Fine Read­ing: Cnidar­i­ans and Di­nofla­gel­lates To­gether

by Merry

The very word in­fec­tion brings to mind the ar­rival of a nasty pathogen coun­tered by a host im­mune re­sponse, a bat­tle­ground strewn with the car­casses of the losers. But how do you de­fine in­fec­tion? Is not the be­gin­ning of an en­dosym­bio­sis, even a mu­tu­ally ben­e­fi­cial one, also an in­fec­tion? Corals, anemones, and other Cnidar­i­ans have been in­fected for mil­lions of years. They are among the nu­mer­ous ma­rine in­ver­te­brates that have es­tab­lished en­dosym­bioses with pho­to­syn­thetic part­ners, their part­ners be­ing di­nofla­gel­late al­gae of the genus Sym­bio­dinium (com­monly re­ferred to as zoox­an­thel­lae, or zoox for short). While some Cnidar­i­ans pass their en­dosym­bionts to their off­spring, oth­ers have to be "in­fected" anew each gen­er­a­tion. The in­tri­ca­cies of this re­la­tion­ship be­tween Cnidar­i­ans and their di­nofla­gel­lates is the sub­ject of an ex­haus­tive re­view (424 cited ref­er­ences!) by Davy, Alle­mand, and Weis. Here you'll find many ques­tions to pon­der, each pre­sented as a mini-re­view that in­cludes what is known,  the re­search that pro­vided those an­swers, and cur­rent hy­pothe­ses about the re­main­ing ques­tions. Here is a sam­pling of those ques­tions.

Zoox­an­thel­lae (brown­ish-green cells) in­side translu­cent coral polyps. CREDIT: © osf.co.uk. Source

Dur­ing the ini­tial es­tab­lish­ment of the sym­bio­sis, how does, for ex­am­ple, a coral host rec­og­nize the right sort of di­nofla­gel­late and re­ject the oth­ers? Af­ter be­ing phago­cy­tosed, how does the di­nofla­gel­late es­cape be­ing di­gested and in­stead man­age to con­vert a phago­some into a hos­pitable mem­brane-bounded com­part­ment. Corals pos­sess an in­nate im­mune sys­tem sim­i­lar to ver­te­brates, in­clud­ing genes for lectins, and, sure enough, gly­cans are found on the sur­face of the Sym­bio­dinium. So, does the coral mount an im­mune re­sponse against the "in­vader" and if so, how does the sym­biont counter that de­fense? As in other "in­fec­tions," MAMPs (mi­crobe-as­so­ci­ated mol­e­c­u­lar pat­terns) meet host PRRs (pat­tern recog­ni­tion re­cep­tors), but here recog­ni­tion of suit­able al­gae leads to a mu­tu­ally sat­is­fac­tory ac­com­mo­da­tion.

This an­i­mal could be lurk­ing in a pond near you: green hy­dras are com­mon in rivers and lakes. Their green hue is con­tributed by their Chlorella al­gal en­dosym­bionts. Di­vi­sion of the Chlorella is syn­chro­nous with that of the host cell. When a hy­dra re­pro­duces sex­u­ally, rather than by bud­ding, al­gal cells pass to the next gen­er­a­tion via the eggs. Source

But even the right part­ner has to be keep in line. When free-liv­ing, the di­nofla­gel­lates di­vide far more rapidly than when in hos­pite.How does the Cnidar­ian host reg­u­late the num­ber of zoox per host cell? Is it by re­strict­ing nu­tri­ents such as ni­tro­gen? Or by mak­ing the zoox re­lin­quish to them most of their pho­to­syn­thate? Are there, as has been pro­posed, "host re­lease fac­tors" that per­suade the res­i­dents to pro­duce more pho­to­syn­thate and re­lease more into the host cy­to­plasm? No­table among the Cnidar­i­ans, it is the reef-build­ing corals with their zoox that are able to thrive in nu­tri­ent-poor wa­ters. How do they take up nu­tri­ents ef­fi­ciently, reuse, and re­cy­cle them? And, for that mat­ter, why are zoox es­sen­tial for reef build­ing? This mat­ters a great deal. As ocean acid­i­fi­ca­tion threat­ens to tip the bal­ance from skele­ton de­po­si­tion to­ward dis­so­lu­tion, we badly need a greater un­der­stand­ing of the role of the en­dosym­bionts in cal­ci­fi­ca­tion.

As this re­view shows, re­search has been ac­tive in this field, pro­vid­ing enough an­swers, or par­tial an­swers, to whet the ap­petite. New techniques—gene ex­pres­sion pro­fil­ing, new imag­ing tech­niques, and metabo­lite pro­fil­ing, among others—promise rapid ad­vances. Per­haps you want to par­tic­i­pate. Of course, you might not have a coral reef avail­able, but hum­bler or­gan­isms also of­fer op­por­tu­ni­ties. Con­sider the lowly fresh­wa­ter green Hy­dra, also a Cnidar­ian. As the au­thors point out, the Hy­dra-Chlorella sym­bio­sis is rel­a­tively well stud­ied and has the po­ten­tial to sub­stan­tially in­form our un­der­stand­ing of the cnidar­ian-di­nofla­gel­late sym­bio­sis, es­pe­cially with re­gard to host-sym­biont recog­ni­tion and phago­cy­to­sis. If these ques­tion in­trigue you, jump in, and start cul­ti­vat­ing a Cnidar­ian in your own back­yard.