Changes in the Abundance and Taxonomic Composition of Benthic Heterotrophic Protists from Atlantic Sublittoral to Deep-Sea Sediments

Protists are the most diverse eukaryotes on our planet and metabarcoding has revealed an enormous diversity even from deep-sea environments. A range of different species has also been isolated from the deep sea and some have proven able to survive and even grow under deep-sea conditions. However, little is known about how the community structure of benthic protists changes from sublittoral down to abyssal depths. This is especially important regarding island and seamount communities which are surrounded by deep-sea areas potentially isolating them. Using a combination of live-counting and cultivation techniques, we investigated the abundance and taxonomic composition of benthic protist communities in sediments from sublittoral to abyssal depths around three islands and two seamounts of the Azores’ archipelago in the North Atlantic. Protist abundance decreased significantly and community composition changed with increasing depth. While some species were found at all depths, others were only detected in sublittoral or lower bathyal depths, indicating that some benthic taxa are limited in their distribution to a certain depth, whereas others are also present at the deep-sea floor. The proportion of unidentified specimens increased with depths pointing towards a high number of so far undetected species in the deep-sea realm.