PROTIST KINGDOM: CHARACTERISTICS, CLASSIFICATION, EXAMPLES - BIOLOGY - 2025

The protist kingdom is made up of eukaryotic unicellular organisms that cannot be included in the other three eukaryotic kingdoms: plants, fungi, or animals. It comprises a series of mostly microscopic and eukaryotic organisms that includes slime fungi, protozoa, and certain algae.

This term was proposed by the German zoologist Ernst Haeckel to encompass lower organisms with a primitive nucleus, lacking a nuclear membrane, to more complex individuals with a well-defined nucleus.

Different types of protists

Protists are a heterogeneous group, with a structural diversity that is not found in any other lineage of organism. Therefore, they have very few general and unique characteristics that distinguish them. Their diversity is so wide that they are similar to fungi, plants and even animals.

In terms of size they are extremely varied, ranging from organisms that cannot be detected with the naked eye, to algae that reach several meters in length.

In general, the organisms that belong to this kingdom are unicellular, although there are multicellular species and some live in colonies. At the cellular level, they are very complex, since they must carry out all the basic vital functions of a multicellular organism in the space corresponding to a single cell.

In the past, the classification of all these organisms was restricted to the protist kingdom. Currently the vision of the protist kingdom is considered obsolete, since modern systematics has restructured the classification of eukaryotes. Following the principles of the cladist school, the “protist” group should not be accepted because it is paraphyletic.

The paraphilia - a group of organisms that contains the most recent common ancestor but not all descendants - of a group implies that some protists are more related to the group of plants, fungi and animals than to other protists. For this reason, several separate lineages are now considered.

Some examples of protists are Paramecium, a ciliated organism whose shape resembles a slipper, and the flagellate parasite Trypanosoma cruzi, the causative agent of Chagas disease.

Characteristics of the protist kingdom

Some examples of protist organisms. By Alejandro Porto, via Wikimedia Commons

It is a very varied kingdom

They have great functional and structural diversity. The main characteristic they have in common is that most are unicellular and that they are not animals, plants, or fungi.

They are a polyphyletic group

The protist kingdom is a group that comes by evolution from various ancestral groups. These organisms are polyphyletic because they do not all descend from a common ancestor. For this reason it is impossible to specify characteristics that determine them in general.

It can be said that the characteristics that protists have in common are to maintain a very simple structure and all those typical of eukaryotic organisms.

Most protists are unicellular

Organisms in the protist kingdom are commonly single-celled with a fairly simple structure. Almost all the members of this kingdom are organisms not visible to the naked eye and are usually identified through a microscope.

There are certain algae, especially red and brown algae that have a slightly more complex organization forming an almost tissue or tissue composition.

They can also form colonies of individuals that behave as if they were a single organism but without becoming a tissue.

They are eukaryotic organisms

A eukaryote is an organism with a complex cell in which the genetic material is organized within a nuclear membrane or nucleus.

Eukaryotes comprise animals, plants, and fungi, all of which are mostly multicellular, as well as various groups that are collectively classified as protists (which are usually unicellular).

Like all eukaryotic cells, protists have a characteristic central compartment called the nucleus that houses their genetic material. They also have specialized cellular machinery called organelles that perform defined functions within the cell.

Photosynthetic protists, like various types of algae, contain plastids. These organelles are the place where photosynthesis (the process of absorbing sunlight to produce nutrients in the form of carbohydrates) takes place.

The plastids of some protists are similar to those of plants. Other protists have plastids that differ in color, the repertoire of photosynthetic pigments, and the number of membranes enclosed by the organelle.

In contrast, prokaryotes are organisms such as bacteria that lack nuclei and other complex cellular structures.

Aquatic or humid habitat

Protists do not have a respiratory system. The mechanism of respiration is carried out by gas diffusion through the plasma membrane.

It occurs mainly through the aerobic process, but some protists that live in the digestive tracts of animals function strictly under the anaerobic process.

Anaerobic respiration is the simplest and occurs when there is a lack of oxygen. This type of respiration differs from the daily ventilation of humans or animals. It is a chemical process in which energy is released from food substances, such as glucose or sugars.

