|Transmission electron micrograph of an O. tauri cell|
C. Courties & M.-J. Chrétiennot-Dinet (1995)
Ostreococcus tauri is a unicellular species of marine green alga about 0.8 micrometres (μm) in diameter, the smallest free-living (non-symbiotic) eukaryote yet described. It has a very simple ultrastructure, and a compact genome.
As a common member of global oceanic picoplankton populations, this organism has a major role in the carbon cycle in many areas. Recently, O. tauri has been the subject of studies using comparative genomics and functional genomics, as it is of interest to researchers because of its compact genome and green lineage.
O. tauri was discovered in 1994 in the Thau lagoon, France, in an year-long study of the picoplankton population of the lagoon using flow cytometry. O. tauri was found to be the main component of the picoplankton population in the lagoon, and images of cells produced by transmission electron microscopy revealed the smallest yet described free-living eukaryotic cells. O. tauri was immediately placed in the class Prasinophyceae based on the presence of characteristic chlorophyll pigments and Chlorophyceae-related carotenoids, and this classification was confirmed by further work.
Cells are roughly spherical (coccoid), averaging about 1 μm long by 0.7 μm wide. The cell's ultrastructure is very simple, lacking a cell wall and consisting of a nucleus, a single mitochondrion, a single chloroplast, and a single Golgi apparatus. Cells also lack flagella.
O. tauri is the dominant algal species, by cell abundance, in the Thau Lagoon in the south of France. The conditions that are thought to lead to this dominance are firstly that the Lagoon is used for intensive mollusc cultivation, and secondly that copper levels in the Lagoon are high. The first consideration selects for smaller cells (picoplankton); larger eukaryotic species of alga and many predators of smaller algae are preferentially consumed by the molluscs, which are filter feeders. The second consideration selects against cyanobacteria, as O. tauri is thought to cope better with "adverse conditions". The excess copper in the lagoon is thought to originate from agricultural chemicals used by surrounding vinyards.
Use as a model organism
In 2006, the O. tauri genome was sequenced by Derelle et al.. The 12.56 Mb genome organized in 20 chromosomes showed extreme gene density and few intron-containing genes. Two chromosomes with outlying characteristics (G+C content, intron structure) were identified, namely chromosome 2 and chromosome 19. Sequencing of other species of the Mamiellales order showed occurrence of similar outlying chromosomes in other species (O. lucimarinus, M. pusilla  & B. prasinos).
- Derelle, E., Ferraz, C., Rombauts, S., Rouzé, P., Worden, A. Z., Robbens, S., … Moreau, H. (2006). Genome analysis of the smallest free-living eukaryote Ostreococcus tauri unveils many unique features. Proceedings of the National Academy of Sciences of the United States of America, 103(31), 11647–52. http://doi.org/10.1073/pnas.0604795103
- Palenik, B., Grimwood, J., Aerts, A., Rouzé, P., Salamov, A., Putnam, N., … Grigoriev, I. V. (2007). The tiny eukaryote Ostreococcus provides genomic insights into the paradox of plankton speciation. Proceedings of the National Academy of Sciences of the United States of America, 104(18), 7705–10. http://doi.org/10.1073/pnas.0611046104
- Blanc-Mathieu, R., Verhelst, B., Derelle, E., Rombauts, S., Bouget, F.-Y., Carré, I., … Piganeau, G. (2014). An improved genome of the model marine alga Ostreococcus tauri unfolds by assessing Illumina de novo assemblies. BMC Genomics, 15(1), 1103. http://doi.org/10.1186/1471-2164-15-1103
- Krumholz, E. W., Yang, H., Weisenhorn, P., Henry, C. S., & Libourel, I. G. L. (2012). Genome-wide metabolic network reconstruction of the picoalga Ostreococcus. Journal of Experimental Botany, 63(6), 2353–2362. http://doi.org/10.1093/jxb/err407
- Courties, C. et al. (1994). Smallest eukaryotic organism. Nature 370: 255.
- Courties, C. et al. (1995). A new marine picoeukaryote - Ostreococcus tauri Gen et Sp-NOV (Chlorophyta, Prasinophyceae). Phycologia 34 (4): 285-292
- Courties, C. et al. (1998). "Phylogenetic analysis and genome size of Ostreococcus tauri (Chlorophyta, Prasinophyceae)" J. Phycol 34 (5): 844-849.
- Vaquer, A. et al. (1996). Standing stock and dynamics of picophytoplankton in the Thau Lagoon (northwest Mediterranean coast). Limnology and Oceanography 41 (8): 1821-1828
- Worden, A. Z., Lee, J.-H., Mock, T., Rouzé, P., Simmons, M. P., Aerts, A. L., … Grigoriev, I. V. (2009). Green evolution and dynamic adaptations revealed by genomes of the marine picoeukaryotes Micromonas. Science (New York, N.Y.), 324(5924), 268–272. http://doi.org/10.1126/science.1167222
- Moreau, H., Verhelst, B., Couloux, A., Derelle, E., Rombauts, S., Grimsley, N., … Vandepoele, K. (2012). Gene functionalities and genome structure in Bathycoccus prasinos reflect cellular specializations at the base of the green lineage. Genome Biology, 13(8), R74. http://doi.org/10.1186/gb-2012-13-8-r74