Mortality and survival of cultured surface-ocean flagellates under simulated deep-sea conditions

•Two surface-isolated flagellates incubated at 2 °C and 50 MPa for 1–2 weeks•Low temperature and high pressure changes both contributed to mortality rate.•Up to a quarter of flagellate cells survived and could “seed” deep sea communities.•Occasionally, eukaryotic cells multiplied under extended deep sea conditions.

The vast majority of the marine environment consists of dark, cold, high-pressure environments into which microbes from the surface ocean are continuously transported by advection or by attachment to sinking particles. Little is known about how the low temperature and high pressure of the deep ocean affect the abundance and activity of surface organisms, in particular microbial eukaryotes. Cultures of two flagellate species, Cafeteria roenbergensis and Neobodo designis, both isolated from surface waters where they are considered cosmopolitan, were incubated in titanium chambers for one to two weeks under typical deep-sea temperature (2 °C) and pressure (50 MPa, representing 5000 m water depth). Samples were taken daily with minimal loss of pressure in the culture vessel, and flagellates were subsequently enumerated, along with prokaryotic prey when possible. The abundance of protists declined in all treatments, with a significantly greater rate of mortality under combined cold temperature and high pressure conditions than in the cold temperature-only conditions. However, 1.6 (SD = 3.8)% of C. roenbergensis and 10.0 (6.1)% of N. designis cells survived on average in the high pressure treatments, indicating that some fraction of sinking protists can survive transport to the deep ocean. In addition, after a period of acclimation, positive growth rates were measured in some cases, suggesting that surface-adapted flagellates cannot only survive under deep-sea conditions but are able to reproduce and potentially provide seed populations to cold, high-pressure environments.