Prokaryotes are organisms whose cells lack a membrane-bound nucleus, and they include every species in the domains Bacteria and Archaea. That two-word answer barely scratches the surface of a group so vast and varied that it accounts for the majority of life’s genetic diversity on Earth. The distinction between prokaryotes and the nucleus-bearing eukaryotes was once treated as the deepest split in biology, but discoveries over the past few decades have revealed that “prokaryote” is more of a structural description than a neat evolutionary category, and the organisms it covers range from familiar gut bacteria to exotic microbes thriving in boiling acid.
How Prokaryotes Differ From Eukaryotes
The defining feature of a prokaryotic cell is what it lacks: a nucleus surrounded by a double membrane. In eukaryotic cells (think animals, plants, fungi, and protists), DNA is packaged inside a nucleus and compartmentalized away from the rest of the cell. In prokaryotes, the DNA sits in a region of the cell called the nucleoid, with no membrane walling it off. Most prokaryotic cells are also much smaller than eukaryotic cells, with typical bacteria measuring around two micrometers in length.
Beyond the missing nucleus, prokaryotes generally lack the elaborate internal membrane systems that eukaryotes rely on: no endoplasmic reticulum, no Golgi apparatus, and no classical mitochondria or chloroplasts. They do have ribosomes for building proteins, but those ribosomes are smaller than their eukaryotic counterparts. Many prokaryotes have a rigid cell wall outside the cell membrane, though the chemistry of that wall differs dramatically between bacteria and archaea. These structural simplifications do not mean prokaryotes are primitive. They are streamlined, fast-reproducing, and metabolically inventive in ways that eukaryotes cannot match.
Bacteria and Archaea Are Two Separate Domains
For most of the twentieth century, all prokaryotes were lumped together as “bacteria.” That changed in 1977, when Carl Woese and George Fox showed that the living world divides into three primary lineages. Their molecular comparisons led to a formal proposal for three domains of life: Bacteria, Archaea, and Eucarya.
1PubMed. Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and EucaryaBacteria and Archaea look superficially similar under a microscope, both small, both lacking a nucleus, but they are as genetically distinct from each other as either is from animals or plants.
One of the clearest chemical differences lies in their cell membranes. In archaea, the membrane is built from isoprenoid chains linked to a glycerol backbone through ether bonds. Bacteria (and eukaryotes) use fatty acid chains linked through ester bonds to a mirror-image version of that backbone.
2PubMed Central. Biosynthesis of archaeal membrane ether lipidsDespite this fundamental chemical difference, both membrane types work well across a wide range of environments, from freezing polar waters to near-boiling hydrothermal vents.
3PubMed Central. Adaptations of archaeal and bacterial membranes to variations in temperature, pH and pressureMajor Groups of Bacteria
The domain Bacteria is staggeringly diverse, with at least 25 phyla containing species that scientists have managed to grow in the lab, and many more known only from DNA sequences found in environmental samples.
4Trends in Microbiology. Diversity of bacterial cell envelopesA few groups dominate the conversation because of their abundance, their medical relevance, or their ecological impact.
Proteobacteria are probably the most familiar bacterial group, even if the name is not. This phylum includes Escherichia coli, the workhorse of molecular biology labs and a common gut inhabitant; Salmonella, responsible for food poisoning; Vibrio cholerae, the cause of cholera; and nitrogen-fixing bacteria in soil. Proteobacteria show enormous metabolic range, from photosynthetic purple sulfur bacteria to the chemolithotrophs that derive energy from inorganic chemicals in mine tailings and ocean sediments.
5PubMed. Chemolithoautotropic Diazotrophy Dominates the Nitrogen Fixation Process in Mine TailingsFirmicutes form the other major bacterial heavyweight. This group includes Staphylococcus and Streptococcus (both common causes of skin and throat infections), Clostridium (which produces the toxin behind botulism and tetanus), and Lactobacillus (essential for making yogurt and sauerkraut). Most Firmicutes have a thick cell wall and stain violet in the classic Gram staining test, which is why they are called Gram-positive. Proteobacteria, by contrast, have a thinner wall sandwiched between two membranes and stain pink (Gram-negative). Structural analyses show a sharp divergence between these two phyla at the protein level, reflecting a long evolutionary separation.
