The smallest known eukaryotes, at Ϸ1-m diameter, are Ostreococcus tauri and related species of marine phytoplankton. The genome of Ostreococcus lucimarinus has been completed and compared with that of O. tauri. This comparison reveals surprising differences across orthologous chromosomes in the two species from highly syntenic chromosomes in most cases to chromosomes with almost no similarity. Species divergence in these phytoplankton is occurring through multiple mechanisms acting differently on different chromosomes and likely including acquisition of new genes through horizontal gene transfer. We speculate that this latter process may be involved in altering the cell-surface characteristics of each species. In addition, the genome of O. lucimarinus provides insights into the unique metal metabolism of these organisms, which are predicted to have a large number of selenocysteine-containing proteins. Selenoenzymes are more catalytically active than similar enzymes lacking selenium, and thus the cell may require less of that protein. As reported here, selenoenzymes, novel fusion proteins, and loss of some major protein families including ones associated with chromatin are likely important adaptations for achieving a small cell size.green algae ͉ picoeukaryote ͉ genome evolution ͉ selenium ͉ synteny P hytoplankton living in the oceans perform nearly half of total global photosynthesis (1). Eukaryotic phytoplankton exhibit great diversity that contrasts with the lower apparent diversity of ecological niches available to them in aquatic ecosystems. This observation, know as the ''paradox of the plankton,'' has long puzzled biologists (2). By providing molecular level information on related species, genomics is poised to provide new insights into this paradox.Picophytoplankton, with cell diameters Ͻ2 m, play a significant role in major biogeochemical processes, primary productivity, and food webs, especially in oligotrophic waters. Within this size class, the smallest known eukaryotes are Ostreococcus tauri and related species. Although more similar to flattened spheres in shape, these organisms are Ϸ1 m in diameter (3, 4) and have been isolated or detected from samples of diverse geographical origins (5-8). They belong to the Prasinophyceae, an early diverging class within the green plant lineage, and have a strikingly simple cellular organization, with no cell wall or flagella, and with a single chloroplast and mitochondrion (4). Recent work has shown that small-subunit rDNA sequences of Ostreococcus from cultures and environmental samples cluster into four different clades that are likely distinct enough to represent different species (6, 9).Here we report on the gene content, genome organization, and deduced metabolic capacity of the complete genome of Ostreococcus sp. strain CCE9901 (7), a representative of surface-ocean adapted Ostreococcus, referred to here as Ostreococcus lucimarinus. We compare it to the analogous features of the related species O. tauri strain OTH95 (10). Our results show that many process...
