Ignicoccus

Ignicoccus is a genus of archaea in the Desulfurococcaceae family, Desulfurococcales order, within the phylum Crenarchaeota. The genus currently includes the species Ignicoccus hospitalis, described from hydrothermal vent samples, and is best known for its distinctive cell biology and its association with the archaeal symbiont Nanoarchaeum equitans.

Morphology and cell envelope: Ignicoccus species are hyperthermophilic cells that possess a two-membrane cell envelope: an inner cytoplasmic membrane and an outer cellular membrane separated by a periplasmic space. The outer membrane can form vesicles and is involved in energy transduction; this arrangement creates a substantial periplasmic compartment unusual among archaea.

Physiology and ecology: They are strict anaerobes found in deep-sea hydrothermal vent ecosystems. They grow best at high temperatures (near 90°C) and in environments rich in reduced sulfur compounds; energy metabolism is linked to sulfur compounds and hydrogen, with CO2 serving as a carbon source in many strains.

Symbiosis: Ignicoccus hospitalis hosts the small archaeon Nanoarchaeum equitans on its surface; N. equitans lacks many biosynthetic pathways and relies on I. hospitalis for lipids and perhaps other metabolites.

Taxonomy and research significance: The Ignicoccus–Nanoarchaeum system provides a rare example of archaeal ectosymbiosis and reveals novel aspects of archaeal cell envelope structure and energy metabolism.