6Computational Biology and Chemistry. Bacterial protein structures reveal phylum dependent divergenceCyanobacteria deserve special mention because they are the prokaryotes that learned to split water and release oxygen through photosynthesis. Through their roles in primary production and oxygen generation, cyanobacteria fundamentally transformed the cycling of carbon, nitrogen, sulfur, and iron on early Earth, reshaping conditions for every other living thing.
7Trends in Microbiology. Early evolution of cyanobacteria and their impact on the Earth’s systemThe evidence supports oxygenic photosynthesis evolving at least several hundred million years before the Great Oxygenation Event, with major cyanobacterial lineages diversifying rapidly around that time roughly 2.4 billion years ago.
8PubMed Central. The Archean origin of oxygenic photosynthesis and extant cyanobacterial lineagesOther bacterial phyla worth knowing include Actinobacteria (the source of most natural antibiotics, plus the genus Mycobacterium that causes tuberculosis), Spirochaetes (the corkscrew-shaped bacteria behind Lyme disease and syphilis), and Bacteroidetes (a dominant group in the human gut that helps break down complex carbohydrates).
Archaea and the Extremophile Reputation
Archaea were first discovered in extreme environments: hot springs, salt flats, deep-sea hydrothermal vents, and the guts of methane-producing organisms. That early discovery history gave them a reputation as exclusively “extremophile” organisms, which turns out to be incomplete. Many archaea thrive in perfectly ordinary soil and ocean water, where they play important roles in nutrient cycling, particularly in ammonia oxidation.
9PubMed. Relative contributions of archaea and bacteria to aerobic ammonia oxidation in the environmentThat said, the extremophiles are genuinely remarkable. The major groups include:
- Methanogens: archaea that produce methane as a metabolic byproduct. They live in oxygen-free environments like swamps, rice paddies, landfills, and the digestive tracts of cattle. They are responsible for a significant share of global methane emissions.
- Halophiles: salt-loving archaea that flourish in brine pools and salt lakes, sometimes at salt concentrations that would kill virtually any other organism. Some poly-extremophilic halophiles thrive at salt concentrations above 10 percent by weight, alkaline pH above 8.5, and temperatures above 50°C simultaneously.
- Thermophiles and hyperthermophiles: archaea that grow best at temperatures most organisms could not survive, sometimes exceeding 100°C near deep-sea vents.
The proteins inside these organisms carry distinct adaptations for their environments. Heat-loving archaea tend to have a strong hydrophobic core and more electrostatic interactions that keep enzymes stable at high temperatures. Cold-adapted species have more flexible proteins with fewer internal attractions. Halophilic proteins are loaded with negatively charged amino acids on their surfaces, compensating for the extreme salt conditions outside the cell.
11PubMed Central. Protein adaptations in archaeal extremophilesMetabolic Tricks That Eukaryotes Cannot Pull Off
If there is one area where prokaryotes leave eukaryotes in the dust, it is metabolic versatility. Eukaryotic organisms overwhelmingly rely on organic carbon for food, with photosynthesis as the main alternative. Prokaryotes have found ways to extract energy from an astonishing range of chemical reactions: oxidizing hydrogen sulfide, iron, ammonia, hydrogen gas, and methane, or reducing sulfate, nitrate, and carbon dioxide. Some use light but not water as an electron source. Others fix atmospheric nitrogen into a biologically usable form, a trick no eukaryote can perform on its own.
Nitrogen fixation is especially significant because it converts inert nitrogen gas into ammonia that other organisms can use to build proteins and DNA. Research in mangrove sediments has found that the dominant nitrogen fixers in the dark, deep layers are chemolithoautotrophic prokaryotes, organisms that fix both carbon and nitrogen using inorganic energy sources like hydrogen, reduced sulfur, and iron.
