What Is the Protista Kingdom?

Before explaining what the Protista kingdom is, its characteristics, classification and other aspects, it is important to emphasize that the Protista kingdom is one of the five kingdoms of living beings.

The system of biological kingdoms is the way in which science classifies living beings according to their lineage in the course of evolution. In addition to the kingdoms of living things, there are other taxonomic categories within the same classification system, such as domain, phylum, class, order, family, genus, and species. The kingdoms are:

• Animalia
• Plantae
• Fungi
• Protista
• Monera

The Protista kingdom includes organisms that are considered the first eukaryotic life forms, such as algae, protozoa, or slime molds, which precede plants, animals, and fungi.

The Protista kingdom is a paraphyletic group, which means that it does not contain all the descendants of a common ancestor. It also includes a very diverse group of organisms, usually unicellular or simple multicellular organisms that do not form tissues. Their enormous diversity makes them difficult to characterize apart from the common features of all eukaryotic organisms, i.e., the possession of cells with a defined nucleus. Apart from their relative evolutionary simplicity, the various protists do not have much in common. They differ in diet, reproduction, locomotion, and cell structures.

You may also be interested in this other article, where we explain the differences between eukaryotic and prokaryotic cells.

To better understand the Protista kingdom, we will mention and explain its main features:

• As eukaryotes, they have a nucleus delimited by a membrane and no differentiated vegetative tissue except for reproductive structures.
  
  
• Their size can range from unicellular microscopic forms to multicellular organisms several meters long.
  
  
• They live in a variety of environments, but usually prefer aquatic or wet environments. They are found in soil, fresh water, on the sea floor, in decaying organic material, etc.
  
  
• Numerous species exhibit flagella, cilia, and pseudopods as locomotion modes, but there are also numerous species that do not move.
  
  
• Many organisms are free-living, while others form symbiotic relationships (mutualists, parasites, etc).
  
  
• Most have mitochondria.
  
  
• Some species have developed the amazing ability to create resistance structures called cysts during their life cycle in times of unfavorable conditions such as lack of water and food. This helps them survive in a state of latency (cessation of their metabolic functions) until more favorable conditions prevail for their reproduction and survival.
  
  
• They are able to create spores at different stages of the life cycle, which ensure their reproduction and dispersal.
  
  
• Depending on their reproductive strategy, they can reproduce asexually through mitosis, dichotomy, budding, or multiple divisions, or sexually through meiosis and mitosis.

The complex and diverse characteristics of protists make their classification one of the most difficult and convoluted in the evolution of living things. Many scientists throughout history have attempted to find the most accurate classification possible for the Protista kingdom.

Currently, the various protists are classified into the following supergroups:

• Opisthokonta: includes animals, fungi, and several protist lineages most closely related to either animals or fungi.
  
  
• Amoebozoa: free-living amoeboid forms with lobed pseudopodia (e.g., amoebae), but also more filose amoebae, some flagellates, and various slime molds.
  
  
• Excavata: comprises Discoba, Metamonada and Malawi monads.
  
  
• Archaeplastida: comprises the photoautotrophic red algae (Rhodophyta), green algae, land plants, and the minor group glaucophytes.
  
  
• Chromalveolata: Alveolata, Stramenopila, Haptophyta and Cryptophyta.
  
  
• Rhizaria: includes a wide variety of amoebae, flagellates, various parasites, and the chlorarachniophyte algae.

Next, we will examine some examples of protist species to learn more about their characteristics and appearances.

Even after learning how this realm is classified, one naturally wonders which organisms belong to the protista realm. For this reason, we have listed some of the best known genera and most representative species of the Protista kingdom:

• Laminaria digitata: Laminaria digitata is a large brown alga that can grow up to two meters long.
  
  
• Sargassum: is a genus of brown macroalgae common in temperate and tropical oceans of the world.
  
  
• Chlamydomonas: is a genus of green algae consisting of about 150 species found in stagnant waters and moist soil, freshwater, seawater, and even snow.
  
  
• Ulva: a genus of edible green algae widely distributed along the coasts of the world's oceans and popularly known as sea lettuce.
  
  
• Trypanosoma cruzi: is a species of parasitic euglenoids that cause Chagas disease in humans, dourine and surra in horses, and brucellosis-like disease in cattle.
  
  
• Entamoeba histolytica: is a parasite that primarily infects humans and other primates and causes amoebiasis, E. histolytica. It is estimated to infect about 35-50 million people worldwide.
  
  
• Balantidium coli: is a parasitic species of ciliates that causes the disease balantidiasis.
  
  
• Paramecium caudatum: is a free-living protozoan. They can grow up to 0.33 mm long and are covered with tiny hair-like organelles called cilia.
  
  
• Plasmodium vivax: is a protozoal parasite and a human pathogen.
  
  
• Giardia lamblia: is a flagellated parasitic microorganism that colonizes the small intestine, causing a diarrheal condition known as giardiasis.
  
  
• Trichomonas vaginalis: is an anaerobic, flagellated protozoan parasite and the causative agent of a sexually transmitted disease called trichomoniasis.

You might also be interested in this other article, where we explain what is the kingdom Monera, its main characteristics and classification criteria.

The organisms of the kingdom of Protista are considered the first link of living things and are important because they contribute to the balance of nature and other living things. They also contain essential components for various algae and their photosynthesis, and are able to decompose and recycle nutrients for other animals.

Some of the key factors that make this realm indispensable for the survival of the planet, are the following:

• Some protists also produce oxygen and can be used to make biofuel.
  
  
• Protists are the main food source for many animals. In fact, Phytoplankton is one of the few food sources for whales. Zooplankton is eaten by various marine animals, such as shrimp and crab larvae.
  
  
• Protozoa play an indispensable ecological role because they form a link in the food chain.
  
  
• They are producers of organic material.
  
  
• They are natural predators of bacteria.
  
  
• Some of them create the zooplankton in seas, oceans and water bodies, where they are the trophic link between nutrient producers and utilizers.
  
  
• They are used in certain industrial processes (bioinsecticides) and transformation (wastewater treatment, bioremediation).
  
  
• In the health sector, they are influential as infectious agents causing malaria or amoebiasis.

You may be interested in this other article, where we discuss the differences between biomes and ecosystems.

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