High genetic diversity in the pelagic deep-sea fauna of the Atacama Trench revealed by environmental DNA

A current paradigm in marine biodiversity states that faunal richness decreases with depth. However, the deep-ocean ecosystem has been significantly under-sampled, hindering a complete view of its biodiversity. This situation is accentuated in ultra-deep waters, where the remote and extreme conditions unfit the traditional sampling methods. Using environmental DNA, we assessed the pelagic metazoan diversity of the Atacama Trench from the high-productive near-surface level down to ~8000 m depth. Our results show that waters deeper than 4000 m contributed up to 50% of the overall genetic diversity. These findings contrast with similar observations in the less-productive Kermadec Trench, where the diversity in deep waters was lower than in shallower waters. Moreover, both deep pelagic ecosystems exhibited some unknown phylogenetic clades within the dominant taxonomic groups: hydrozoans and copepods. The deep-ocean biota may thus contribute to global biodiversity far more than hitherto suggested, especially in zones influenced by high primary production. Our results underline the need for increased effort to study these remote ecosystems and improve our understanding of their contribution to the ecology and biogeochemistry of the deep-sea pelagic and benthic realms.