Ice nucleating particles (INP) are aerosol particles that catalyze the freezing of atmospheric water at temperatures warmer than homogeneous freezing, which occurs at -36 ?C. INP contained in marine sea spray aerosols may be important drivers of atmospheric ice formation in clean marine areas with low concentrations of terrestrial INP. The characteristics of these INP and how they are formed is not well understood and may be linked to phytoplankton populations in the underlying water. Three phytoplankton batch cultures were grown in a marine aerosol reference tank (MART): Thalassiosira weissflogii, Synechococcus elongatus, and Emiliania huxleyi. Aerosols generated from the phytoplankton cultures were measured for INP activity. All taxa of marine phytoplankton investigated produce INP early in their growth, with maximum average freezing temperatures ranging from -18.0 +- 2.3 ?C to -24.6 +- 1.9 ?C. There was no link between INPs and chlorophyll a concentrations, unlike past studies. High ice nucleation temperatures occurred at times with a high growth rate. Three types of icosahedral marine phytoplankton viruses (31 nm to 170 nm diameter) were also tested for ice nucleation ability. These viruses froze at temperatures between -25.4 +- 0.6 ?C and -20.8 +- 1.6 ?C, showing the first evidence of viruses acting as INP. Field samples were taken to compare the INP activity of phytoplankton sampled during the spring phytoplankton bloom as part of the North Atlantic Aerosols and Marine Ecosystems Study (NAAMES). Samples showed the same moderate INP activity seen in the MART samples taken at times of fast growth. Samples were also removed from natural seawater samples by flow cytometry. These samples each contained a single group of phytoplankton (Synechococcus, nanoeukaryotes, and picoeukaryotes). The sorted samples had average freezing temperatures between -33.8 +- 1.5 ?C and -22.1 +- 0.9 ?C. Diluted samples showed an increase of the INP activity, demonstrating that the salt contained in the samples lowered the freezing temperature of the samples by up to 10 ?C. Although not every sample tested had freezing temperatures above homogeneous freezing, these results show that many groups of phytoplankton and viruses produce INP that are active at atmospherically relevant temperatures.
