Diatoms account for roughly 20 percent of global primary production, while making up less than 0.2 percent of primary producer biomass. Additionally, they are the key drivers of biogeochemical silica cycling and have acquired a diverse set of metabolic pathways including a complete urea cycle (previously only known from animals), iron-concentrating mechanisms, and polyamine biosynthesis. Surprisingly, these diverse functional traits were all enabled by the acquisition of genes transferred horizontally from bacteria. We have sequenced the nuclear genomes of five diatom species and are using these data to reconstruct the diversification of diatoms, both in species numbers and in functional diversity, and to characterize the relative importance of endogenous (novel genes acquired through duplications, e.g., polyploidy) and exogenous (transferred from bacteria) sources of genetic variation in the speciation and functional diversification of diatoms (Wickett and outside collaborators).