The model marine diatom Thalassiosira pseudonana likely descended from a freshwater ancestor in the genus Cyclotella

The natural distribution of T. pseudonana spans both marine and fresh waters, and phylogenetic analyses of morphological and molecular datasets show that, 1) T. pseudonana marks an early divergence in a major freshwater radiation by diatoms, and 2) as a species, T. pseudonana is likely ancestrally freshwater. Marine strains therefore represent recent recolonizations of higher salinity habitats. In addition, the combination of a relatively nondescript form and a convoluted taxonomic history has introduced some confusion about the identity of T. pseudonana and, by extension, its phylogeny and ecology. We resolve these issues and use phylogenetic criteria to show that T. pseudonana is more appropriately classified by its original name, Cyclotella nana. Cyclotella contains a mix of marine and freshwater species and so more accurately conveys the complexities of the phylogenetic and natural histories of T. pseudonana.The multitude of physical barriers that likely must be overcome for diatoms to successfully colonize freshwaters suggests that the physiological traits of T. pseudonana, and the genes underlying those traits, might differ from those of strictly marine diatoms. The freshwater ancestry of T. pseudonana might therefore confound generalizations about the physiological and metabolic properties of marine diatoms. The freshwater component of T. pseudonana's history merits careful consideration in the interpretation of experimental data collected for this important model species.Diatoms are unicellular photosynthetic algae with secondary, red-algal-derived plastids [1]. With total diversity estimates in the tens to hundreds of thousands of species, diatoms are one of the most diverse lineages of eukaryotes [2] and are critically important to the ecology of both marine and fresh waters. Marine diatoms alone account for roughly one-fifth of global net primary production [3]. Efforts to understand the ecology and evolution of diatoms were catapulted forward when the fi