Aerobic respiration needs oxygen to function. Most chemical reactions occur in the mitochondria.

Varied locomotion

Most protists are endowed with mobility and can move, either by creeping, by pseudopods or by flagella and cilia.

Cilia and flagella are microtubule structures that help them to move in a moist environment.

Other protists move through temporary extensions of their cytoplasm known as pseudopodia. These extensions also allow protists to capture other organisms on which they feed.

They can be pathogenic organisms

There is a group of protists that, due to their characteristics, act as pathogens in plants, animals and humans. Among them are:

-Amebic dysentery, which is an intestinal infection caused by a type of amoeba called Entamoeba hystolytica.

-Chagas disease, caused by Trypanosoma cruzi, a flagellate that infects humans through an insect (the snout bug).

-Malaria or malaria, caused by plasmodium, a protist that is transmitted through the bite of infected mosquitoes.

Nutrition

The mode of feeding of these organisms is as varied as their members. They can be autotrophic or heterotrophic. Some individuals can feed by both forms in a facultative way.

Autotrophs

Autotrophic organisms, like plants, are capable of synthesizing their own food from an inorganic substrate. One way to convert an inorganic compound to organic matter is photosynthesis. This process occurs in chloroplasts and requires the presence of sunlight.

Some protists capable of synthesizing their own food through photosynthesis are the euglena (Euglena gracilis) and Volvox aureus. This last organism has the ability to form colonies, they are grouped in a gelatinous matrix and each individual is called a zooid.

Euglena and other species such as Ochromonas mutabilis and Petalomonas mediocanellata are capable of using more than one type of nutrients at the same time or on different occasions.

Heterotrophs

In contrast, heterotrophs obtain the organic molecules necessary for their nutrition from other sources.

This form of food is much more varied and can occur due to the phenomenon of phagocytosis in which the unicellular organism surrounds the food particle with its cell membrane and thus remains trapped inside the cell. Some examples are Amoeba histolytica and Paramecium caudatum.

In addition, they are capable of consuming decomposing matter and this mode of feeding is called "saprobiotic". Depending on the type of matter, they can be differentiated into saprophytic and saprozoic. The first group consumes decaying plants and the second eats animals. Some examples are Astasia klebsi and Polytoma uvella.

In this group of organisms, coprozoic organisms that feed on excrement have also been reported, among them Oikomonas thermo, Bodo caudatus and Copromonas subtilis.

Reproduction

Organisms of the protist kingdom can reproduce asexually by mitosis, followed by processes of bipartition, budding or divisions or sexually.

Asexual reproduction

Budding is a form of asexual reproduction and lies in the formation of bumps in an individual parent or mother. This cellular overhang begins to grow and develop.

When it reaches the necessary size it can separate from the mother individual, thus creating a new organism. It is also possible that the new organism becomes attached to it.

Similarly, binary fission is another way of asexual reproduction. This phenomenon begins with DNA replication, then the cytoplasm divides, giving rise to two daughter cells. Depending on the way the division occurs, the process can be regular, with the two daughter cells being of similar size, longitudinal or transverse.

Another type of asexual reproduction is fragmentation, in which the individual is able to divide into pieces and each one is capable of generating a separate individual.

Sexual reproduction

On the other hand, there are species that can form their gametes through mitosis processes. Sex cells can come together in a standard fertilization process or self-fertilization can occur.

In most flagellates, algae, amoeboids and certain parasites, they can reproduce sexually by fertilization of the gametes.

Ciliates, in contrast, reproduce primarily by conjugation, which consists of the exchange of genetic information.

There is a phenomenon called alternation of generations, where the haploid phase is interspersed with a diploid phase.

Origin

Protists are organisms that often go unnoticed, since they are microscopic beings. However, they are of vital importance for life in rivers and seas because they represent the food in the chain of animals.

It is complex to know which was the first eukaryotic cell that arose in the world. Despite this, scientists say that there was a protist ancestor that evolved to form colonies, which is known as foraminifera.

