Archaea are exclusively unicellular organisms. The organisms you may have heard called “Archaebacteria” were historically grouped with bacteria due to their shared single-celled structure and lack of a nucleus. However, modern genetic analysis revealed they represent a distinct branch on the tree of life, prompting scientists to rename them simply Archaea. This domain, along with Bacteria and Eukarya, constitutes one of the three primary classifications of all known life, set apart by their fundamental molecular and genetic characteristics.
Unicellular Structure
Archaea are classified as prokaryotes, similar to bacteria, because their cells lack a membrane-bound nucleus and other internal compartments called organelles. This fundamental structure means that each cell contains its genetic material and machinery within a single, boundary-defined space. They are typically microscopic, with an average size range that often falls between 0.1 and 15 micrometers, making them invisible to the naked eye.
The shapes of archaeal cells vary widely, encompassing spheres (cocci), rods (bacilli), and spirals, but some species exhibit unique forms, such as flat, square sheets. Although Archaea often live in dense colonies or aggregates, they do not form true multicellular organisms. In a true multicellular organism, cells differentiate into specialized tissues that are interdependent for survival, a feature that is absent in the Archaea domain. Each archaeal cell remains an independent unit, capable of carrying out all life functions on its own.
Key Differences Between Archaea and Bacteria
The separation of Archaea from Bacteria stems from profound differences at the molecular level, particularly in their cell membranes and walls. The cell membranes of Archaea are constructed with unique ether-linked lipids. By contrast, the membranes of Bacteria and Eukarya use ester-linked lipids, which are less chemically stable. This ether linkage is an adaptation that allows many Archaea to thrive in extremely harsh environmental conditions.
The structure of the cell wall also marks a clear division between the two domains. Bacteria possess cell walls made primarily of peptidoglycan, a polymer that is absent in Archaea. Instead, Archaea may have cell walls composed of pseudopeptidoglycan, complex polysaccharides, or simply a layer of surface proteins known as the S-layer.
Beyond the cell envelope, the internal machinery for processing genetic information shows that Archaea are more closely related to Eukaryotes than to Bacteria. For instance, the enzyme responsible for transcribing DNA into RNA, known as RNA polymerase, is a complex enzyme composed of multiple subunits in Archaea, resembling the eukaryotic version. This is different from the simpler, single RNA polymerase found in Bacteria. Furthermore, the DNA of Archaea is often condensed and organized by histone-like proteins, a feature characteristic of Eukaryotes but not found in Bacteria.
Habitats and Metabolism
Archaea are famous for their ability to survive in environments that are hostile to most other life forms, leading to their classification as extremophiles. Organisms such as thermophiles thrive in high-temperature habitats, including deep-sea hydrothermal vents and hot springs. Other types include halophiles, which require extremely high salt concentrations, such as those found in hypersaline lakes. Some acidophiles can tolerate environments with a pH near zero.
The metabolism of Archaea includes some processes that are unique to the domain, most notably methanogenesis. Methanogens produce methane gas as a byproduct of their energy metabolism in anaerobic environments. This process is a significant part of the global carbon cycle and occurs in places like marshlands, landfill sites, and the digestive tracts of ruminant animals. Ammonia-oxidizing archaea also play a role in the global nitrogen cycle, contributing to the conversion of ammonia into nitrite in marine and soil environments.
While many Archaea are extremophiles, the domain also includes species that live in more moderate environments, such as soils, open oceans, and the microbiota of the human gut. Their metabolic diversity allows them to utilize a broad range of energy sources, from organic compounds to inorganic substances like hydrogen gas or sulfur.