Nucleoid
Circular DNA without a nuclear envelope, packed with archaeal histones in many species — a eukaryotic-like hint.
Energy strategies range from hydrogen-eating methanogens to light-using haloarchaea with bacteriorhodopsin. The membrane is both battery and identity card.
CRISPR systems were first understood in archaea and bacteria as memories of viruses. That is information ecology, not a home-engineering lesson.
Parts
70S-class ribosome
Prokaryote-sized but with proteins and antibiotic sensitivities closer to eukaryotes in several sites.
Ether lipid membrane
Resists heat and acidity better than many bacterial bilayers. A chemical reason extremophiles can hold a gradient.
S-layer
A crystalline protein coat common on archaea. Not peptidoglycan, not a Gram-stain story.
Flagellum (archaellum)
A rotary motor built from different proteins than the bacterial flagellum — convergent swimming.
Cofactors (F420, methanopterin)
Methanogens use unusual cofactors that glow under UV — a field trick for spotting them in sludge.
Life cycle
1Sense the gradient
Cells track hydrogen, salt, heat or light. Haloarchaea can even use sunlight via bacteriorhodopsin.
2Harvest electrons
Methanogens reduce CO2 or methyl compounds. Sulfolobus oxidises sulphur in hot acid.
3Divide
Binary fission or budding, depending on lineage. Generation times in cold sediments can be months.
4Persist or sporulate rarely
True endospores are a bacterial speciality. Archaea more often persist as hardy vegetative cells.
If a headline says archaea 'invaded a hospital', read twice: contamination of a sample is not the same as a pathogen.
Keep exploring
More in Tree of microbes
Bacteria
Single-celled prokaryotes with cell walls ? the oldest widespread life form, and both partners and pathogens.
Familiarity 95Viruses
Packets of DNA or RNA that are not cells — they replicate only inside living hosts.
Familiarity 96Fungi
A eukaryotic kingdom of yeasts, moulds and mushrooms — chitin walls, external digestion, partners of plants.
Familiarity 80Bacteriophages
Viruses that infect bacteria — Earth's most abundant biological entities, and a concept-level idea in medicine.
Familiarity 42Protozoa
Motile unicellular eukaryotes — a teaching grade that includes pond ciliates and a minority of serious parasites.
Familiarity 58