2. Domain Archaea (Kingdom: Archaebacteria)

These are unicellular prokaryotic organisms that resemble bacteria in their appearance, and hence were fallaciously placed under bacteria before the rise of three domain systems. These are the foremost primitive and ancient forms among the three domains and are known to be present within the most extreme conditions of the environment. For example, acidophiles- live at pH below 1, alkaliphiles- live in very salty environments, thermophiles- live at high temperatures (113°C), psychrophiles- live in cold temperatures (4°C), methanogens- produce the gas methane, thermoacidophiles-withstand acidic high-temperature water, etc. Contrary to this, all archaea are not extremophiles as it has been recorded from normal environments such as soil, and ocean, and also found to cohabit with bacteria like within the human gut, the occurrence of methane-producing archaea. Their unique characteristics include their membranes chemistry which consists of branched hydrocarbon chains attached to glycerol by ether linkages that helps to stabilize them even under extreme environments. They share a number of of the properties that are common to either bacteria or eukaryotes.

Similarities with bacteria

• Unicellular prokaryotic nature
• Absence of membrane-bound nucleus and other internal structures
• Occurrence of single circular chromosome- a chunk of circular, double-stranded DNA (dsDNA)
• Asexual reproduction by the fission process
• Presence of flagella for locomotion in their environment

Archaea show resemblance to eukaryotes specifically for the enzymatic machinery associated with the processing of genetic information like DNA packaging and replication, RNA transcription, and protein translation. These don’t have a peptidoglycan layer in their cell envelope and are immune to antibiotics that affect the bacteria but are sensitive to some antibiotics that influence the Eukaryotes. Recent conclusions made by biologists after Woese proposed three equal domains, suggested eukaryotes to be the direct descendants of archaea rather than sister groups. The three main phyla that come under Archaea are Crenarchaeota, Euryarchaeota, and Korarchaeota. 

1. Crenarchaeota includes mainly hyperthermophiles and thermoacidophiles. 
2. Similarly, methanogens constitute the members of the Euryarchaeaota phylum.
3. Very little discovery has been done about Korarchaeota. These are found very rarely in nature and are thought to be the oldest lineage of archaebacteria.