Os­tre­o­coc­cus tauri, the Small­est Known Eu­kary­ote

Pic­tures Con­sid­ered #24

by Elio

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.

Left: To­mo­graphic slice (2D sec­tion com­pu­ta­tion­ally ex­tracted from the 3D to­mo­gram) from an O. tauri cell. La­bels: nu­cleus N, chloro­plast C, Golgi body G, and mi­to­chon­drion M. Right: 3‑D seg­men­ta­tion of this cell. This re­mark­able free-liv­ing cell has evolved to carry out all the con­served func­tions in a tiny and com­pact vol­ume. Source

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.