Distribution and Drivers of Benthic Biodiversity in Hadal and Abyssal Realms through eDNA based Inventorie

Biodiversity in the marine environment still mostly remains to be revealed. However, the difficulty to access it, particularly in the deep sea, and the time needed to deliver rigorous morphological descriptions of the many new species discovered during each oceanographic expedition render this task unrealistic at the scale of human lifetime. Environmental DNA (eDNA) offers, adopting a phylogenetic species concept, a parallel avenue to deliver rapid and standardized biodiversity inventories of the different biotic compartments forming deep sea communities, including prokaryotic and eukaryotic diversity. In the framework of the project “Pourquoi Pas les Abysses” (Ifremer) and its twin project eDNAbyss (CEA, Génoscope), we adopted a standardized protocol to sample and analyse molecularly and bioinformatically eDNA from sediment of bathyal, abyssal and hadal environments. Our main goal was to contribute to a molecular inventory of the deep sea biodiversity, in relation with abiotic descriptors of the environments. This protocol was deployed in several abyssal areas of the Atlantic and the Pacific, and in hadal environments during the Sonne cruise So261 on Atacama trench, in collaboration with the HADES (EU, ERC, Nordcee) project. Our aim was to explore the compared the levels and taxonomic composition of eukaryotic biodiversity along the trench axis and in reference abyssal areas. The first results showed decreasing biodiversity with site depth, despite a trend toward higher biomass in hadal than in abyssal systems. The relative influence of site and sediment depth on benthic communities, as well as geography (latitude/distance) was also shown to vary significantly depending on taxa. We will provide an overview of the extend of hadal versus abyssal biodiversity, the composition of communities, and the main relationship emerging with abiotic descriptors of the environment.