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Six isolates of deep-sea flagellated protozoa \yere grown in culture at 1 to 300 atm to measure their growth response to increasing hydrostatic pressure. Three kinetoplastid flagellates and 1 choanoflagellate were isolated from deep-sea hydrothermal vent samples and 2 chrysomonads were isolated from deep continental shelf sediments. The growth rates of 2 species isolated from the vent, CaeciteUus parvulus and Rhynchomonas nasuta, were compared to the growth rates of shallow-water strains of the same species. Deep-sea isolates of C. parvulus and R. nasuta had a higher rate of growth at higher pressures than did their shallow-water counterparts. Thls feature could result from adaptation to higher pressure upon sinking to depth or with time on the ocean bottom. Four of the 6 deep-sea isolates-C. parvulus, R. nasuta and the 2 chrysomonads-were capable of growth at pressures corresponding to their respective depths of collect~on, indicating that these species could be metabolically active at these depths. C. parvulus and R. nasuta encysted at pressures greater than their depth of collection. The choanoflagellate isolate was observed to encyst at pressures greater than 50 atm. These findings suggest a potential ecological role for encystment In deep-sea/water-column coupling. Cosmopolitan or epipelag~c species, such as C. parvulus and R nasuta, may be transported on sedimenting particles into the deep sea, where encystment at high pressure could serve as a mechanism for pelagic renewal by advection via hydrothermal plume entrainment and thermohaline circulation. KEY WORDSProtozoa. Flagellate. Chyrysomonad . Hydrostatic pressure. Growth ra te . Encystment . Hydrothermal vent Continental shelf . Extreme environment

marine ecology. progress series

Effect of hydrostatic pressure on the growth rates and encystment of flagellated protozoa isolated from a deep-sea hydrothermal vent and a deep shelf region