Ignicoccus gen. nov., a novel genus of hyperthermophilic, chemolithoautotrophic Archaea, represented by two new species, Ignicoccus islandicus sp. nov. and Ignicoccus pacificus sp. nov

This study describes the discovery and characterization of Ignicoccus, a new genus of hyperthermophilic archaea within the Crenarchaeota. Two species were isolated from submarine hydrothermal systems: Ignicoccus islandicus from the Kolbeinsey Ridge near Iceland and Ignicoccus pacificus from the East Pacific Rise. Both species are obligate chemolithoautotrophic organisms that grow as irregular cocci at temperatures between 70-98°C with optima around 90°C. These organisms obtain energy exclusively by reducing elemental sulfur using molecular hydrogen as the electron donor, producing hydrogen sulfide. They are strictly anaerobic and cannot use organic substrates as sole energy sources, though some organic compounds stimulate growth. Phylogenetic analysis based on 16S rRNA sequences confirms their membership in the Desulfurococcaceae family within the order Desulfurococcales. DNA-DNA hybridization showed low similarity between the two species (11%) and with other Desulfurococcaceae members (5-9%), supporting their classification as a distinct new genus. The organisms possess characteristic archaeal lipids and a unique cell envelope architecture with periplasmic vesicles. Their chemolithoautotrophic lifestyle suggests they serve as primary producers in deep-sea hydrothermal vent ecosystems.

Key findings

• Two new hyperthermophilic archaeal species, Ignicoccus islandicus and Ignicoccus pacificus, were isolated from submarine hydrothermal vents in the Atlantic and Pacific, respectively, representing the first obligate chemolithoautotrophic sulfur-reducers in the Desulfurococcaceae family.
• These organisms grow optimally at 90°C using only hydrogen as electron donor and elemental sulfur as electron acceptor, making them obligate hydrogen-sulfur autotrophs that produce hydrogen sulfide.
• 16S rRNA sequence analysis and DNA-DNA hybridization data demonstrate that the isolates form a distinct new genus within the crenarchaeal order Desulfurococcales, with sequence differences of at least 6% from other family members.
• The organisms possess an unusual cell envelope architecture with a periplasm containing numerous membrane-bound vesicles (50-600 nm) and contain both acyclic diphytanyl glycerol and tetraether lipids typical of archaea.
• Their chemolithoautotrophic metabolism and presence in geographically distant hydrothermal systems suggest these organisms are widespread primary producers in deep-sea hydrothermal ecosystems.