Unexpected discovery of a serpentinite-hosted chemosynthetic ecosystem on Asùt Tesoru Seamount, Mariana Forearc

Chemosynthetic ecosystems powered by microbial primary production are rare ‘hot spots’ of biological activity in the deep-sea characterized by dense aggregations of specially adapted animal species. Among settings where such systems have been found, serpentinite-hosted seep systems supported by alkaline geofluid are particularly understudied with just a few known sites worldwide. Mariana Forearc hosts the world's only series of active serpentinite mud volcanoes, but seep communities have only been reported from South Chamorro Seamount where large bathymodioline mussels dominate. Here, we report the discovery of a serpentinite-hosted seep on the conical summit of Asùt Tesoru Seamount, Mariana Forearc. Named the ‘Big Blue Seep’, this field features white, likely carbonate crusts inhabited by animals, under which fluid seepage could be seen. We confirm 16 animal species, including typical seep-associated fauna such as Desbruyeresia gastropods and Acharax awning-clams. This is surprising as previous research expeditions did not notice any sign of chemosynthesis-based ecosystems on this seamount, although the community is indeed difficult to spot due to the lack of large-bodied epifauna such as mussels. The Big Blue Seep is adjacent to three drill holes made by the International Ocean Discovery Program expedition 366 (Holes U1496A-C), which may have impact on seepage. Our findings represent the second chemosynthesis-based ecosystem associated with serpentinite mud volcanism, suggestive of further such communities on other Mariana Forearc mud volcanoes.