Extended Data Figure 6: Ice-nucleating activity of diatoms cells and their exudates.

WACIFE frozen fraction curves derived from 60–129-μm-sized droplets (∼0.4 nl volume) as a function of temperature. Green symbols indicate diatom exudates in 0.1-μm-filtered sea water. Blue and red symbols represent 0.1-μm-filtered sea water devoid of exudates with and without the addition of growth media, respectively. All temperatures have been corrected for freezing point depression to pure water conditions from their initial aqueous solution water activity, a_w = 0.985 (open circles), 0.97 (open squares), 0.96 (filled diamonds), 0.95 (open diamonds), 0.94 (filled circles), 0.925 (open triangle), 0.90 (asterisks). Shaded areas illustrate ranges of observed heterogeneous ice nucleation of intact and fragmented diatom cells (green) and homogeneous ice nucleation of aqueous NaCl droplets (grey) for similar a_w values^6,32. Error bars represent the instrumental uncertainty in temperature measurement. Predicted homogeneous freezing temperatures for similar sized water droplets are indicated by the grey bar^31,40.