A Whiff of Tax­on­omy – Ra­di­o­laria

by Roberto

The men­tion of Ra­di­o­laria al­ways brings to mind Ernst Haeckel's as­ton­ish­ing hand-drawn ima­ges, com­piled in his clas­sic plate col­lect­ion Art Forms in Na­ture. I'm al­ways mes­mer­ized by the sym­metry and de­tail of his draw­ings of mi­cros­cop­ic ske­le­tons. Haec­kel's abi­li­ty to com­bine sci­ence and art is one as­pect of his re­mark­able ca­reer. His life pre­sents a ra­ther bi­zarre mix­ture of bril­liance and con­tro­ver­sy, very much wor­thy of a fu­ture post here. But when it comes to draw­ing and ca­ta­log­uing Ra­dio­la­ria, he is se­cond to none. He de­scribed near­ly 4000 dif­fer­ent or­gan­isms and grouped them as Ra­dio­la­ria. These abun­dant ma­rine mi­crobes, en­cased in si­li­ca shells, catch prey by ex­tend­ing parts of their cell bo­dy through holes in the shells. Many of them also ob­tain some of their ener­gy from pho­to­syn­the­tic sym­bionts such as zoo­xan­thel­lae. But re­la­ti­ve­ly lit­tle else is known about the bio­lo­gy of these beau­ti­ful ocean dwel­lers. In the end, what is best un­der­stood is their evo­lu­tio­na­ry his­to­ry as re­cord­ed in the ocean se­di­ments. The fos­sil re­cord shows them ap­pear­ing ear­ly in the Cam­bri­an, some 540 mil­lion years ago. As such, ra­dio­la­ri­an re­mains are used in geo­lo­gi­cal da­ting and in the de­ter­mi­na­tion of an­cient cli­mates.

Plate 91, de­pict­ing Ra­di­o­laria, from Ernst Haeckel's Art Forms in Na­ture. Source. Fron­tispiece: Light mi­croscopy im­age of a Ra­di­o­laria shell. Source

Who are the Ra­dio­la­ria? In the big scheme of things they are pro­tists, uni­cel­lu­lar eu­kar­yotes. With­in Eu­kar­ya, they be­long to the su­per­group Rhi­za­ria that also in­cludes Cer­co­zoa and Fo­ra­mi­ni­fe­ra, which Elio de­scribed in a prior post. As it turns out, mo­le­cu­lar phy­lo­gen­et­ic ana­lys­es now show that the or­gan­isms that Haec­kel grouped as Ra­dio­la­ria are po­ly­phy­le­tic, with some be­long­ing to the Cer­co­zoa. As one set of au­thors put it the ti­tle of a 2013 pa­per, this meant "a fare­well to Haec­kel's Ra­dio­la­ria." For any­one in­ter­est­ed in ex­plor­ing ra­dio­la­ri­an ta­xo­no­my in great de­tail, here is a good start­ing place.

What on Earth do these beau­ti­ful yet mys­te­ri­ous crea­tures do? In re­search­ing ma­teri­al for my re­cent post on the bio­lo­gi­cal pump (the trans­port of car­bon from the ocean sur­face to great depths and the ocean floor) I came across what I con­sider­ed a most sur­pris­ing find­ing. A pa­per with au­thors from six count­ries, with An­dres Gu­tier­rez-Ro­­dri­guez as first au­thor, de­scribes the mi­cro­bi­al com­pon­ents of the par­ti­cu­late or­gan­ic mat­ter (POM) that pas­sive­ly sinks through the wa­ter co­lumn to bring car­bon that is fixed at the sur­face in­to the ocean in­ter­ior. Their me­thod was to set traps to catch sink­ing POM and then ana­lyze the nu­cle­ic acids pre­sent to de­ter­mine mi­cro­bi­al com­mu­ni­ty com­po­si­tion. Sur­pri­sing­ly, Ra­dio­la­ria do­mi­nat­ed among the eu­kar­yotes pre­sent. Who would have gues­sed it? Si­li­ca en­closed mi­cro­bes bring­ing down most of the car­bon. We know so lit­tle about Ra­dio­la­ria that seem to be do­ing so much for Earth.