Protist-dominated hard substrate faunas thrive at the deepest ocean depths.

Deep-sea hard substrates host faunal novelties and distinct evolutionary lineages. However, sessile organisms on rocks are difficult to sample and largely unknown at extreme hadal depths. Here, we report a deep hard-substrate fauna (9000 to 10,898 meters), comprising 32 species of six protist and metazoan phyla, most millimeter-sized and new to science, from the Kermadec and Mariana trenches, using the manned submersible Fendouzhe. We show that the filamentous organisms dominating these assemblages are heterotrophic foraminiferans, challenging the earlier chemolithoautotrophic hypothesis. Large-scale seafloor imaging and sampling suggest that similar protistan-dominated sessile communities thrive in seven hadal regions around Oceania. These faunas open new perspectives on biodiversity at the deepest ocean depths and unveil widespread, but previously unrecognized, carbon hotspots in global hadal trenches. Editor's summary: Characterizing life at the deepest depths of the ocean is extremely challenging, involving sampling at depths between 9000 and 11,000 meters. Our knowledge of these communities is thus relatively limited. Song et al. report a wide array of species sampled by a manned submersible in the Kermedec and Mariana trenches. They describe a new fauna characterized by mostly filamentous and sedentary protists and forams. Particularly surprising was the presence of heterotrophic, rather than chemolithoautotrophic, organisms feeding on terrestrial pine pollen. The authors argue that similar communities occur across the deepest ecosystems, with implications for understanding carbon hotspots at these depths. —Sacha Vignieri [ABSTRACT FROM AUTHOR]