THE 3 DOMAINS IN BIOLOGY (WOESE CLASSIFICATION) - BIOLOGY - 2025

The three domains of biology or the three-domain system is a classification proposed by the biologist Carl Woese in the late 1970s, which divided organic beings into the Bacteria, Archaea and Eukaryota domains.

This classification into "domains" is superior to the traditional system of division into five or six kingdoms, with which we are most familiar. The fundamental division of domains is to divide prokaryotes into two domains, where archaea are more related to eukaryotes, than to the other group of prokaryotes - bacteria.

Source: Rock 'n Roll, from Wikimedia Commons

This phylogenetic arrangement is widely accepted by most biologists. However, with the development of bioinformatics and statistical tools, some authors have proposed new relationships between living beings, which defies Woese's classification.

Classification history

Division into two kingdoms: Animalia and Plantae

Before the publication of the works of Woese and his colleagues, biologists used a "traditional" classification, using a simple and intuitive dichotomy that divided plants from animals - formally Animalia and Plantae.

In this division, all bacteria, fungi, and photosynthetic protists were considered "plants," while protozoa were grouped together with animals.

With the advancement of science, the development of modern methodologies and a more thorough analysis of organic beings, it became clear that the division into plants and animals did not fit the true evolutionary history of these. In fact, it was a "rustic" and inconsistent simplification of the relationships between them.

Division into three kingdoms:

With the aim of remedying this situation, the renowned evolutionary biologist and ornithologist Ernst Haeckel added a new kingdom to the list: the Protista Kingdom.

This classification achieved a clearer division of forms that obviously should not be grouped. However, the classification remained alarmingly problematic.

Division into five kingdoms

In 1969 the American ecologist Robert Harding Whittaker proposed the division scheme into five kingdoms: Animalia, Plantae, Fungi, Monera and Prostista.

This system is based mainly on the cell types that make up organisms. The members of Monera are unicellular and prokaryotic beings, while the protists are also unicellular, but eukaryotic.

The remaining three kingdoms - Animalia, Plantae, and Fungi - are classified in terms of their mode of nutrient acquisition. Plants have photosynthetic capabilities, fungi secrete enzymes into the environment, followed by absorption of nutrients, and animals consume their food, with internal or external digestion.

The division of organisms into five kingdoms was widely accepted by the systematists of the time, since they considered that the classification was increasingly adjusted to the real evolutionary relationships of living beings.

Division into three domains

In the 1970s, University of Illinois professor Carl Woese began to find evidence for a certain unknown group of very striking single-celled organisms. These lived in environments with extreme conditions of temperature, salinity and pH, where it was thought that life could not be maintained.

At first glance, these organisms were classified as bacteria, and were called archaea. However, a deeper and more detailed look at the archaebacteria made it clear that the differences with the bacteria were so striking that they could not be classified within the same group. In fact, the resemblance was merely superficial.

In this way, the molecular evidence allowed this group of researchers to establish a classification system of three domains: Bacteria, Archaea and Eukaryota.

Proposing novel genealogical relationships between organisms marked an event of great importance in modern biology. This important discovery led Woese to win the National Medal of Science in 2000.

The three domains of life

The tree of life proposed by Carl Woese establishes the possible genealogical relationships between organic beings, suggesting the existence of three domains of life.

This hypothesis was proposed thanks to the analysis of 16S ribosomal RNA - abbreviated as 16S rRNA.

This marker is a component of the 30S subunit of the prokaryotic ribosome. Following Woese's work, it has been widely used for phylogenetic inference. Today it is very useful to establish the classification and identification of bacteria.

Below we will describe the most notable characteristics of each of the members that make up the three domains of life:

Archaea Domain

Archaea

Archaea are organisms that are characterized mainly by inhabiting environments with extreme conditions of temperature, acidity, pH, among others.

In this way, they have been found in waters with significantly high saline concentrations, acidic environments and hot springs. In addition, some archaea also inhabit regions with "average" conditions, such as the soil or the digestive tract of some animals.

From the cellular and structural point of view, archaea are characterized by: they do not have a nuclear membrane, the lipids of the membranes are linked by ether bonds, they present a cell wall - but this is not composed of peptidoglycan, and the structure of the genes is similar to eukaryotes on circular chromosomes.

The reproduction of these prokaryotes is asexual, and horizontal gene transfer has been evidenced.

