Domain – Concept and domains of biological kingdoms

We explain what a domain is and its relationships with the biological kingdoms. Also, some of its main features.

Domains in biology
All known life fits into one of the three known domains.

What is a domain?

In biology, domain, sometimes also called empire or superkingdom, is understood as the broader taxonomic category into which known living things are classified. In other words, it is the broadest category into which the different kingdoms of life can be organized, according to the most recent classification models and those most widely accepted in the specialized scientific community.

The current system in the matter is the one proposed by the American microbiologist Carl Richard Woese in 1990, and known as the three-domain system, since it organizes the different kingdoms of life (which are generally animalia, plantae, fungi, protista, bacteria and archaea) into three large groups or domains, based on their fundamental cellular characteristics: the bacterium domain, the archaea domain and the eukarya domain.

The first two domains, bacteria and archaea, encompass the world of prokaryotic organisms, that is, those that lack a cell nucleus and are much simpler and smaller than those belonging to the remaining domain, the eukaryotes. The latter have larger, more complex cells equipped with a cell nucleus where their DNA is found, and therefore they can be unicellular or multicellular organisms.

A) Yes, all known life fits into one of these domains, perhaps with the exception of viruses, whose parasitic and acellular existence remains so mysterious that it is still not possible to determine if they are really living beings.

Dominion and kingdom

Bacteria - biological domains
In the bacterium kingdom are prokaryotic organisms.

Domains are the broadest category of life, into which the various known kingdoms are organized. These, for their part, are the immediately lower categories (although in some systems super-kingdoms are also understood as an intermediate category between domain and kingdom, or even as an alternative: two super-kingdoms, eukaryota and prokaryota, instead of three domains), among which living beings are distributed based on their evolutionary, metabolic, cellular and behavioral similarities.

There are various systems of classification of life that propose 3, 4, 5, 6 and up to 7 different kingdoms. The most common includes the following:

  • Bacteria kingdom. Where are the simplest and most primitive prokaryotic organisms of all, the most predominant on the planet, dedicated to all kinds of nutritional functions: photosynthesis, chemosynthesis, parasitism, predation, etc.
  • Archaea kingdom. Initially considered part of the bacterial kingdom (and called archaebacteria) it was later found that they have substantial evolutionary differences that allow them to be a kingdom (and a domain) apart from bacteria, with whom they share their prokaryotic existence, but with different behaviors (extreme habitats, chemisynthetic nutrition) and cellular characteristics that resemble eukaryotes.
  • Proctist kingdom. Also called protista and formerly moneras, it is the kingdom where all eukaryotic unicellular organisms are contemplated, a sort of step between prokaryotic life and multicellular kingdoms. This is where protozoa, unicellular algae and other eukaryotic microorganisms of various eating habits come in.
  • Kingdom plantae. The plant kingdom, that is, that of plants, those immobile multicellular eukaryotic organisms that feed on photosynthesis: the biochemical composition of sugars from water, carbon dioxide and sunlight, thanks to a pigment they possess, called chlorophyll. Its cells house it in their plastids, and they also have a rigid cellulose cell wall.
  • Fungi kingdom. The kingdom of fungi, intermediate between plant and animal, since they are not autotrophs like plants, but they are immobile. They feed on the decomposition of organic matter, either in a saprophytic or parasitic way, and reproduce by means of spores. Their eukaryotic cells have a cell wall, but made of chitin.
  • Animalia Kingdom. The animal kingdom, with its enormous variety of genera and species of multicellular, heterotrophic, eukaryotic organisms, endowed with mobility, sexual reproduction and a metabolism based on respiration, that is, the oxidation of glucose obtained from matter organic consumed by other living things. Their cells lack a cell wall.

Bacteria domain

The bacterium domain coincides with the kingdom of the same name, within which are exclusively prokaryotic organisms, with a simple and primitive cellular structure, which are considered the most abundant forms of life on the planet, and surely the first to emerge in the evolutionary soup of the primitive Earth.

They can be obtained in practically all habitats, even within (in symbiotic or parasitic relationship) of some multicellular organisms, and dedicated to various types of metabolic activity: photosynthesis, such as cyanobacteria (blue-green algae), decomposition of matter organic, etc.

Archaea domain

Archaea domain
In the archaea domain are prokaryotes with similarities to eukaryotic life.

Along with the bacterium domain, the archaea domain covers the entire prokaryotic world. It also coincides with the kingdom of the same name, in which archaebacteria or archaea are included, prokaryotic organisms that exhibit certain similarities with eukaryotic life, despite existing in very specific and generally hostile habitats (leading an Extremophilic life) such as boiling subterranean waters, although they have also been found among the microorganisms that make up marine plankton.

Eukarya domain

The eukarya or eukaryotic domain is the broadest of the three, in the sense that it groups together a diverse set of kingdoms: animals, plants, fungi, and all protists, that is, all forms of eukaryotic life, possessing cells with a specific cell nucleus (where the DNA is housed) and other complex cellular organelles.

The evolutionary step from prokaryotes to eukaryotes is still difficult to understand, but it is also key in the formation of more complex organisms, such as multicellular, in which cells sacrifice their independence to form a more complex and interconnected organized whole. The creatures in this domain are called eukaryotes.