Phaeodactylum tricornutum is a diatom. It is the only species in the genus Phaeodactylum. Unlike other diatoms P. tricornutum can exist in different morphotypes (fusiform, triradiate, and oval), and changes in cell shape can be stimulated by environmental conditions. This feature can be used to explore the molecular basis of cell shape control and morphogenesis. Unlike most diatoms P. tricornutum can grow in the absence of silicon, and the biogenesis of silicified frustules is facultative. This provides opportunities for experimental exploration of silicon-based nanofabrication in diatoms.

Further reading:

The Phaeodactylum genome reveals the evolutionary history of diatom genomes. Nature 456(7219), 239-244 (2008). [pubmed]