Ostreococcus tauri
Transmission electron micrograph of an O. tauri cell
Scientific classification Edit this classification
(unranked): Viridiplantae
Division: Chlorophyta
Class: Mamiellophyceae
Order: Mamiellales
Family: Bathycoccaceae
Genus: Ostreococcus
Species:
O. tauri
Binomial name
Ostreococcus tauri
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 and often uses light receptors called phototropins to sense external light.[1] Recently, O. tauri has been the subject of studies using comparative genomics and functional genomics,[2][3][4][5] as it is of interest to researchers because of its compact genome and green lineage.

History

Ostreococcus tauri was discovered in 1994 in the Thau lagoon, France, in a 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.[6] O. tauri was immediately placed in the class Prasinophyceae based on the presence of characteristic chlorophyll pigments and Chlorophyceae-related carotenoids.[6][7][8] However, it has been classified together with its order Mamiellales into an own class, Mamiellophyceae meanwhile (Marin and Melkonian, 2010).[9][10][11]

Anatomy

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.[6] Cells also lack flagella.

Initially described as containing 14 chromosomes,[8] it is now known that the nucleus contains 20 chromosomes, in all about 33 fg of DNA.[6]

Ecology

Ostreococcus 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 vineyards.[12]

Use as a model organism

As early as 1998, O. tauri was identified as "a good candidate for biological models such as cell division and/or genome sequencing studies".[8]

Cryptic sex

When grown in the laboratory, O. tauri cells are haploid[13]. Although no sexual life cycle has been described, the occurrence of cryptic sex is indicated by the expression of core genes for meiosis[13]. Sexual exchanges, as indicated by formation of genetic recombinants, appear to occur, but are infrequent.

Genomics

In 2006, the O. tauri genome was sequenced by Derelle et al..[2] 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,[3] Micromonas pusilla [14] & B. prasinos[15]). The mitochondrial genome consist of 44,237 base pairs and 65 genes, and the chloroplast genome of 71,666 base pairs and 86 genes.[16]

References

  1. ↑ Veetil, S.K; Mittal, C; Ranjan, P; Kateriya, S (July 2011). "A conserved isoleucine in the LOV1 domain of a novel phototropin from the marine alga Ostreococcus tauri modulates the dark state recovery of the domain". Biochim Biophys Acta. 1810 (7): 675–82. doi:10.1016/j.bbagen.2011.04.008. PMID 21554927.
  2. 1 2 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
  3. 1 2 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
  4. ↑ 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
  5. ↑ 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
  6. 1 2 3 4 Courties, C. et al. (1994). Smallest eukaryotic organism. Nature 370: 255.
  7. ↑ Courties, C. et al. (1995). A new marine picoeukaryote - Ostreococcus tauri Gen et Sp-NOV (Chlorophyta, Prasinophyceae). Phycologia 34 (4): 285-292
  8. 1 2 3 Courties, C. et al. (1998). "Phylogenetic analysis and genome size of Ostreococcus tauri (Chlorophyta, Prasinophyceae)" J. Phycol. 34 (5): 844-849.
  9. ↑ NCBI: Mamiellales Moestrup, 1984 (order)
  10. ↑ NCBI: Mamiellophyceae B.Marin & Melkonian, 2010 (class)
  11. ↑ WoRMS: Mamiellophyceae
  12. ↑ 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
  13. 1 2 Grimsley N, Péquin B, Bachy C, Moreau H, Piganeau G. Cryptic sex in the smallest eukaryotic marine green alga. Mol Biol Evol. 2010 Jan;27(1):47-54. doi: 10.1093/molbev/msp203. PMID: 19734297
  14. ↑ 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, 324(5924), 268–272. http://doi.org/10.1126/science.1167222
  15. ↑ 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
  16. ↑ The complete chloroplast and mitochondrial DNA sequence of Ostreococcus tauri: organelle genomes of the smallest eukaryote are examples of compaction
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