-pathogenic ones as well) in the intestine or digestive tract of a person. These protozoa cover the intestinal wall and make it difficult for nutrients to be absorbed, causing diarrhea of ​​varying degrees.
  • Toxoplasmosis Caused by protozoa of the genus toxoplasma, which are transmitted to humans through contact with infected cats and other types of felines, or with infected animal or human feces. Its symptoms are confused with those of the flu, but it also causes inflammation of the lymph nodes, spleen, liver and cysts in the tissues, being its greatest danger in pregnant women, since it affects the fetus causing malformations and other problems.

Examples of protozoa

Some common protozoa are:

  • Paramecium. A fast-moving, oval-shaped, ciliated, free-living protozoan.
  • Giardia. Parasitic protozoan that invades the human intestines, causing foul gas, inflammation and diarrhea.
  • Amoeba. A genus of predatory protozoa, which may or may not parasitize other multicellular living beings, or live freely in aquatic spaces.
  • Trichomona. Another genus of parasitic protozoa, which invade the vagina and are sexually transmitted, causing foul discharge, itching and painful urination, and even risk of premature labor.