PROTOZOOLOGY: HISTORY, FIELD OF STUDY AND RESEARCH - BIOLOGY - 2025

The protozoología is a branch of zoology that studies the protozoan, a large and heterogeneous group of unicellular organisms, mobile and heterotrophic. The etymology of the term comes from the Greek words proto (first) and zoo (animal). Euglena, Paramecium, and Amoeba are widely known genera of microorganisms studied by protozoology.

Defining protozoology is a complex task, because the definition of the object of study of this branch of knowledge, that is, protozoa, has been a controversial issue since its origins.

Euglena gracilis. The Euglenophyceae are the only photosynthetic protozoa. Source: Ellis O'Neill, via Wikimedia Commons

The history of this discipline dates back to the second half of the 17th century, when the microscopic world began to become visible to the human eye, thanks to the invention of the first optical instruments.

Protozoology is considered an integrative science, which addresses basic research in the areas of taxonomy, systematics, evolution, physiology, ecology, molecular biology, cell biology, among others.

While the controversy over the definition of the group continues, recent research continues to address old questions that provide the foundation for classification. Thus, currently highly relevant topics are addressed, such as oil prospecting or bioremediation.

History

First observations and descriptions

The earliest observations and descriptions of protozoa are attributed to the Dutch naturalist A. van Leuwenhoek, who built simple microscopes for observing the natural world during the second half of the 17th century.

Painting by A. van Leeuwenhoek, 1686. Source: See page for author, via Wikimedia Commons

The first systematic description of protozoan organisms was made by the Danish scientist OF Müller, in 1786.

In 1818, Georg Goldfuss proposed the term protozoan to group the unicellular organisms considered by him as primitive.

In 1841, Dujardin's studies on sarcoda (later known as protoplasm) allowed the interpretation of cell structure, which later made it easier to understand that protozoa are unicellular organisms.

Between 1880 and 1889 Otto Bütschli published three volumes on the Protozoa that earned him the qualification of architect of protozoology, by giving structure to modern protozoology.

Protozoology as a discipline

In the mid-19th century, important events in the history of protozoology took place that gave recognition and prestige to this branch of zoology.

In 1947 the first journal of protozoology was founded in Jena, Germany; Archiv für Protistenkunde. That same year, the Protozoology Society was born in the city of Chicago, USA. Another important event was the holding of the First International Protozoology Congress held in Prague, Czechoslovakia, in 1961.

The improvement of microscopes, at the beginning of the 20th century, increased the number of known microorganisms and allowed to expand the knowledge about this group of organisms.

The creation, diversification and massification of the use of electronic microscopes in the middle of the 20th century, promoted great advances in the study of the taxonomy, systematics, morphology and physiology of protozoa.

Protozoa in the first classifications

Classifications of organisms by ancient Greek philosophers did not include microscopic organisms. Technologies and the advancement of knowledge resulted in increasingly novel classification proposals, following the incessant search for a natural classification.

In 1860 Hogg proposed the Protoctist Kingdom to group primitive plants and animals. Later Haeckel (1866) proposed the Protista Kingdom to group unicellular organisms.

In 1938, HF Copeland proposed the use of four kingdoms: Monera, Protista, Plantae, and Animalia. The Monera Kingdom groups the cyanobacteria and bacteria that had been included by Haeckel within the Protista. This regrouping was based on its enucleated character, discovered by Chatton.

Starting from the Coperland classification, RH Whittaker separated the mushrooms from Protista and created the Fungi Kingdom, establishing the traditional classification of five kingdoms.

Woese, in 1977, recognized only three evolutionary lineages: Archaea, Bacteria, and Eukarya. Subsequently, Mayr in 1990 proposed the Prokaryota and Eukaryota Domains.

Margulis and Schwartz, in 1998, reintroduced the five-kingdom system, with two super-kingdoms.

Classifications in the 21st century

During the XXI century, new proposals for the classification of living beings have emerged in the incessant search for a phylogeny based on evolutionary relationships.

The results of a project called the Catalog of Life System (2015) support the proposal of two super-kingdoms: Prokariota and Eukaryota. In the first superkingdom they include the kingdoms Archaea and Bacteria. In the second they include the kingdoms Protista, Chromista, Fungi, Plantae and Animalia.

In this classification, protozoa are the common ancestor of all eukaryotes, and not only of animals, as was originally proposed.

Fields of study

Protozoa as an object of study

Protozoa are eukaryotic organisms. They are formed by a single cell with a differentiated nucleus that performs all the functions of a complete organism.

Their average size can range from 2 or 3 microns to 250 microns long. However, Spirostomun, a ciliated protozoan, can reach 3 mm and Porospora gigantea, a sporozoan, can measure 16 mm in length.

Protozoa are mainly heterotrophic, and can be phagotrophs, predators or detritivores. An important exception is the Euglenophyceae, the only photosynthetic protozoa that obtain their chloroplasts from captured and exclavised green algae.

Their reproduction is mainly asexual through binary fission or multiple fission. However, a minority possess sexual reproduction by syngamy or autogamy (fusion of haploid gametes), or by exchange of genetic material (conjugation).