12The ISME Journal. Chemolithoautotrophic diazotrophs dominate dark nitrogen fixation in mangrove sedimentsSimilar patterns appear in mine tailings, where prokaryotic nitrogen fixers preferentially use inorganic electron donors like elemental sulfur rather than organic carbon, playing essential roles in nutrient cycling and ecological recovery in these barren environments.
5PubMed. Chemolithoautotropic Diazotrophy Dominates the Nitrogen Fixation Process in Mine TailingsProkaryotes Inside You
Your body hosts trillions of prokaryotic cells, overwhelmingly bacteria, with the densest communities in the gut. The intestinal mucosal barrier, composed of epithelial cells and mucus layers, houses commensal bacteria that form the first line of defense against pathogenic microbes.
13PubMed Central. Interactions of commensal and pathogenic microorganisms with the mucus layer in the colonThese resident bacteria help digest fiber, synthesize certain vitamins, train the immune system, and compete with harmful species for space and resources.
A relatively small number of prokaryotic species cause disease, but those few have shaped human history. Tuberculosis (Mycobacterium tuberculosis), plague (Yersinia pestis), cholera (Vibrio cholerae), and bacterial pneumonia are all caused by prokaryotes. Even so, the vast majority of prokaryotic species are harmless or actively beneficial. The “bad” ones get disproportionate attention for obvious reasons, but they represent a tiny fraction of overall prokaryotic diversity.
How Prokaryotes Talk to Each Other
Despite being single-celled, many prokaryotes coordinate their behavior through a chemical signaling system called quorum sensing. Bacteria release small signaling molecules into their environment; as the population grows and concentrations of those molecules rise, they trigger collective changes in gene expression. This allows bacteria to behave differently when they are in a crowd versus alone.
14PubMed Central. Bacterial quorum sensing: its role in virulence and possibilities for its controlOne of the most visible outcomes of quorum sensing is biofilm formation. When bacteria sense enough neighbors nearby, they switch from a free-swimming lifestyle to a surface-attached community encased in a self-produced matrix of sugars and proteins. Biofilms coat everything from river rocks to medical implants, and they are notoriously hard to eliminate because the matrix protects the cells inside from antibiotics and immune attack.
15PubMed Central. Communication is the key: biofilms, quorum sensing, formation and preventionQuorum sensing also regulates virulence: some pathogenic bacteria hold off on producing toxins until their numbers are high enough to overwhelm the host’s defenses, making a coordinated “group attack” instead of tipping off the immune system too early.
Gene Swapping and Rapid Evolution
Prokaryotes do not reproduce sexually, but they have their own way of shuffling genetic material: horizontal gene transfer. Instead of passing genes only from parent to offspring, bacteria and archaea routinely pick up DNA from unrelated organisms. The three classical mechanisms are transformation (absorbing free DNA from the environment), transduction (receiving DNA via a virus), and conjugation (direct transfer between cells through a physical bridge). Beyond these well-known routes, researchers have identified a range of non-classical strategies, including hybrid mobile genetic elements and mechanisms that can even transfer molecules other than DNA.
16PubMed. Beyond the canonical strategies of horizontal gene transfer in prokaryotesHorizontal gene transfer is not a minor footnote. The rate at which prokaryotes gain and lose genes is comparable to their rate of point mutations and far exceeds the rate of gene duplication. Theoretical models suggest that without horizontal gene transfer, prokaryotic populations would deteriorate over time because harmful mutations would accumulate with no way to purge them. In other words, gene swapping is not just common; it may be essential for prokaryotic survival over evolutionary time.
17PubMed Central. Horizontal gene transfer: essentiality and evolvability in prokaryotes, and roles in evolutionary transitionsThis has practical consequences: antibiotic resistance genes spread among bacteria through horizontal transfer, which is why a resistance trait that emerges in one species can appear in a completely unrelated species disturbingly fast.
Prokaryotes Gave Rise to Eukaryotic Organelles
One of the most consequential events in evolutionary history was a prokaryote ending up inside another cell and staying there. Endosymbiotic theory, which goes back over a century, explains the similarity of mitochondria and chloroplasts to free-living prokaryotes by proposing that these organelles originated as prokaryotic cells engulfed by a host cell.
