Prokaryotes represent a vast and ancient group of single-celled organisms. These microbes are defined by their simple cellular structure, which lacks a true nucleus and other complex, membrane-enclosed compartments. Their evolutionary history dates back billions of years, and their adaptability has allowed them to colonize nearly every environment on Earth, making them fundamental components of the global ecosystem.
Core Features of Prokaryotic Cells
The defining characteristic of a prokaryotic cell is its internal simplicity. Unlike eukaryotic cells, prokaryotes do not have a nucleus and lack complex organelles like mitochondria or an endoplasmic reticulum. Instead, their genetic material, a single circular chromosome, is concentrated in a region of the cytoplasm called the nucleoid. This region is not separated from the rest of the cell by a membrane, allowing for direct interaction between the DNA and the cell’s internal machinery.
The cytoplasm is a gel-like substance that fills the cell and houses ribosomes, the structures responsible for protein synthesis. Encasing the cytoplasm is the cell membrane, a phospholipid layer that controls the passage of substances into and out of the cell. Nearly all prokaryotes also possess a rigid cell wall outside the membrane, which provides structural support and helps maintain its shape.
The Challenge of Defining a Species
Defining a species in the prokaryotic world presents a challenge. The traditional Biological Species Concept, which defines a species as a group of organisms that can interbreed, is not applicable. Prokaryotes reproduce asexually, so concepts of interbreeding and gene flow between populations do not directly apply.
To overcome this, scientists developed a genetics-based framework for classifying prokaryotic species. This approach relies on measuring the similarity of DNA sequences between organisms. The most widely used genetic marker for this purpose is the 16S ribosomal RNA (rRNA) gene, as its sequence changes slowly over evolutionary time, making it a reliable tool for comparison.
This genetic comparison leads to the classification of microbes into Operational Taxonomic Units (OTUs). A common guideline is that prokaryotes sharing 97% or more identity in their 16S rRNA gene sequence are considered to belong to the same species. More recently, a method called Average Nucleotide Identity (ANI) has gained traction; it compares the entire genome of organisms and uses a threshold of around 95% identity to delineate species.
Major Domains of Prokaryotes
The world of prokaryotes is divided into two major domains: Bacteria and Archaea. Although they are both prokaryotic, they are as different from each other as they are from eukaryotes. This split was first revealed through molecular analysis of ribosomal RNA, which showed deep evolutionary divergence between the two groups.
Bacteria are characterized by cell walls that contain peptidoglycan, a polymer of sugars and amino acids that provides structural integrity. This domain is known for its metabolic diversity, with bacteria capable of deriving energy from sunlight, inorganic chemicals, or organic matter. Familiar examples include Escherichia coli, a resident of the human gut, and various species of Streptococcus.
Archaea, on the other hand, lack peptidoglycan in their cell walls. Their cell membranes are also chemically distinct, composed of different lipids that can form a monolayer, which helps them survive in extreme conditions. This has earned them a reputation as “extremophiles,” with many species thriving in environments like boiling hot springs or highly saline waters. However, archaea are not restricted to these harsh habitats and are also abundant in more common environments like soil and oceans.
Genetic Diversity and Adaptation
Prokaryotic evolution is driven by mechanisms that generate genetic diversity and allow for rapid adaptation. The primary mode of reproduction is binary fission, a form of asexual reproduction where a cell divides into two identical daughter cells. While this process produces clones, mutations during DNA replication can introduce new genetic variations into the population. Given their rapid generation times, even a low mutation rate can lead to significant diversity over time.
A source of genetic novelty is Horizontal Gene Transfer (HGT), the movement of genetic material between unrelated organisms. This process allows prokaryotes to acquire new genes and traits quickly. HGT occurs through three primary mechanisms: transformation, where a cell takes up DNA from its environment; transduction, where viruses transfer bacterial DNA between hosts; and conjugation, where DNA is passed directly between cells.
For instance, HGT is the primary mechanism responsible for the rapid spread of antibiotic resistance genes among bacterial populations. By acquiring a resistance gene, a previously susceptible organism can instantly gain the ability to survive antibiotic treatment.
Ecological and Industrial Roles
Prokaryotes have a significant impact on the planet and human society, filling many ecological and industrial roles. In the environment, they are decomposers, breaking down dead organic material and recycling nutrients back into the ecosystem. They are also primary producers, forming the base of many food webs, and perform nitrogen fixation, a process that converts atmospheric nitrogen into a form usable by plants.
Within the human body, prokaryotes are important for health. The gut microbiome is a complex community of bacteria that aids in digestion, synthesizes vitamins, and helps train the immune system. These beneficial microbes protect against pathogenic invaders.
Humans have also harnessed the metabolic capabilities of prokaryotes for numerous industrial applications. They are used in food production to make yogurt and cheese, in biotechnology to produce medicines like insulin, and in bioremediation to clean up pollutants like oil spills and toxic waste.