An axenic culture of P. tricornutum CCMP 632 (National Center for Marine Algae and Microbiota, Bigelow Laboratory, Boothbay, Maine) was grown at 18 °C in polycarbonate bottles under continuous illumination (90 µmol m -2 s -1 ). The culture medium was prepared using 0.2 µm filtered coastal seawater that was autoclaved to sterilize then amended with macro-and micronutrients as detailed in the supplementary information (SI Table ). All nutrients were sterile filtered (0.2 µm PES) prior to addition to sterilized seawater. Cultures were maintained by sterile technique and the absence of bacteria was confirmed via DAPI stained samples. Relative chlorophyll fluorescence was monitored (Turner TD-700) as a proxy for growth (SI Fig. ). Cultures were harvested during the exponential growth phase.