Classification of archaea

They are classified as methanogenic, halophilic and thermoacidophilic. The first group uses carbon dioxide, hydrogen, and nitrogen to produce energy, producing methane gas as a waste product. The first arch to be sequenced belongs to this group.

The second group, the halophiles are "lovers of salt." For its development, it is necessary for the environment to have a saline concentration about 10 times greater than that of the ocean. Some species can tolerate concentrations up to 30 times higher. These microorganisms are found in the Dead Sea and in evaporated ponds.

Finally, thermoacidophils are able to withstand extreme temperatures: greater than 60 degrees (some can tolerate more than 100 degrees) and less than the freezing point of water.

It is necessary to clarify that these are the optimal conditions for the life of these microorganisms - if we expose them to room temperature it is quite possible that they will die.

Bacteria Domain

Mycobacterium tuberculosis bacteria

The bacterium domain comprises a large group of prokaryotic microorganisms. In general, we usually associate them with diseases. Nothing is further from reality than this misunderstanding.

While it is true that certain bacteria cause fatal diseases, many of them are beneficial or live in our body establishing commensal relationships, forming part of our normal flora.

Bacteria do not have a nuclear membrane, they lack organelles themselves, their cell membrane is made up of lipids with ester-type bonds and the wall is made up of peptidoglycan.

They reproduce asexually, and horizontal gene transfer events have been evidenced.

Classification of bacteria

Although the classification of bacteria is really complex, here we will discuss the fundamental divisions of the domain, cyanobacteria and eubacteria.

The members of cyanobacteria are blue-green photosynthetic bacteria that produce oxygen. According to the fossil record, they appeared about 3.2 billion years ago and were responsible for the drastic change from an anaerobic environment to an aerobic (oxygen-rich) environment.

Eubacteria, for their part, are the true bacteria. These appear in varied morphologies (cocci, bacilli, vibrios, helical, among others) and have modified structures for their mobility, such as cilia and flagella.

Eukarya Domain

Eukaryotic human cell representation. You can see the core

Eukaryotes are organisms that are distinguished primarily by the presence of a well-defined nucleus, delimited by a complex biological membrane.

Compared to the other domains, the membrane has a variety of structure and the lipids exhibit ester-like bonds. They present true organelles, delimited by membranes, the structure of the genome is similar to archaea, and it is organized in linear chromosomes.

Group reproduction is extraordinarily varied, exhibiting both sexual and asexual modalities, and many group members are capable of reproducing in both ways - they are not mutually exclusive.

Classification of eukaryotes

It includes four kingdoms with very varied and heterogeneous forms: the protists, fungi, pantas and animals.

Protists are single-celled eukaryotes, like euglena and paremecia. The organisms that we commonly know as fungi are the members of the Fungi kingdom. There are uni and multicellular forms. They are key elements in ecosystems to degrade dead organic matter.

Plants are made up of photosynthetic organisms with a cell wall mainly made of cellulose. Its most conspicuous characteristic is the presence of the photosynthetic pigment: chlorophyll.

It includes ferns, mosses, ferns, gymnosperms and angiosperms.

Animals comprise a group of heterotrophic multicellular organic beings, the majority with the capacity for movement and displacement. They are divided into two large groups: invertebrates and invertebrates.

Invertebrates are made up of porifers, cnidarians, nematodes, mollusks, arthropods, echinoderms, and other small groups. Similarly, vertebrates are fish, amphibians, reptiles, birds, and mammals.

Animals have managed to colonize virtually all environments, including oceans and air environments, exhibiting a complex set of adaptations for each.

References

1. Forterre P. (2015). The universal tree of life: an update. Frontiers in microbiology, 6, 717.
2. Koonin EV (2014). Carl Woese's vision of cellular evolution and the domains of life. RNA biology, 11 (3), 197-204.
3. Margulis, L., & Chapman, MJ (2009). Kingdoms and domains: an illustrated guide to the phyla of life on Earth. Academic Press.
4. Sapp, J. (2009). The new foundations of evolution: on the tree of life. Oxford University Press.
5. Sapp, J., & Fox, GE (2013). The singular quest for a universal tree of life. Microbiology and molecular biology reviews: MMBR, 77 (4), 541-50.
6. Staley JT (2017). Domain Cell Theory supports the independent evolution of the Eukarya, Bacteria and Archaea and the Nuclear Compartment Commonality hypothesis. Open biology, 7 (6), 170041.