It is believed that the origin of this kingdom was in eukaryotic unicellular organisms that, over time and thanks to the laws of nature, transformed into simple colonies and then into more complex groups.

Metabolism

The protist kingdom is of aerobic origin, this means that organisms use oxygen to extract energy from organic substances.

Despite this characteristic, some developed the secondary ability of anaerobic metabolism to survive in low-oxygen habitats.

Classification

There are mainly three groups of protists: protozoa, euglenozoa and archaezoa.

Protozoa or protozoa

They are microscopic-sized unicellular organisms that usually live in humid or aquatic areas. They have a free life and have a heterotrophic metabolism.

These organisms breathe through the cell wall, so they are usually sensitive to oxygen deprivation. Although they are composed of a single cell, similar to the eukaryotes of the metazoans, they can form colonies.

However, each individual behaves differently and does not depend on their group to survive, this characteristic allows them to function if the colony becomes separated.

The body of these organisms takes various forms. Sometimes they have no cover, as is the case with amoebae; in others there is presence of skeletal covers.

They have an encyst capacity that can be used as a method of protection against water scarcity or for reproductive purposes.

The main source of food for protozoa are bacteria, other organisms and organic debris, supplies that it digests through the digestive vacuole and whose indigestible parts it expels through the same vacuole, which is called the fecal vacuole.

Regarding its reproduction, it can be sexual or asexual. Almost all protozoa use the asexual form to duplicate themselves.

The process consists of the division of the organism into two or more daughter cells. If these cells are similar it is known as binary fission. If, however, one is smaller than the other, it is a budding.

The group of protozoa or protozoa at the same time is subdivided into polyphyletic groups such as:

- Rhizopods

They are amebic protozzos. They are transported through temporary appendages from their surface, which are called pseudopods.

These are deformations of the cytoplasm and the plasma membrane that occur in the direction of displacement and that drag the rest of the body.

- Ciliates

They are organisms surrounded by cilia, filiform structures, and that have a complex internal structure: they can surround all or part of the cell.

Through the cilia they can move and also create currents to place food in your mouth.

- Flaggles

It has one or more flagella; that is, filaments longer than the cilia and whose movement helps to move the cell.

They are composed of unicellular forms without cell walls and occur in small numbers.

- Sporozoans

They are parasites in the phase of multiple division. They do not have much mobility, which causes that there are several groups without any relation.

Euglenozoa or chromist

They are protists that possess mitochondria. They have characteristics similar to plants, because some are photosynthetic and have chloroplasts.

They are flagellated and unicellular in a variable way, this means that they can go from an immobile state, changing shape to a spherical one and becoming encrusted. Many times they group together to form colonies. In this case, each cell can be linked by a gelatinous, sessile or free matrix.

These organisms feed on smaller ones such as bacteria. In the case of those that have chloroplasts, they are also nourished by absorption.

Euglenozoa have two flagella: one forward and one backward. Their reproduction is asexual via bipartition, even when they are in the flagellated phase.

First there is a duplication of all organelles and then cytokinesis follows the helical lines of the periplast bands. In the case of closed mitoses, the nuclear membrane does not fall apart.

These organisms are skilled in their environment. For example, when conditions are unfavorable, they stick up and germinate when they return.

As well as protozoa or protozoa, euglenozoa have four groups:

-

They live in fresh water, especially when it is rich in organic matter. However, they can also be seen in salt water, although it is not very common.

Some have chloroplasts and are photosynthetic, and others feed by phagocytosis or pinocytosis.

-

In this classification are several parasites that are responsible for serious diseases in humans and animals, such as Chagas and Leishmaniasis.

-

Free-living fogotrophs and some parasites. They especially inhabit marine water where they feed on algae and other aquatic elements.

- Postgaardea

They are flagellate protists that live in low oxygen space. This situation has forced them to develop characteristics that facilitate the absorption of nutrients by bacteria and other organisms.

Archaezoa

Protists that do not have mitochondria are called organelles that were added to the eukaryotic cell by endosymbiosis.

This classification is modern, since it was previously believed that the absence of mitochondria was the result of an evolution by parasitism, which is called secondary absence.

