Ostreococcus tauri is about as small as a free-living eukaryote can get. Its disk-shaped cells are as small as 0.8 µm, which makes it possible to examine them by electron cryotomography. As reported in a paper from the Jensen lab, there is barely enough room in these cells for the nucleus, one mitochondrion, one chloroplast, Golgi body, plus a few other cell constituents. This small size does not mean that it lacks in genes as its DNA is ~12,000,000 base pairs in size, divided among 20 chromosomes.
O. tauri is a green alga that, in some areas, is a prominent part of the picoplankton, the biome of entities 0.2 to 2 µm in size that is usually dominated by prokaryotes. It can be obtained in pure cultures, where it grows quite fast (up to 7 doublings per 24 hours). Its division can be partly synchronized by shifts between light and darkness. And it auto-fluoresces because of its chlorophyll content. A useful model system for many types of studies, it has been previously featured in this blog.