Micromonas is a eukaryotic, photosynthetic microorganism found throughout marine ecosystems - from polar waters to the tropics. It is classified as a member of the picophytoplankton because of its small size (< 2 µm in diameter). Currently, picophytoplankton are known to be important to primary production, but the individual contributions of each of the different groups, or species (with Micromonas being one of those), aren't well known. To date, Micromonas strains have been described as belonging to a single species. Micromonas pusilla was reported as early as 1951 to be highly abundant in marine systems (Knight-Jones & Walne 1951). However, it is likely that the designation Micromonas pusilla harbors several different (cryptic) species (Slapeta et al. 2006, Worden 2006). Genome sequences from different strains of this organism will help to answer such questions.

Further reading:

Green evolution and dynamic adaptations revealed by genomes of the marine picoeukaryotes Micromonas. Science. 324(5924), 268-272 (2009). [pubmed]