Kingdom Protista – Concept, types, characteristics and examples

We explain what the protist kingdom is, its characteristics and how it is classified. Also, how is their nutrition, reproduction and examples.

Protist kingdom
The protist kingdom groups together a set of generally single-celled organisms.

What is the protist kingdom?

Protista kingdom, also called protoctista, is understood to be one of the groups in which biology classifies living beings, more specifically eukaryotes, together with the animal, plant and fungi kingdom: all are called protists those eukaryotes that cannot be classified as animals, plants or fungi.

The protist kingdom is a paraphyletic group (they do not contain all the descendants of a common ancestor) and includes a very diverse set of organisms generally single-celled, both autotrophic and heterotrophic. This variety makes it difficult to characterize them, except for the common features of every eukaryotic being, that is, possessing cells with a defined cell nucleus.

The existence of a protist kingdom was proposed in 1969 in the theory of the five kingdoms of life, but at the moment it is considered a term in disuse, and there is a tendency to classify its integral members within the other branches of eukaryotic life.

The word protista comes from the Greek and means “primordial” or “first of the first”. Protoctista, similarly, translates “first creatures”.

Characteristics of protists

Protist kingdom
There is not much in common between the different forms of protists.

As they are not a monophyletic group, that is, they share an evolutionary history, the members of the protista kingdom they do not have fundamental common traits with which to characterize them, as they are not those of eukaryotic life: the same ones that plants, fungi and animals can present, but in the biological classification that attributes them the status of kingdom (that of Robert Whittaker of 1969), their fundamental condition would be to be “Eukaryotic unicellular or unicellular-colonial organisms, which do not make up any type of tissues.”

Thus, except for their relative evolutionary simplicity, there is not much in common between the various forms of protists, and present different models of nutrition, reproduction, locomotion and cellular structures.

Classification of protists

The kingdom of the protists is traditionally divided into very different supergroups, as follows:

  • Archaeplastida or Primoplantae. It contains the most primitive green and red algae, precursors of plant life, especially terrestrial life. For this reason many include them today within the kingdom plantae.
  • Stramenophiles or Heterokonta. Algae of different types, from unicellular to multicellular, as well as other saprophytic protists or parasites that lack chlorophyll, but resemble molds, since they would have evolutionarily lost their chlorophyll.
  • Alveolata. Aquatic organisms with and without chlorophyll, parasitic in some cases, but in any case presenting cortical alveoli (hence the name), vesicles that form a flexible film supporting the plasma membrane.
  • Rhizaria. Various amoeboid or flagellate organisms, with or without chlorophyll, such as sharp amoebas or mucous molds.
  • Excavate. Organisms formerly classified as flagellates, which have a central feeding groove, thus being heterotrophic, although many can present chlorophyll as a consequence of endosymbiosis with green algae. The classification of this group is a matter of debate.
  • Amoebozoa. A populated group of amoebae and amoeboids, characterized by forming pseudopods (“fingers”) with their cytoplasms. Some may be multinucleated and others may be pre-multicellular (dictyostelids).
  • Opisthokonta. Various protists that would have given rise to the kingdoms of animalia and fungi, endowed with a flagellum located in a position posterior to that of cell movement (opisthtocont).

Protist nutrition

Protist kingdom
Some protists lead a parasitic life.

Protists can have autotrophic or heterotrophic metabolisms, depending on whether they possess chloroplasts (chlorophyll) to carry out photosynthesis, or if instead they lack them and must feed on the surrounding organic matter (through osmosis or ingestion or phagocytosis).

Many of them have master nutrition mechanisms simultaneously, and some of them lead a parasitic existence, entering multicellular organisms and feeding on them, causing diseases.

However, the protists they are originally aerobic (They use oxygen for their metabolic processes), with the exception of those that evolved to live in environments where this element is scarce.

Reproduction of protists

The reproduction of protists can be both sexual and asexual, and sometimes the same species can alternate between one model and another, according to environmental conditions. Sexual reproduction occurs through the generation of gametes and cell fusion, while asexual occurs by cellular fission and mitosis. In no case are embryos produced.

Importance of protists

Protist kingdom
Thanks to the protists the different kingdoms of eukaryotes would be produced.

Protists are a diverse group and difficult to compartmentalize, but fundamental for the emergence of life as we know it. They were the first eukaryotic organisms to emerge in the primitive seas of the Earth, and from them life took various evolutionary courses that would produce the different kingdoms of eukaryotes: plants, animals and fungi.

Thus, protists constitute a previous step and relatively common to all these forms of eukaryotic life, including the human, and its study is also the study of eukaryogenesis, or the appearance of the cell nucleus in the evolutionary history of primitive prokaryotes.

Examples of protists

Protist kingdom
Plasmodia are transmitted through the bite of an infected mosquito.

Some well-known protists are as follows:

  • Paramecia. Ciliated, free-living unicellular organisms, abundant in stagnant waters and puddles, and is perhaps one of the most and best-studied protists in the world.
  • Amoebas. Called amoebas or amoebae, they are unicellular protists that move and feed through the generation of pseudopos or “fingers” with their cytoplasm, which gives them a changing and dispersed shape. They can be free-living or parasitic.
  • Plasmodia. A genus of parasitic protists of which up to 175 species are known, which are transmitted to vertebrate hosts through the bite of an infected mosquito. It is the cause of the disease known as malaria or malaria.
  • Glaucophytes. Unicellular freshwater algae, of which some 13 species are known, and which at times are included among plants. They are generally composed as individual cells, but they can also share the cell wall of their parent, since their reproduction is always asexual.
  • Coanozoans. This group of protists is linked to animals and fungi, constituting a kind of intermediate step between these two groups of eukaryotes. It can be divided into holomycota (similar to mushrooms) and holozoa (similar to animals).