makes a significant contribution to global organic carbon production through carbon dioxide assimilation in the oceans. Eukaryotic picophytoplankton has a world- wide distribution and is an important contributor to biogeochemical cycles. Of picoeukaryotes, three abundant ubiquitous genera from prasinophytes, Ostreococcus, Bathycoccus and Micromonas have been the most studied. The first picoeukaryote to be identified was Ostreococcus tauri, a species initially identified as a major component of the picophytoplankton in the Thau Correspondence: [email protected] 5 UPMC University Paris 06, UMR7621 Laboratoire d’Ocanographie Microbienne, Observatoire Ocanologique, F-66651, Banyuls/mer, France lagoon. O. tauri has been described as the smallest free-living eukaryote with the simplest ultrastructure that is one chloroplast, one mitochondrion, one Golgi body in addition to the nucleus. In recent years, several genomes of Ostreococcus have been sequenced including O. tauri and Ostreococcus lucimarinus. Two genomes of Micromonas became available recently and a deep strain of Ostreococcus is now being sequenced. The analysis of Ostreococcus genomes has led to several hypothesis about physiological features, such as the occurrence of an atypical light harvesting complex and C4 photosynthesis. An unusual high number of selenocysteine-containing proteins and a reduction of chromatin protein have been described in both strains of Ostreococcus but their significance is yet unknown. Unusual features of O. tauri and O. lucimarinus genomes include high gene density, 2010 Monnier et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Monnier et al. BMC Genomics 2010, 11:192 http://www.biomedcentral.com/1471-2164/11/192 Page 2 of 13 heterogeneous genome structure with two atypical chromosomes and high genome compaction. Analysis of gene content and evolution rates have suggested that lack of recombination and thus a lack of GC Biased Gene Conversion may be the origin of the lower GC content of the atypical chromosomes. Phylogenetic footprints size distributions depend on gene orientation of neighboring genes and suggest a lower frequency of bidirectional regulatory elements in promoters 
in Ostreococcus as compared to budding yeast. Clustering of genes involved in nitrogen assimilation on chromosome 10 suggest a possible link between gene localization on chromosomes and transcription. However all the above conclusions were inferred exclusively from in silico studies and the impact of genome structure on the transcription mechanisms remains to be addressed. Little is known on the biology and physiology of eukaryotic picophytoplankton, which might explain their ecological Indirubin-3′-oxime site success. In budding yeast, a genomewide transcriptomic approach has revealed transcriptional networks associated with the temporal compartmentalization of cellular processes such as cell division during the metabolic cycle. We have recently shown in Ostreococcus, that cell division is temporally regulated in cells exposed to diurnal cycle. Most of the cell cycle regulators, including cyclin and cyclindependent kinase family, were found to be transcriptionally regulated. Therefore, to gain insight in the biology and PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19793655 physiology of Ostreococcus as well as the transcriptional basis
