Diatoms are unicellular algae with plastids acquired by secondary endosymbiosis. They are responsible for ∼20% of global carbon fixation. We report the 34 million - base pair draft nuclear genome of the marine diatom Thalassiosira pseudonana and its 129 thousand - base pair plastid and 44 thousand - base pair mitochondrial genomes. Sequence and optical restriction mapping revealed 24 diploid nuclear chromosomes. We identified novel genes for silicic acid transport and formation of silica-based cell walls, high-affinity iron uptake, biosynthetic enzymes for several types of polyunsaturated fatty acids, use of a range of nitrogenous compounds, and a complete urea cycle, all attributes that allow diatoms to prosper in aquatic environments.