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Abstract. Even though studies of Arctic ice forming particles suggest that a bacterial or viral source derived from open leads could be important for ice formation in Arctic clouds (Bigg and Leck, 2001), the ice nucleation potential of most polar marine psychrophiles or viruses has not been examined under conditions more closely resembling those in the atmosphere. In this paper, we examined the ice nucleation activity (INA) of several representative Arctic and Antarctic sea-ice bacterial isolates and a polar Colwellia phage virus. High-resolution ice nucleation spectra were obtained for droplets containing bacterial cells or virus particles using a free-fall freezing tube technique. The fraction of frozen droplets at a particular droplet temperature was determined by measuring the depolarized light scattering intensity from solution droplets in free-fall. Our experiments revealed that all sea-ice isolates and the virus nucleated ice at temperatures very close to the homogeneous nucleation temperature for the nucleation medium – which for artificial seawater was –42.2±0.3°C. Our results suggest that immersion freezing of these marine psychro-active bacteria and viruses would not be important for heterogeneous ice nucleation processes in polar clouds or to the formation of sea ice. These results also suggested that avoidance of ice formation in close proximity to cell surfaces might be one of the cold-adaptation and survival strategies for sea-ice bacteria. The fact that INA occurs at such low temperature could constitute one factor that explains the persistence of metabolic activities at temperatures far below the freezing point of seawater.

High-resolution ice nucleation spectra of sea-ice bacteria: implications for cloud formation and life in frozen environments