Ostreococcus belongs to the Prasinophyceae, an early-diverging class within the green plant lineage, and is reported as a globally abundant picoeukaryotic group (tiny algae) throughout the oceanic euphotic zone (the top of the water column, where green plants live). The most striking feature of O. tauri and related species is their minimal cellular organization: a naked, nearly 1-micron cell, lacking flagella, with a single chloroplast and mitochondrion. Three different ecotypes or potential species have been defined, based on their adaptation to light intensity. One is adapted to high light intensities and corresponds to surface-isolated strains. A second has been defined as low-light and includes strains from deeper in the water column. The third corresponds to strains isolated from a coastal lagoon and can be considered light-polyvalent. The light-polyvalent and high-light ecotypes or species have been sequenced (at Genescope and JGI, respectively). JGI will now sequence the genome of the low-light strain (RCC141), to complete the set.
This project will provide the opportunity to study the gene content of marine eukaryote species that are phylogenetically closely related but adapted to various ecological niches, a boon for the field of comparative and environmental genomics. For example, comparison between the genomes of “high-light” and “low-light” Prochlorococcus strains (cyanobacteria) showed that 1/3 of the genes were specific to a strain. Among these strain-specific genes, many corresponded either to hypothetical sequences or to proteins potentially involved in photosynthesis, UV resistance, or nitrogen metabolism that were consistent with the strain’s reported niche. These Prochlorococcus genenomes, in combination with genomes from the sister group Synechococcus, are now an important model for niche differentiation in cyanobacteria and other prokaryotes. Ostreococcus, because it is widespread and has a relatively small genome, is an ideal comparative model for understanding niche differentiation in unicellular eukaryotes. This genome will also contribute to our understanding of the evolution of size in eukaryotes, a topic addressed in general by Ostreococcus.
CSP project co-leaders: Brian Palenik (UC San Diego) and Hervé Moreau (CNRS and Univ. of Paris VI, Banyuls sur mer ).