We use the described methods for determining the internal cellular quota of thiamin and its\ndifferent phosphorylation states in marine algae, in the below case for the haptophyte Emiliania\nhuxleyi. The chemical assay used \u2013 the thiochrome assay - was developed in the 1930s but has\nbeen modified over the decades to include instrumentation with improved detection (Backstrom et\nal. 1995, Reddick et al. 2001). The assay is based on cyclization\/oxidation of thiamin to\nthiochrome under basic conditions in the presence of an oxidizing agent such as potassium\nferricyanide. Thiochrome is intensely fluorescent and easy to detect and quantify with high\nsensitivity in an HPLC assay. This method has previously been used to determine cellular quotas\nof thiamin and its different phosphorylation states in algae from brackish waters (Pinto et al. 2002,\nSylvander et al. 2013).