Influence of hydrostatic pressure on the behaviour of three ciliate species isolated from the deep-sea floor

Abstract Locomotion is considered to be the main form of expression of ciliate behaviour regarding their overall life activity. But how ciliates behave under deep-sea conditions is still unclear. Data on the occurrence of ciliates in the deep sea are scarce and mostly based on molecular studies. We isolated three different ciliates,Aristerostomasp.,Euplotes dominicanusandPseudocohnilembus persalinusfrom two stations located in abyssal depths of the North Atlantic Ocean (≥ 4000 m; 15° 55.89′ N, 68° 53.34′ W; 23° 33.23′ N, 48° 5.04′ W) during the deep-sea expedition with the research vessel R/V Meteor (Cruise M139, 08.07.–08.08.2017). We observed their behaviour directly under high hydrostatic pressures up to 500 bar. The three ciliate species behaved normally up to a pressure of 200 bar, but showed disturbances of the normal behaviour at higher pressures. For all three isolated deep-sea ciliates, additional long-term survival experiments were carried out for 6 days at 200, 350 and 430 bar. Several specimens showed an ability to survive the entire experimental time interval at the highest pressure and to recover from pressure release (returning to their normal movement) indicating their barotolerance. Our results suggest that ciliates are active in the deep sea even in regions deeper than 2000 m and might be an important part of the deep-sea microbial food web.