Ciliate diversity and distribution across horizontal and vertical scales in the open ocean.
Ciliates are globally distributed eukaryotic organisms inhabiting virtually all environments on Earth. Although ciliates range from 10 µm to few mm in cell size, they are repeatedly reported in the pico-sized fraction (smaller than 2-3 µm) of molecular surveys. Here, we used existing datasets (BioMarKs and Tara Oceans) with different size fractions to demonstrate that the ciliate pico-sized signal, likely derived from cell breakage during filtration, is informative and reliable to study marine ciliate biodiversity and biogeography. Then, we used sequences from the picoeukaryotic fraction of two circumnavigation expeditions, Malaspina-2010 and Tara Oceans, to give insights into the taxonomic composition and horizontal and vertical distribution of ciliates in the global ocean. Results suggested a high homogeneity of ciliate communities along the ocean surface from temperate to tropical waters, with ciliate assemblages dominated by a few abundant and widely distributed taxa. Very few taxa were found in a single oceanic region, therefore suggesting a high level of ciliate cosmopolitanism in the global ocean. In vertical profiles, ciliates were detected up to 4000 m depth, and a clear vertical community structuring was observed. Our results provided evidence supporting ciliates as deeply integrated organisms in the deep-sea trophic web, where they may play a relevant role as symbionts of metazoans and grazers of prokaryotes and small eukaryotes in the water column and in aggregates.