Diatoms are eukaryotic, photosynthetic microorganisms found throughout marine and freshwater ecosystems and are responsible for as much as 20% of global primary productivity. A defining feature of diatom is their ornately patterned silicified cell wall or frustule, which displays such species-specific fine scale nano-structures that diatoms have long been used to test the resolution of optical microscopes. The marine centric diatom Thalassiosira pseudonana was chosen as the first eukaryotic marine phytoplankton for whole genome sequencing because this species has served as a model for diatom physiology studies, the genus Thalassiosira is cosmopolitan throughout the world's oceans, and the genome is relatively small at 34 mega base pairs.

Further reading:

The genome of the diatom Thalassiosira pseudonana: ecology, evolution, and metabolism. Science. 306(5693), 79-86 (2004). [pubmed]