Archaea were long thought kissing cousins to bacteria, as they look like them, though weirdly so. Both are “primitive” prokaryotes, with selfsame structure and function. Many have similar lifestyles.
Looks are deceiving. RNA analysis revealed archaea more closely related to eukaryotes and only a distant relation to bacteria. That makes sense because eukaryotes arose from archaeal origin: archaea incorporated bacteria that begat eukaryotic cells. Such evolutionary endosymbiosis transpired numerous times, begetting a variety of eukaryotic life.
Archaeans exist everywhere that life can survive. They are an extremely robust and versatile life form, with both extremophiles and ubiquity in their favor.
Archaea are so prevalent as to play roles in the carbon and nitrogen cycles. All told, archaea account for 10–20% of Earth’s biomass. That’s particularly impressive when considering that the average archaean is 1 micrometer (µm; one-millionth of a meter) tiny, and the largest is 15 µm. A human hair is 100 µm thick.
Archaea may be autotrophic, heterotrophic, or saprotrophic. Autotrophic archaea eat photons: phototrophs. Photosynthesis is a more evolved process.
Some archaea are lithotrophs: consuming inorganic substrates, such as ammonia, hydrogen sulfide, and sulfur. Many lithotrophs are extremophiles. Nothing beats a hot sulfur Sunday to a Sulfolobus, a thermoacidophile which lives in hot springs and hydrothermal vents, where the water is as acidic as stomach acid (pH = 2–3) and near boiling.
Other archaea gas themselves up with methane (methanotrophs), nitrogen (nitrifiers), or carbon dioxide. Some archaea outgas what others eat. Methanogens – methane makers – are carbon dioxide consumers that exude methane waste.
Archaean saprovores play an essential role in decomposition and recycling organic nutrients. This is an analogous role to the stone eaters that convert inorganic materials into energy. Lithotrophs introduce new material into the food web, while saprotrophs reintroduce.
The diversity of archaeal lifestyles highlights that they represent a catchall empire of wildly successful organisms. Their classification remains controversial because they defy the typical methods of categorization, such as by reproduction style, as all archaeans are asexual.
While bacteria can be classified to some degree by shape, archaea tend to be pleomorphic: able to alter shape in response to environmental conditions. There are also shape-shifting bacteria.
Deinococcus is an extremophilic bacterium that is pleomorphic. It is also one of the most radioresistant (radiation-resistant) organisms known, as well as being able to survive dehydration, cold, vacuum, and acid. Its extreme reluctance to die earned it a listing in The Guinness Book Of World Records as the world’s toughest bacterium.
Categorizing by differences in genomes is thwarted by archaeal facility with horizontal gene transfer: otherwise similar archaea creating genomes that are not closely related. In many ways, archaeal versatility defies their easy classification.
More than any other domain, archaeal ubiquity and adaptive fluidity emphasize that the potentiality of life’s manifestations is nearly unlimited. Archaea exclaim that life is bound to arise whenever and wherever inorganic resources and environmental stability exceed a minimal threshold.
Social to a fault, archaea are commonly mutualists or commensals. No archaeal pathogens or parasites are known. Archaea are the most moral organisms.