What is "methanococcus"

Methanococcus is a genus of coccoid methanogens of the family Methanococcaceae. 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 archaeal genome to be completely sequenced, revealing many novel and eukaryote-like elements.