In taxonomy, Methanocaldococcus is a genus of the Methanocaldococcaceae. Methanocaldococcus formerly known as Methanococcus is a genus of coccoid methanogens. They are all mesophiles, except the thermophilic M. thermolithotrophicus and the hyperthermophilic M. jannaschii. The latter was discovered at the base of a “white smoker” chimney at 21°N on the East Pacific Rise and it was the first archaean genome to be completely sequenced, revealing many novel and eukaryote-like elements. The name Methanocaldococcus has Latin and Greek roots, methano for methane, caldo for hot, and the Greek kokkos for the spherical shape of the cells. Overall, the name means spherical cell that produces methane at hot temperatures. All species in Methanocaldococcus are obligate methanogens. They use hydrogen to reduce carbon dioxide. Unlike many other species within Euryarchaeota, they cannot use formate, acetate, methanol or methylamines as substrates.