Stimulation of ice nucleation by marine diatoms

Atmospheric aerosol particles serve as nuclei for ice-crystal formation. As such, these particles are critical to the generation of cirrus clouds, which form from gas and liquid water(1). Atmospheric aerosols also initiate ice formation in warmer, mixed-phase clouds, where ice crystals coexist with aqueous droplets(1). Biogenic aerosol particles of terrestrial origin, including bacteria and pollen, can act as ice nuclei(2). Whether biogenic particles of marine origin also act as ice nuclei has remained uncertain(3-5). We exposed the cosmopolitan planktonic diatom species Thalassiosira pseudonana to water vapour and supercooled aqueous sodium chloride under typical tropospheric conditions conducive to cirrus-cloud formation. Ice nucleation was determined using a controlled vapour cooling-stage microscope system. Under all conditions, diatoms initiated ice formation. The presence of diatoms in water increased the temperature for ice formation up to 13 K, and in aqueous sodium chloride, ice formed at temperatures up to 30 K higher than when diatoms were not present. In addition, diatoms initiated ice formation from water vapour at relative humidities as low as 65%. The rate of ice nucleation was rapid and independent of surface area. We suggest that marine biogenic particles such as diatoms help explain high values and seasonal variations(6,7) in ice-nuclei concentrations(8-10) in subpolar regions.