Protozoa classification and types
Protozoa are a diverse group of unicellular eukaryotic organisms that primarily exhibit heterotrophic feeding behaviors. Traditionally classified within the kingdom Protista, protozoa are informally divided into several subgroups, including algae, which are photoautotrophs, and various types of protozoa, which are primarily ingestive heterotrophs. The classification of protozoa is continually evolving as new genetic and cytochemical analyses reveal more about their relationships. Protozoa inhabit a range of environments, from freshwater and marine ecosystems to damp soils and as parasites within other organisms.
Common types of protozoa include excavates, which have distinctive flagella and modified mitochondria; alveolates, recognized by their membrane-bound vesicles; and amebozoans, which use pseudopodia for movement. Many protozoa are known pathogens, causing diseases in humans such as malaria from Plasmodium spp., dysentery from Entamoeba histolytica, and vaginitis from Trichomonas vaginalis. Their impact on human health, particularly in developing countries, highlights the necessity of understanding protozoa and their classification for better disease management and prevention.
Protozoa classification and types
Definition
Protozoa are members of an informal grouping of simple, usually unicellular, heterotrophic phyla that share similar characteristics. Some protozoa are pathogenic.
Classification
Traditionally, the kingdom Protista (also known as Protoctista) is made up of simple eukaryotic organisms that do not fit neatly into any of the other kingdoms. Often its members are more closely related to members of other kingdoms than to each other. For many years, this kingdom has been informally divided into three subgroups: the algae, which are photoautotrophs; the protozoa, which are ingestive heterotrophs; and the fungus-like protists, which are absorptive heterotrophs. These informal groupings sometimes break down, as among the euglenids and dinoflagellates, in which there are species that are photoautotrophs, species that are either absorptive or ingestive heterotrophs, and species that can switch between autotrophy and heterotrophy. Most taxonomists agree that the kingdom Protista is artificial (not monophyletic) and should be split into multiple kingdoms.
Like the taxonomy of the protists in general, the taxonomy of the protozoa is continually changing, as more species are subjected to rigorous cytochemical and genomic analysis. All protozoa are nucleated and are considered to be eukaryotic, although some may show both eukaryotic and prokaryotic characteristics. The normal classification hierarchy that starts with kingdom is often not used with Protista. The species are simply placed in taxa according to possible evolutionary and biochemical similarities without labeling the taxa.
Characteristics and Habitat
Protists can live in fresh- or marine-aquatic habitats, in damp soil, or in other organisms such as parasitic or commensal organisms. All can reproduce asexually, while some, like the alveolates, can also reproduce sexually.
Most excavates reproduce asexually and have flagella, a cytoskeleton of microtubules, modified mitochondria, and a large ventral groove used in feeding. Diplomonads, like Giardia spp., have two haploid nuclei, multiple flagella, and a modified mitochondrion, the mitosome, that is unable to perform cellular respiration because it lacks electron transport. All are anaerobic with bacteria-like anaerobic metabolism, and some are parasitic in the guts of animals. The related parabasalids also have a modified mitochondria-like structure called a hydrogenosome. This double membrane structure produces hydrogen as a by-product of the oxidation of pyruvate, much like hydrogen-producing bacteria. Parabaslids also have a prominent Golgi body (or Golgi apparatus) called the parabasal body that is involved in protein synthesis. Parabasalids also have an undulating membrane thought to be involved in locomotion. Many digest cellulose in the guts of termites and ruminants, while others, such as Trichomonas spp., are commensal or parasitic in animals.
The euglenoid kinetoplastids have a single large mitochondrion with discoid inner membranes. The mitochondrial deoxyribonucleic acid (DNA) is enclosed in a structure called the kinetoplast. Kinetoplastids also have Golgi bodies and a haploid nucleus. Some are parasitic, such as Trypanosoma spp., Leishmania spp., and Chrythidia spp. Among the nonparasitic kinetoplastids are some of the few colonial protozoa.
Alveolates are recognized by their alveoli, the membrane-bound vesicles just inside the plasma membrane. Almost all have plastids of red algal origin. Among the heterotrophic dinoflagellates are no important human pathogens, although many photosynthetic dinoflagellates produce toxins that can affect humans and other animals. All apicomplexans, formerly called sporozoa, are nonmotile parasites, contain a modified plastid called the apicoplast, produce spores, and have sexual reproduction. The most prevalent protistan disease of humans, malaria, is caused by the apicomplexan Plasmodium spp. All ciliates, as the name implies, have cilia, either on the entire surface or clustered in tufts or rings. They also have two types of nuclei, macronuclei and micronuclei, and reproduce both sexually and asexually. All, except for the occasionally pathogenic Balantidium, are nonpathogenic.
Amebozoans all have lobe- or tube-like pseudopodia for locomotion and lack cilia or flagella at any stage of the life cycle. Most cannot reproduce sexually. The nucleus has a prominent nucleolar region called the endosome, and many can produce resting stages called cysts. Gymnamebas, like Amoeba proteus, are free-living, unicellular amoeba that produce multiple pseudopods. They usually are aquatic and feed on bacteria or detritus. Entamoeba spp. are also unicellular, produce a single pseudopod at a time, and are parasitic in animals.
Impact
Many protozoa are parasitic pathogens. They cause a number of diseases in humans and exact a huge toll on the peoples of developing countries. The various Plasmodium spp., for example, cause the different forms of malaria, a disease that has had a devastating effect on the developing world.
The protozoan Giardia intestinalis (also known as G. lamblia) causes diarrhea in hikers and backpackers, especially, and Trichomonas vaginalis causes vaginitis and urethritis. Trypanosomes are responsible for sleeping sickness (T. brucei) and Chagas’ disease (T. cruzi), and because they can change their surface proteins, they often elude the host’s immune system. Leishmania spp. most commonly cause cutaneous leishmaniasis, an ulcerative disease of the skin. Toxoplasma gondii can be transmitted to humans through cat feces and can cause toxoplasmosis. Entamoeba hystolytica is the only major amebic pathogen in humans and causes amebic dysentery.
Bibliography
Katz, Laura, and Debashish Bhattacharya, eds. Genomics and Evolution of Microbial Eukaryotes. New York: Oxford University Press, 2008.
Margulis, Lynn, and Michael Chapman. Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth. 4th ed. New York: Academic Press/Elsevier, 2009.
Parker, Steve. Protozoans, Algae, and other Protists. Mankato, Minn.: Compass Point Books, 2009.
"Protozoa." LibreTexts, 1 Aug. 2021, bio.libretexts.org/Learning‗Objects/Laboratory‗Experiments/Microbiology‗Labs/Microbiology‗Labs‗I/48%3A‗Protozoa. Accessed 4 Feb. 2025.
Rogers, Kara. Fungi, Algae, and Protists. New York: Britannica Educational Press, 2011.