Archaea
Archaea are a domain of single-celled prokaryotic microorganisms that are distinct from bacteria despite lacking a membrane-bound nucleus. They are notable for inhabiting extreme environments such as hot springs, salt lakes, and deep-sea hydrothermal vents, earning many members the label "extremophiles." Archaea share some features with eukaryotes (such as similar transcription machinery) and others with bacteria, making them a unique lineage with significant implications for understanding the evolution of life.
Major Groups of Archaea and Their Environments
| Group | Environment | Characteristic | Example |
|---|---|---|---|
| Thermophiles | Hot springs (>80 °C) | Heat-stable enzymes | Pyrococcus furiosus |
| Halophiles | Salt lakes (>25% NaCl) | Requires high salt | Halobacterium salinarum |
| Methanogens | Anaerobic sediments | Produces methane (CH₄) | Methanobacterium thermoautotrophicum |
| Acidophiles | pH < 3 environments | Thrives in acid | Sulfolobus acidocaldarius |
| Psychrophiles | Polar ice (−20 °C) | Cold-active enzymes | Methanogenium frigidum |
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Related Terms
Bacteria
Bacteria are single-celled prokaryotic microorganisms that lack a membrane-bound nucleus and reproduce primarily by binary fission. They are among the most abundant life forms on Earth, inhabiting nearly every environment including soil, water, and the human body. Bacteria play essential roles in nutrient cycling, decomposition, and digestion, and certain species cause infectious diseases while others are harnessed in biotechnology and food production.
Biofilm
A biofilm is a structured community of microorganisms — predominantly bacteria — that are attached to a surface and enclosed within a self-produced extracellular matrix of polysaccharides, proteins, and nucleic acids. Bacteria within biofilms exhibit markedly different gene expression from their planktonic (free-floating) counterparts and can be up to 1000 times more resistant to antibiotics and immune responses. Biofilms are clinically significant as causes of chronic infections (e.g., on medical implants and catheters) and are also exploited beneficially in wastewater treatment and bioremediation.
Quorum Sensing
Quorum sensing is a cell-density-dependent chemical communication system used by bacteria to coordinate gene expression across a population by producing, releasing, and detecting small signalling molecules called autoinducers (AIs). When the concentration of autoinducers exceeds a threshold — indicating that a quorum (sufficient population density) has been reached — bacteria collectively switch on genes controlling biofilm formation, virulence factor production, sporulation, and bioluminescence. Quorum sensing allows bacteria to act as a multicellular unit, and disrupting it is being investigated as a novel anti-virulence strategy to combat antibiotic-resistant pathogens.
From Greek "archaios" (ἀρχαῖος) meaning "ancient" or "primitive", reflecting the hypothesis that archaea represent an ancient lineage close to the ancestor of all life. The domain was formally proposed by Carl Woese and George Fox in 1977 based on ribosomal RNA sequence analysis.