•They are thermophilicand extremely acidophilic (one of the most acidophilic of all known microorganisms).
•Examples: Thermoplasma(optimal temp. 55oC, pH2), Picrophilus(pH below 0), Ferroplasma(optimal temp. 35oC).
•They lack cell walls and are facultative aerobes.
•Their cell membrane contains lipoglycanand glycoprotein, but not sterol.