Unicellular organisms living in a marine environment sometimes have to cope with massive fluctuations in inorganic phosphate (Pi) availability. We studied phosphatases and phosphate transporters of the diatom Phaeodactylum tricornutum with respect to the extracellular Pi-concentration depending expression as well as their extra- or intracellular localization. These studies, which add a further level of knowledge to already existing transcriptomic data, highlight the capacity of the diatom to distribute Pi intracellularly and to mobilize Pi from extra- and intracellular resources