Pico Who?

by Elio

You may have heard that the small­est free-liv­ing eukar­yote is Os­tre­o­coc­cus tauri. So they say. O. tauri is in­deed a tiny green alga. It is en­dowed with just one chloro­plast, one mi­to­chon­drion, a Golgi body, and not much cyto­plasm. Some­times it is only 1.0 μm in di­am­e­ter – yes, smaller even than quite a few prokary­otes. But it is not alone in this re­spect. In the oceans and else­where, one can find lots of other very small eu­kary­otes.

An un­named French in­ves­ti­ga­tor ex­am­ines a cul­ture of O. tauri. Source: Le Jour­nal du CNRS

Col­lec­tively they are known as the pi­coeukary­otes; when com­bined with bac­te­ria, they make up the ma­rine pico­plankton (that is, cells 0.2–2 μm in di­am­e­ter). Al­though most pi­coeukary­otes look pretty much alike, they en­com­pass a very di­verse group within that catch-all cat­e­gory, the pro­tists. By and large, they have been known for only five or six years, so this is news to many of us. Big news in fact, be­cause they are present in num­bers rang­ing be­tween 10³ and 10⁴ cells/ml of sea­wa­ter. A rough cal­cu­la­tion says that their cu­mu­la­tive weight in the ocean (us­ing around 10²⁴ ml as a guess for the vol­ume of the likely habi­tats) amounts to about 10⁹ met­ric tons (that's the weight of some ten mil­lion blue whales). Say we're wrong by a cou­ple of or­ders of mag­ni­tude, there are still lots of them. This is also the es­ti­mated weight of all ma­rine phages, which are some 1000 times more abun­dant but also about 1000 times smaller than the "pi­cos."

Elec­tron cry­oto­mo­graphic re­con­struc­tion of an O. tauri cell. n=nucleus; c=chloro­plast; p=peroxisome; er=endoplasmic re­ticulum; purple=mitochondrion. Source

What do these minia­tur­ized eu­kary­otes do? The ma­rine pi­coeukary­otes seem to be about equally di­vided be­tween pho­totrophs and het­erotrophs. The for­mer con­tribute much of the to­tal oceanic pho­to­syn­the­sis, ap­parently out­do­ing the most abun­dant pho­totrophic cyanobac­te­ria, Syne­chococ­cus and Prochloro­coc­cus. In a study of cen­tral North At­lantic sur­face wa­ters, only 10% of the sur­face pi­coplank­ton con­sisted of eu­kary­otes. But be­ing larger than bac­te­ria, they ac­counted for 60% of the bio­mass and 68% of the pho­to­syn­thetic pro­duc­tion. Eco­log­i­cally speak­ing, cell size sug­gests the level a uni­cellular or­gan­ism oc­cu­pies in the food chain. Given that phago­cytic or­gan­isms tend to prey on those smaller than them­selves, pi­coeukary­otes would be found near the base of food webs.

Some pi­coeukary­otes, such as Mi­cromonas pusilla, can be grown in pure cul­ture, but so far most can­not. Even when they can­not be cul­tured, we know they are there thanks to flu­o­res­cence and elec­tron mi­croscopy, flow cy­tom­e­try, and metage­nomic analy­sis. Sev­eral of these or­gan­isms ap­peared ideal for use as model sys­tems and thus are be­ing in­ten­sively in­ves­ti­gated. O. tauri is one of those whose genomes have al­ready been se­quenced. Be­fit­ting its size, it has a small and very com­pact genome. Its 8000 or so genes make up a hefty 82% of its 12.5 Mb genome. To date, O. tauri also has the dis­tinc­tion of be­ing the only eu­kary­otic cell that has been ob­served in its en­tirety by elec­tron cry­oto­mog­ra­phy.

Epi­flu­o­res­cence mi­croscopy of Cyani­dio­schyzon mero­lae 10D. Red = auto-flu­o­r­es­cence due to chloro­phyll; blue=DAPI-stain­ing of DNA. Source

We have pre­vi­ously men­tioned Cyani­dioschy­zon mero­lae in these pages. This has been the or­gan­ism of choice for study­ing how sin­gle or­ganelles in a cell are repli­cated and then ac­cu­rately dis­trib­uted to the daugh­ter cells upon cell di­vi­sion. It turns out that its sin­gle mi­to­chon­drion be­comes at­tached to the mi­totic spin­dle! An­other star­tling find­ing is that some of its tRNA genes are cir­cu­larly per­muted, as are the genomes of some phages. But C. me­rolae does it dif­fer­ently, their halves be­ing in a back­ward ori­en­ta­tion with the 3' half of the gene be­ing up­stream of the 5' one.

Pi­coeukary­otes are among the most abun­dant of our fel­low trav­el­ers on this planet. They de­serve our at­ten­tion.