Bacteria and Archaea are two of the three fundamental domains of life, both single-celled microorganisms. While sharing the prokaryotic characteristic of lacking a membrane-bound nucleus and other internal organelles, they are fundamentally distinct at a molecular level. These distinctions are important for understanding the vast diversity of microbial life and their unique evolutionary paths.
Cellular Structure and Composition
Significant differences exist in the cellular components of Archaea and Bacteria, particularly their cell walls and membranes. Bacterial cell walls are primarily composed of peptidoglycan, a unique polymer of sugars and amino acids that provides structural support and protection. This layer can vary in thickness and is a key feature in bacterial classification, such as Gram staining.
In contrast, Archaea lack peptidoglycan in their cell walls. Instead, archaeal cell walls exhibit diverse compositions, including pseudopeptidoglycan, which is structurally similar but chemically distinct from bacterial peptidoglycan, or layers made of proteins (S-layers), glycoproteins, or other polysaccharides. The cell membranes of Bacteria and Archaea also differ in their lipid structure. Bacterial membranes, like those of eukaryotes, are formed from ester-linked fatty acids.
Archaea possess ether-linked isoprenoid chains in their membrane lipids, contributing to their stability in various environments. These archaeal lipids can sometimes form a single monolayer instead of the typical lipid bilayer found in Bacteria and Eukaryotes, enhancing their resilience in extreme conditions.
Genetic Information and Expression
The ways Archaea and Bacteria organize and express their genetic material also show notable differences. Both domains generally have a single circular chromosome located in a region called the nucleoid, but archaeal DNA is often associated with histone-like proteins. This feature, which aids in DNA compaction, is more akin to eukaryotic DNA packaging, whereas most bacteria do not utilize histones.
Regarding gene expression machinery, Bacteria employ a single RNA polymerase. Archaea, however, possess multiple, more complex RNA polymerases that structurally and functionally resemble those found in eukaryotes. These similarities extend to other aspects of gene expression. For instance, some archaeal genes contain introns, non-coding sequences removed during gene expression, a characteristic common in eukaryotes but generally absent in bacteria.
The initiation of protein synthesis (translation) in Archaea also resembles the eukaryotic process, using methionine as the initiator amino acid, unlike bacteria which use formyl-methionine. These molecular distinctions highlight the closer evolutionary relationship between Archaea and Eukarya compared to Bacteria.
Energy Production and Environments
Archaea exhibit unique metabolic pathways, such as methanogenesis, the biological production of methane, exclusively found within this domain. Methanogenic archaea play a significant role in the global carbon cycle, thriving in oxygen-depleted environments like wetlands, deep-sea sediments, and animal digestive tracts. Bacteria, in contrast, display a vast and diverse array of metabolic capabilities, encompassing various forms of photosynthesis, chemosynthesis, and respiration, allowing them to utilize a wide range of energy sources.
While Bacteria are ubiquitous in virtually every habitat on Earth, Archaea are particularly well-known for thriving in extreme environments. These extremophilic archaea inhabit harsh conditions, such as hot springs, highly saline lakes, and deep-sea hydrothermal vents. Archaea are also present in more moderate environments, including oceans, soils, and the human gut, demonstrating a broader distribution than initially thought.