Archaea are a domain of single-celled microorganisms. For years, they were classified as bacteria and called “archaebacteria,” which translates to “ancient bacteria,” from their discovery in extreme environments thought to mirror the early Earth. As scientific understanding evolved, it became clear they are a distinct group of life. The term “archaebacteria” is now largely outdated, with the name “Archaea” preferred to reflect this unique standing as one of the three primary domains of life.
The Three Domains of Life
The modern understanding of life’s classification was reshaped in the 1970s by the work of biologist Carl Woese. He analyzed the genetic sequences of ribosomal RNA (rRNA), which challenged the long-standing division of life into prokaryotes and eukaryotes. Woese’s genetic data revealed that prokaryotes were not one cohesive group but were composed of two entirely different kinds of organisms. This led to the establishment of the three-domain system for classifying life.
The domains were named Bacteria, Archaea, and Eukarya. This reclassification formally separated Archaea from Bacteria, recognizing them as a group with an independent evolutionary history.
While Archaea resemble bacteria by lacking a nucleus, they also possess features more aligned with eukaryotes. This is particularly true of their machinery for processing genetic information, which shows a closer relationship to eukaryotic systems.
Distinguishing Features of Archaea
The justification for separating Archaea into their own domain comes from several molecular and structural differences when compared to bacteria. A primary distinction lies in the composition of the cell membrane. In Bacteria and Eukarya, membranes are built from lipids with fatty acids connected to a glycerol backbone through ester linkages. Archaeal membranes, conversely, use ether linkages, which are chemically more robust and contribute to their ability to withstand harsh environmental conditions.
Another defining characteristic is the structure of the cell wall. Bacterial cell walls contain a polymer called peptidoglycan, which provides structural integrity and is often the target of antibiotics. Archaeal cell walls completely lack peptidoglycan, instead being constructed from different substances like pseudopeptidoglycan.
Furthermore, the genetic processes within Archaea set them apart. Their systems for transcription and translation are more complex than those in bacteria and share traits with eukaryotes, including introns and histones. These features are absent in bacteria, underscoring the unique evolutionary path of Archaea.
Life in Extreme Environments
Many archaeal species are known as “extremophiles,” a term that describes organisms that flourish in conditions hostile to most other forms of life. This resilience enables them to inhabit some of the most inhospitable places on the planet. One prominent group of extremophiles is the thermophiles, or “heat-lovers,” which thrive in places like geothermal hot springs and volcanic vents.
In contrast, halophiles, or “salt-lovers,” are found in hypersaline environments such as the Great Salt Lake and the Dead Sea. Other archaea, known as acidophiles, prosper in highly acidic waters like those found in acid mine drainage.
A particularly interesting group of archaea is the methanogens. These organisms live in anaerobic, or oxygen-free, environments such as wetlands, swamps, and the digestive systems of animals like cattle and humans. Methanogens perform a unique metabolic process where they produce methane gas as a byproduct of their energy generation.
Major Groups and Their Roles
The domain Archaea is divided into several major phyla, with two of the most well-studied being Euryarchaeota and Crenarchaeota. The Euryarchaeota are a metabolically diverse group that includes the methane-producing methanogens and the salt-loving halophiles. The Crenarchaeota contain a large number of thermophilic species, including many that live in extremely hot, acidic environments.
Beyond their ability to survive in extreme places, archaea play roles in global nutrient cycles. Methanogens, for instance, are a source of atmospheric methane and are integral to the carbon cycle in anaerobic ecosystems. Other archaea are involved in the nitrogen and sulfur cycles, transforming these elements into forms that can be used by other organisms.
The unique biochemistry of archaea has also made them valuable in biotechnology. Because their enzymes can function at extreme temperatures and pH levels, they have been harnessed for various industrial and scientific applications. For example, heat-stable DNA polymerase enzymes from thermophilic archaea are used in the polymerase chain reaction (PCR), a technique used to amplify specific segments of DNA.