A Worm­ful of Bugs

by Elio

Let's start out with a lit­tle quiz. What ex­am­ples can you name of en­dosym­bi­otic bac­te­ria so tightly packed that they're nearly wall-to-wall within cells of their host? If you said legume root nod­ules, you can claim a prize. (Ask some­one else for it, as we don't have any.) You would get bonus points if you also men­tioned the bac­te­ria-filled bac­te­ri­o­cytes of cer­tain in­sects or the cel­e­brated gi­ant tube worms found near the deep sea black smok­ers. By the way, the root nod­ule sym­bioses, though not manda­tory, are highly ben­e­fi­cial to the host, but the other two are oblig­a­tory.

Para­catenula cf. poly­hym­nia. Light mi­cro­scope micro­graph. The light por­tion is the 'head' or ros­trum, the dark por­tion the tropho­some. Source

Here is an­other re­cently de­scribed ex­am­ple to add to this thought-pro­vok­ing reper­toire. In the sandy bot­toms be­neath shal­low ocean wa­ters one finds worms that have nei­ther a mouth nor a gut. Al­though such ar­eas seem sparse in life, in fact they're home to a rather rich biota. Mi­crobes, both pro– and eu­kary­otic, are present in abun­dance and so are an­i­mals, such as the worms we're talk­ing about here. So, how can an­i­mals be mouth­less and gut­less? Tak­ing a page from the gi­ant tube worms, a group of in­ves­ti­ga­tors found that in­deed these worms are filled with a sack of bac­te­ria. As in the tube worms, these bac­te­ria pro­vide food for the host by ox­i­diz­ing H₂S us­ing oxy­gen as the elec­tron ac­cep­tor. This kind of bio­chem­i­cal en­dow­ment is wide­spread in na­ture and is found in groups as di­verse as cil­i­ates, arthro­pods, mol­lusks, and other kinds of worms. There is even a gi­ant ar­chaeon that is cov­ered with sul­fur ox­i­diz­ing bac­te­ria. De­spite this huge va­ri­ety of hosts and the many other sites where sul­fur me­tab­o­lism takes place in na­ture, the great ma­jor­ity of the bac­te­r­ial sul­fide-re­duc­ers are ei­ther Gamma- or Ep­silon­pro­teobac­te­ria. Here, the sym­bionts are Al­phas. The Al­phapro­teobac­te­ria in­clude oth­ers prone to in­tra­cel­lu­lar life such as the ni­tro­gen-fix­ing rhi­zo­bia and the rick­ettsiae (from which the mi­to­chon­dria ap­pear to be de­rived). Should we ex­pect more ex­am­ples of sym­bionts in this class?

EM mi­cro­graph of a com­plete cross-sec­tion of Paraca­tenula cf. poly­hym­nia. Source

The worms in ques­tion, Para­catenula galateia, be­long to the platy­helminths or flat­worms, but in­side they don't look like your usual fluke or tape­worm. Their bac­te­ria-filled sac, called the tropho­some, ac­counts for over 90% of the worm's body and has cells called bac­te­ri­o­cytes that are filled with bac­te­ria. In some species, the bac­te­ria make up close to 50% of the to­tal vol­ume of the worm. By the way, the bac­te­ria are quite big—5 to 8 μm in di­am­e­ter. In­ci­den­tally, such large sizes are also seen in some in­sect bac­te­r­ial en­dosym­bionts.

So, how did these re­searchers find out what the bac­te­ria do for their host? The first hint that they ox­i­dize sul­fur came from just look­ing at the worms. They are full of white gran­ules. By phys­i­cal analy­sis, these gran­ules in­deed con­sist of el­e­men­tal sul­fur and make up 5–19% of the tis­sue mass. More hints: the symbiont's genome con­tains the gene for a sub­unit of DsrAB, the dis­sim­i­la­tory sul­fite re­duc­tase, a key en­zyme that works in re­verse to par­tic­i­pate in sul­fur ox­i­da­tion. The bac­te­ria also carry the gene for AprA, a sub­unit of adenosine‑5'-phosphosulfate re­duc­tase, an­other en­zyme in sul­fur en­ergy me­tab­o­lism. And they en­code Ru­bisCO, the main player in au­totrophic car­bon fix­a­tion. The sul­fur gran­ules and the pres­ence of these en­zymes strongly sug­gest that the bac­te­ria can carry out a sul­fide ox­i­diz­ing, en­ergy-pro­duc­ing me­tab­o­lism. And, just like in the tube worms, this en­ergy-gen­er­at­ing mech­a­nism ul­ti­mately al­lows the bac­te­ria to syn­the­size the meta­bolic build­ing blocks that pro­vide the host with needed or­ganic nu­tri­ents. Lit­tle is known about how H₂S and oxy­gen en­ter the tropho­some. The worms not hav­ing a res­pi­ra­tory or cir­cu­la­tory sys­tem, this pre­sum­ably oc­curs by dif­fu­sion through the worm's body wall.

