The are a domain or kingdom of single-celled microorganisms

The are a domain or kingdom of single-celled microorganisms. These microbes are prokaryotes, meaning they have no cell nucleus or any other membrane-bound organelles in their cells just as do not have either..

Archaeal cells have unique properties separating them from the other two domains of life: Bacteria and Eukaryota.

Archaea were initially classified as bacteria, receiving the name archaebacteria but this classification is outdated. As you read this, you will see that they just do not fit in the same domain as bacteria.

R.S. Gupta has proposed that the archaea evolved from Gram-positive bacteria in response to natural antibiotic selection pressure. This is suggested by the observation that archaea are resistant to a wide variety of antibiotics that are primarily produced by Gram-positive bacteria and that these antibiotics primarily act on the genes that distinguish archaea from bacteria.

Archaeal biochemistry is unique, such as their reliance on ether in their cell membranes. Bacteria have phospholipid membranes. Archaea use more energy sources than eukaryotes: these range from organic compounds such as sugars, to ammonia, metal ions or even hydrogen gas. Salt-tolerant archaea (the Haloarchaea) use sunlight as an energy source, and other of archaea fix carbon; however, unlike plants and cyanobacteria, no species of archaea does both.

Archaea reproduce asexually by , fragmentation, or budding; unlike bacteria and eukaryotes, no species form spores.

Archaea are particularly numerous in the oceans, and the archaea in plankton may be one of the most abundant groups of organisms on the planet.