The most significant photosynthetic eukaryotes in the marine environment are the diatoms, which contribute around 40% of marine primary production, thereby providing approximately one fifth of the oxygen we breathe. Knowledge about diatom biology is currently being revolutionized by the availability of two complete genome sequences, from the centric diatom Thalassiosira pseudonana and the pennate diatom Phaeodactylum tricornutum. Unlike T. pseudonana, P. tricornutum can be routinely transformed and a range of reverse genetics tools have been developed. The current funding request is to utilize these resources to begin comprehensive functional genomics approaches in P. tricornutum, specifically the generation of a physical map of the genome, to facilitate genome assembly and classical genetics approaches, and the development of fluorescent subcellular markers. Such resources will lead to new insights about diatom evolution and about their basic biology.