Despite this, the biologist Thomas Cavalier-Smith proposed this type of protist to give names to groups that are originally devoid of mitochondria and that he considered isolated descendants of eukaryotes.

This group is the subject of research by scientists to verify if the absence of mitochondria is for original reasons or if it is an evolution of the protist kingdom.

Protist algae

The protist kingdom also includes the so-called protist algae, which are autotrophic organisms that photosynthesize. They usually live in water or in very humid environments.

In principle, there were doubts about whether or not to include them in the protist kingdom given that they have cell walls and chloroplasts, elements more related to the plantae kingdom.

Most algae are unicellular, although there are some multicellular as well. There are three types: brown, green and red.

Examples of protist organisms that transmit disease

During in-depth work on the protist kingdom, it has been said that many of these organisms are responsible for spreading diseases and viruses. The most typical are the following:

Entamoeba histolytica

Entamoeba histolytica

It is an anaerobic protozoan that causes amoebic dysentery or amoebiasis, a serious intestinal disease for humans that causes diarrhea and large ulcers on the walls of the intestines.

It is a condition that must be treated medically, because if it progresses it can spread to other organs such as the liver, lungs or brain, causing abscesses.

Dysentery is characterized by bloody stools and mucus. One of the first symptoms is abdominal pain and is diagnosed through a stool exam.

Trypanosoma

Trypanosoma

It is a genus of unicellular protist parasites that parasitize the tsetse fly, which can transmit sleeping sickness to humans.

In addition to the rise in temperature, this condition is reflected with severe headaches and joint pain. If it is not treated medically in time, it can cause fatal damage to the heart and kidney.

It is also common to have symptoms of confusion, sleepwalking during the day, and insomnia at night if you cross the blood-brain barrier; that is, if it reaches the central nervous system.

Trypanosomiasis or African sleeping sickness is fatal if not treated under medical care.

Sporozoans

Plasmodium malariae

Parasitic protozoa responsible for diseases such as malaria or malaria, the most widespread infection in the world according to the World Health Organization.

According to studies, the contagion arose from parasitic dinoflagellates that live in the marine intestine. Approximately 300 to 500 cases of malaria occur a year and more than 800 thousand people die.

Plasmodium is the name of the parasite that gives life to malaria. This disease is transmitted by the female anopheles mosquito. However, the parasite has two factors: a mosquito that acts as a vector and a vertebrate host.

Once the infection enters the body, it matures in the liver and blood cells. Symptoms include fever, anemia, bloody stools, chills, seizures, headaches, and extreme sweating.

Toxoplasma gondii

Toxoplasma gondii

It is a parasitic protozoan that causes toxoplasmosis. The infection enters the human body from eating contaminated meat, accidentally eating cat feces, or eating unwashed vegetables.

Its physical manifestations are confusing, because in healthy people it can be asymptomatic or could even be confused with the flu.

However, in HIV patients it is fatal, as it can trigger necrotizing encephalitis or retinochoroiditis.

Trichomonas vaginalis

Trichomonas vaginalis

It is a pathogenic protozoan that transmits trichomoniasis, a sexually transmitted disease. Although its symptoms are not bothersome, since they resemble vaginitis, it should be treated with a doctor, since its infection facilitates the spread of HIV.

The warning sign par excellence is the secretion of a white liquid in women and burning urination in men.

Ecological importance

From the ecological point of view, protists are indispensable components of plankton and soil communities, being a crucial element in food chains.

Specifically, autotrophic protists play an important role as primary producers in seas and bodies of water. Plankton serve as food for an immense variety of fish, echinoderms, and crustaceans. Therefore, certain species serve as indicators of environmental quality.

Protists are capable of establishing symbiotic relationships with other organisms. There are several examples of typical microbiological relationships between a protist that inhabits the digestive tract of animals and participates in the digestion of food.

Furthermore, protists with a parasitic way of life are considered key elements in the maintenance of the ecological diversity of different ecosystems, because they exert a regulatory role over the populations of their hosts and in the structure of the communities.

References

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