18PubMed. Endosymbiotic theory for organelle originsThe evidence now firmly establishes that mitochondria descended from a bacterium with alpha-proteobacterial ancestry, while chloroplasts descended from a cyanobacterium.
19Current Biology. The Eukaryotic Cell Evolution GuideThe host cell in this story was likely an archaeon. Recent phylogenomic analyses, drawing on a greatly expanded set of archaeal genomes, strongly support the “two-domain” tree of life, in which eukaryotes branch from within Archaea rather than sitting as a completely separate lineage. Specifically, eukaryotes appear most closely related to a group called the Asgard archaea.
20PubMed Central. Expanded diversity of Asgard archaea and their relationships with eukaryotesIf this picture holds, every plant, animal, and fungus on the planet descends from a merger between an archaeal host and a bacterial endosymbiont. Prokaryotes did not just precede complex life; they built it.
Prokaryotes in Biotechnology
The metabolic versatility of prokaryotes makes them useful well beyond their natural habitats. Industrial applications range from food production (fermentation by Lactobacillus, vinegar production by Acetobacter) to pharmaceutical manufacturing (antibiotics from Streptomyces, insulin from engineered E. coli). Heat-stable enzymes from thermophilic bacteria, such as Geobacillus species, are used in high-temperature industrial processes and bioremediation, including the cleanup of oil spills.
21IntechOpen. Geobacillus Bacteria: Potential Commercial Applications in Industry, Bioremediation, and Bioenergy ProductionArchaea are getting increasing attention in this space as well. Halophilic archaea, with their metabolic adaptations to extreme salt, are being explored as tools for bioremediation of brines and saline soils contaminated with toxic compounds, including heavy metals, hydrocarbons, and aromatic pollutants.
22PubMed Central. Halophilic archaea as tools for bioremediation technologiesConventional cleanup methods often fail in high-salt environments because most organisms cannot survive there. Haloarchaea can.
When a Prokaryote Breaks the Rules
The textbook image of a prokaryote is a tiny, simple cell a few micrometers long. In 2022, researchers described a bacterium that makes that image look laughably incomplete. Candidatus Thiomargarita magnifica, found in the mangroves of Guadeloupe, has an average cell length greater than 9,000 micrometers, nearly a centimeter, making it visible to the naked eye. For reference, most bacteria are around two micrometers long, and the previous record-holders topped out at about 750 micrometers.
23PubMed. A centimeter-long bacterium with DNA contained in metabolically active, membrane-bound organellesSize is not the only boundary T. magnifica pushes. Its genome is stored in membrane-bound compartments containing ribosomes, which function as something eerily like organelles. The cell also carries over half a million copies of its genome, a level of polyploidy never seen in a bacterium. This organism is still a prokaryote by phylogeny, firmly placed in the bacterial domain, but its internal complexity challenges the neat separation between “simple prokaryote” and “complex eukaryote.” It suggests that the evolutionary transition from one to the other may have been less of a sharp jump and more of a gradient.
Bacteriophages and the Invisible War
Prokaryotes are themselves prey. Bacteriophages, viruses that specifically infect bacteria, are thought to be the most abundant biological entities on Earth, outnumbering their bacterial hosts roughly ten to one in most environments. Phages shape bacterial communities by killing cells, driving population shifts, and transferring genetic material between hosts. They modify bacterial abundance, diversity, physiology, and virulence through what amounts to an ongoing evolutionary arms race.
24PubMed Central. Bacteriophages presence in nature and their role in the natural selection of bacterial populationsThis predator-prey dynamic has practical implications. Phage therapy, the use of bacteriophages to treat bacterial infections, is being revisited as antibiotic resistance makes conventional drugs less effective. The approach has a long and uneven history, largely sidelined in the West after the discovery of antibiotics but continuously practiced in parts of Eastern Europe. As multidrug-resistant infections become a bigger public health problem, phages are attracting renewed interest from researchers and regulators. In a way, the oldest enemies of prokaryotes may become one of our newest medical tools.