They are motile organisms, which have organs of locomotion such as flagella, cilia or pseudopods. They can also move through amoeboidal movements, typical of the cell, achieved by contraction and relaxation of the same.

They are distributed in all humid environments on Earth. For example, we can find them among the grains of sand on the beach, in rivers, seas, sewers, springs, in the litter of forests, in the intestines of invertebrates and vertebrates or in the blood of human beings.

They are able to survive a lack of moisture; they have resistance structures that allow them to become entrenched until they come back into contact with the aqueous medium.

They can be free-living or maintain symbiotic relationships with other species such as commensalism, mutualism or parasitism. Parasites are causative agents of diseases in plants, animals and humans.

Model systems

Protozoa are ideal as study models that allow addressing various questions in biology. Some characteristics that make them useful are: short generation times, great diversity of fundamental properties and life cycles, generalized geographic distribution and manageable genetics.

Basic studies

Protozoology encompasses the study of the natural history of protozoa. This includes knowledge about the structure, taxonomy, behavior, life cycles and physiology of these organisms.

Basic ecological studies on protozoa cover the dynamics within individuals of the same species and between individuals of different species. The latter has special relevance due to the existence of parasitic protozoa.

Applied studies

Protozoology addresses important areas of applied research in fields as diverse as medicine, veterinary medicine, petrochemistry, biotechnology and many others of interest to humanity.

Protozoology studies protozoa as causative agents of diseases in humans, animals and plants. Thus, it overlaps with basic protozoology in the study of the natural history of parasitic protozoa.

It studies the diseases themselves through knowledge of the mechanisms of colonization of parasites in healthy hosts, of the infectious processes, of the diagnosis, treatment and prevention of these diseases.

In the field of petrochemicals, the study of protozoa is useful in oil exploration. Identifying the presence of some species can shed light on the presence of oil in that exploration layer.

Likewise, the composition of protozoa can be an indicator of the state of recovery of an ecosystem after oil spill events.

On the other hand, the management of protozoan populations can help in the bioremediation of contaminated water bodies and soils. The ability of protozoa to ingest solid particles accelerates the degradation of toxic waste and hazardous agents.

Recent Research Examples

Diversity of protozoa in tropical forests

It is widely known that tropical forests have a great diversity of plant and animal species.

During 2017, Mahé and collaborators published the results of a research project that aimed to learn about the great diversity of forest microorganisms that live on a microbial scale.

The project was developed in forests in Costa Rica, Panama and Ecuador, where they took samples of flowers and lianas fallen on the ground. The results showed that protozoa are much more diverse than forest microorganisms.

Parasitic protozoan viruses in humans

The interaction between parasites and their hosts has received much attention in medical protozoology. However, there are new interactions discovered that complicate the study system and demand even more research.

Recently Grybchuk and collaborators (2017) published a work that identifies several viruses of the Totiviridae family implicated in the increase in the pathogenicity of protozoa from the trypanosome group, linked to the human parasite Leishmania.

The results show several previously unidentified viruses. They also present important information on the origin, diversity and distribution of viruses in a group of protists.

References

1. Beltran, E. (1941). Felix Dujardin and his «histoire naturelle des zoophytes. Infusoires », 1841. Rev. Soc. Mex. Hist. Nat., II. (2-3): 221-232, 1941.
2. Beltrán, E. 1979. Notes of protozoological history V. The rebirth of protozoology seven fertile lustrums: 1941-1976. Mex. Hist. Science. and Tec., No. 5: 91-114.
3. Corliss, JO (1989). The Protozoon and the Cell: A Brief Twentieth-Century Overview. Journal of the History of Biology Vol. 22, No. 2 pp. 307-323.
4. Grybchuk, D et al. (2017). Viral discovery and diversity in trypanosomatid protozoa with a focus on relatives of the human parasite Leishmania. PNAS 28: E506-E5015.
5. Iturbe, U. and Lazcano, A. The natural method of classification and the characteristics of universal comparison. In: Contreras-Ramos, A., Cuevas-Cardona, MC, Goyenechea, I. and Iturbe U., (editors). Systematics, the knowledge base of biodiversity. Autonomous University of the State of Hidalgo, 2007. Pachuca, Hidalgo, Mexico.
6. Leadbeater, BSC and McCready, SMM The fagellates: historical perspectives. edited by Barry SC Leadbeater, John C. Green. Flagellates: Unity, Diversity and Evolution.
7. Mahé, F. et al. (2017). Parasites dominate hyperdiverse soil protist communities in Neotropical rainforests. Nature Ecology & Evolution 1 (91): 1-8
8. Rodríguez, DJG, JL Olivares and J. Arece. (2010). Evolution Of Protozoa. Rev. Salud Anim. Vol. 32 No. 2: 118-120.
9. Rothschild, LJ (2004). Introductory Remarks: Protozoology (Protistology) at the Dawn of the 21st Century. The Journal of Eukaryotic Microbiology 51 (1).