Me­tab­o­lism that de­pends on the ox­i­da­tion of sul­fides us­ing oxy­gen in ma­rine en­vi­ron­ments poses a prob­lem. H₂S is found in the sed­i­ments, oxy­gen in the wa­ter col­umn. Nei­ther is com­monly abun­dant in the other's realm. The worms in the rifts do not have this prob­lem be­cause they live near the black smok­ers that spew large amounts of sul­fides. How about these worms? The au­thors have this to of­fer: By mi­grat­ing through the re­dox po­ten­tial gra­di­ent in the up­per­most 5- to 15-cm sed­i­ment layer, mil­lime­ter-sized worms can sup­ply chemoau­totrophic sym­bi­otic bac­te­ria al­ter­nately with spa­tially sep­a­rated elec­tron donors and ac­cep­tors such as sul­fide and oxy­gen, as has been de­scribed for Ne­ma­toda and Oligochaeta. They may even move up and down, in the style of some sheathed bac­te­ria we have dis­cussed be­fore.

Well worth men­tion­ing is the evo­lu­tion of host and sym­biont. If you place the den­dro­grams for the bac­te­ria (based on 16 S rRNA ) and that of the var­i­ous worms species stud­ied (based on 18S and 28S rRNA) side by side, they line up most sat­is­fac­to­rily. This is known as co­clado­ge­n­e­sis, and it sug­gests that the worms had a com­mon an­ces­tor who ac­quired a bac­te­r­ial prog­en­i­tor, and that the two fur­ther evolved to­gether, each one chang­ing as the other did. The au­thors posit that the orig­i­nal pair­ing took place 500 to 620 mil­lion years ago. If so, this would be the old­est known bac­te­r­ial-meta­zoan sym­bio­sis.

Co­clado­ge­n­e­sis be­tween Para­catenula and Can­di­da­tus Riege­ria. Tan­gle­gram of strict con­sen­sus clado­grams of four re­con­struc­tion meth­ods for both sym­biont 16S rRNA and host con­cate­nated 18S and 28S rRNA. No con­flict­ing nodes are sta­tis­ti­cally sup­ported in the re­sults of the four phy­lo­ge­netic re­con­struc­tion algo­rithms, in­di­cat­ing close co­evo­lu­tion be­tween the part­ners. Source

The sym­bionts have ben given a name, Can­di­da­tus Riege­ria galateiae. The genus name is in honor of the late zo­ol­o­gist Rein­hard Rieger, who de­scribed the host genus. (I'll re­mind you of the prac­tice of bac­te­r­ial tax­on­o­mists not to al­low bac­te­ria into the guild un­less they have been cul­ti­vated. Can­di­da­tus means just that. I will have more to say about this some­time later.)

The more ex­am­ples we have of sym­bioses in­volv­ing mi­crobes, the more our juices start run­ning. They are vivid ex­am­ples of in­ge­nious and in­tri­cate chore­o­gra­phies be­tween dif­fer­ent or­gan­isms, all lead­ing to the con­clu­sion that no op­por­tu­nity for evo­lu­tion is left un­ex­plored.

Ref­er­ences

Gru­ber-Vod­icka HR, Dirks U, Leisch N, Baranyi C, Stoecker K, Bul­gheresi S, Heindl NR, Horn M, Lott C, Loy A, Wag­ner M, Ott J. (2011). Para­catenula, an an­cient sym­bio­sis be­tween thiotrophic Al­phapro­teobac­te­ria and catenulid flat­worms. Pro­ceed­ings of the Na­tional Acad­emy of Sci­ences of the United States of Amer­ica, 108 (29), 12078−12083. PMID 21709249

Leisch N, Dirks U, Gru­ber-Vod­icka HR, Schmid M, Ster­rer W, Ott JA. (2011). Mi­croanatomy of the tropho­some re­gion of Para­catenula cf. poly­hym­nia (Catenul­ida, Platy­helminthes) and its in­tra­cel­lu­lar sym­bionts. Zoomor­phol­ogy, 130 (4), 261−271. PMID 22131640