Cell counts and cell specific chlorophyll autofluorescence were mea sured using a BDAccuri C6 flow cytometer. Culture samples were dilut- ed (100 ul culture in 900 HL medium) and passed through 30 pm filters to remove debris. Flow cytometry was carried out at a flow rate of 35 HL min 1 and a core size of 16 um. Phaeodactylum cells were quanti fied after gating from forward scatter and relative fluorescence emission (excitation at 488 nm, emission detection at 670 nm).