Fine Read­ing: Houses Made by Pro­tists

by Elio

If there is a limit to what uni­cel­lu­lar or­gan­isms can do, you can't prove it by me. As Michael Hansell writes in an ar­ti­cle en­ti­tled Houses Made by Pro­tists, some sin­gle cells can make ex­ceed­ingly in­tri­cate struc­tures. Us­ing a Q&A for­mat, Hansell, who bears the in­ter­est­ing ti­tle of Pro­fes­sor of An­i­mal Ar­chi­tec­ture at the Uni­ver­sity of Glas­gow, dis­cusses ex­am­ples that defy the imag­i­na­tion.

Eu­g­lypha ro­tunda test show­ing some 120 ex­tremely reg­u­lar, over­lap­ping tiles that, prior to cell di­vi­sion, be­come arranged around the newly bud­ding or­gan­ism. A fur­ther 8 to 14 slightly thicker, toothed tiles sur­round the aper­ture. Source

Take for in­stance the pyra­mids of Egypt. They are made of huge lime­stone blocks of fos­sil shells of an­cient foraminifera (forams). These are true gi­ants among the pro­tists, mak­ing shells that reach 10 cm across. That's some amoeba! (The an­cient Greeks thought they were pet­ri­fied lentils. That's some lentils!) A lovely ac­count of this gi­gan­tism can be seen in a site called Pyra­mids, forams, and Red Sea reefs: Field notes from Lor­raine Casazza. Foraminifera are not the only ones to make houses; the so-called tes­tate amoe­bas do, too. ("Tes­tate" refers to tests, the name of such shells. A con­fus­ing term, that. Try giv­ing a test about pro­tist tests.)

A shell of Dif­flu­gia corona, a tes­tate amoeba. Source

These pro­tist ar­chi­tects have two con­struc­tion meth­ods at their dis­posal. One is for the or­gan­isms to in­gest par­ti­cles of sand, or in some cases to make in­sol­u­ble pro­tein struc­tures, and then spit them out to form the shell. Sev­eral hun­dred such grains may be re­quired for mak­ing one shell. This isn't all. The shells of Dif­flu­gia corona have a de­fined aper­ture, in some cases sur­rounded by a num­ber of "teeth" (in it­self a story for an­other day). The other method is to ap­pro­pri­ate pre­formed par­ti­cles and as­sem­ble them, much like a ma­son would make a wall with bricks. These may be struc­tures de­rived from other or­gan­isms, such as plates from the shells of mi­croal­gae or the spicules of sponges. Re­gard­less of the tech­nique used, the struc­tures of each species tend to have a dis­tinc­tive size and shape. For in­stance, cer­tain forams make mul­ti­cham­bered shells that re­sem­ble cham­bered Nau­tilus shells in minia­ture. How do they do it? It's likely that pseudopods come in handy in the con­struc­tion.

Fos­sil case of a trocham­minid foram con­structed from al­gal coc­col­iths (re­pro­duced with per­mis­sion of Erik Thom­sen). Source

Some of these house builders are enor­mously so­phis­ti­cated. Take the ex­tinct foram shown in the mi­cro­graph. Its shell con­sists of plates from the shell of a coc­col­ithophore alga. These pho­to­syn­the­siz­ers are ex­tremely abun­dant in the oceans to­day and were prob­a­bly avail­able in the early cre­ta­ceous. Here is the amaz­ing part: not only do these cells know how to col­lect the algae's plates and to arrange them in a geo­met­ric pat­tern, but they even trim their edges so that they are no longer cir­cu­lar but be­come hexag­o­nal, a shape that al­lows them to be packed in a closed geom­e­try. Just like ma­sons chip­ping away at bricks to make them fit bet­ter.

Houses? What's next?

Ref­er­ence

Hansell M (2011). Houses made by pro­tists. Cur­rent Bi­ol­ogy, 21 (13). DOI 10.1016/j.cub.2011.05.050