A moderately thermophilic origin of a novel family of marine group II euryarchaeota from deep ocean

Marine group II (MGII) is the most abundant planktonic heterotrophic archaea in the ocean. The evolutionary history of MGII archaea is elusive. In this study, 13 new MGII metagenome-assembled genomes were recovered from surface to the hadal zone in Challenger Deep of the Mariana Trench; four of them from the deep ocean represent a novel group. The optimal growth temperature (OGT) of the common ancestor of MGII has been estimated to be at about 60 degrees C and OGTs of MGIIc, MGIIb, and MGIIa at 47 degrees C-50 degrees C, 37 degrees C-44 degrees C, and 30 degrees C-37 degrees C, respectively, suggesting the adaptation of these species to different temperatures during evolution. The estimated OGT range of MGIIc was supported by experimental measurements of cloned beta-galactosidase that showed optimal enzyme activity around 50 degrees C. These results indicate that MGIIc may have originated from a common ancestor that lived in warm or even hot marine environment, such as hydrothermal vents.