MEPS 171:85-95 (1998)  -  doi:10.3354/meps171085

ABSTRACT: Six isolates of deep-sea flagellated protozoa were 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, Caecitellus 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. This 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 collection, 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 epipelagic 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 WORDS: Protozoa · Flagellate · Chyrysomonad · Hydrostatic pressure · Growth rate · Encystment · Hydrothermal vent · Continental shelf · Extreme environment