Ostreococcus lucimarinus is one of three strains of Ostreococcus that are sequenced (next to O.tauri and a low-light/deep-sea strain). This striking feature of these organisms is their incredible small size and compact cellular organisation with a single chloroplast and a single mitochondrion.

Further reading:

Palenik, B. et al. The tiny eukaryote Ostreococcus provides genomic insights into the paradox of plankton speciation.Proc Natl Acad Sci U S A 104, 7705-10 (2007). [pubmed]