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Sym­bionts without bor­ders: Bac­terial part­ners travel the world

Jul 12, 2021
Sym­bi­otic bac­teria liv­ing in the gills of cer­tain clams are true cos­mo­pol­it­ans.

This pandemic year has seen us confined to our homes and restricted from travelling the world. Not so for some microscopic bacteria in the ocean: Throughout the globe, they partner up with clams from the family Lucinidae, which live unseen in the sand beneath the shimmering blue waters of coastal habitats. This partnership is the clams’ passport to their extensive global reach. The bacteria can also travel a long way. According to research by scientists from the Max Planck Institute for Marine Microbiology in Bremen and the University of Vienna now published in PNAS, the bacterial symbionts living in lucinid gills can travel the world without borders.

Lucinid on the lookout: Lucinids are the most species rich and widely distributed family of marine bivalves hosting bacterial endosymbionts. In this picture, a large specimen of Ctena imbricatula is checking out its environment with its foot that it can enlarge ten times its body size. (© Laetitia Wilkins)
Lucinid on the lookout: Lucinids are the most species rich and widely distributed family of marine bivalves hosting bacterial endosymbionts. In this picture, a large specimen of Ctena imbricatula is checking out its environment with its foot that it can enlarge ten times its body size. (© Laetitia Wilkins)

The Lu­cin­idae fam­ily, lu­cin­ids for short, com­prises ap­prox­im­ately 500 liv­ing spe­cies of bi­valves. They are at least 400 mil­lion years old, ac­cord­ing to fossil re­cords, and have man­aged to col­on­ize a wide vari­ety of hab­it­ats, from beau­ti­ful beaches to the abyssal depths un­touched by the sun over a kilo­meter be­low the sea sur­face. Their abil­ity to thrive in a wide vari­ety of hab­it­ats is made pos­sible by their ‘part­ner in crime’, a sul­fur-ox­id­iz­ing bac­terial sym­biont that util­izes hy­dro­gen sulf­ide, bet­ter known as ‘rot­ten egg gas’, as an en­ergy source to power primary pro­duc­tion. This pro­cess is not un­like pho­to­syn­thesis used by plants, yet not de­pend­ent on sun­light, and gen­er­ates enough sug­ars to feed both the sym­biont and the lu­cin­ids them­selves.

Striking up partnerships from near or far

Find­ing a suit­able part­ner out in the wild is a mat­ter of life and death for lu­cin­ids. They have to pick up their bac­terial part­ners at a very early life stage when they settle in the sed­i­ment after their lar­val stage. From this time on, they rely on their bac­terial sym­bionts for nu­tri­tion. However, bac­terial cells are min­is­cule and the oceans are awash with a mul­ti­tude of pos­sible can­did­ates. Typ­ic­ally, an­im­als that rely so heav­ily on bac­teria are ex­pec­ted to strike up part­ner­ships with local res­id­ents. These mi­crobes are likely to work best un­der the unique con­di­tions of their local hab­it­ats. A new study based on meta­ge­n­omic ana­lyses of sym­bi­otic bac­teria in lu­cin­ids now re­veals that this is not al­ways the case: Some bac­terial sym­bionts travel the globe and are true cos­mo­pol­it­ans. 

Fluorescence microscopy reveals that lucinid gills are packed with symbionts. Lucinids host them in specialized cells called bacteriocytes. Bacterial symbionts are labeled in green and magenta, host nuclei in gold. (© Lukas Leibrecht)
Fluorescence microscopy reveals that lucinid gills are packed with symbionts. Lucinids host them in specialized cells called bacteriocytes. Bacterial symbionts are labeled in green and magenta, host nuclei in gold. (© Lukas Leibrecht)

Globally distributed symbionts

Using state-of-art DNA-sequencing and genome assembling, we dis­covered that a single bac­terial sym­biont spe­cies was the most abund­ant sym­biont in eight lu­cinid spe­cies span­ning three oceans  – the At­lantic, Pa­cific and In­dian Oceans – across the trop­ics of both hemi­spheres,” said Laetitia Wilkins from the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy in Bre­men, Ger­many, shared first au­thor of the pub­lic­a­tion to­gether with Jay Os­vatic from the Uni­versity of Vi­enna, Aus­tria. “These sym­bionts are vir­tu­ally all over the place.” No other known sym­biont is so suc­cess­ful at dis­persal and es­tab­lish­ing sym­bi­oses with lu­cin­ids, the re­search­ers re­port. They named it Candidatus Thi­od­iazo­tropha taylori – ”to ac­know­ledge the wis­dom of John Taylor from the Nat­ural His­tory Mu­seum in Lon­don, who has de­voted 25 years of his life to study­ing lu­cinid bio­logy and tax­onomy”, as Os­vatic poin­ted out.

“This un­ex­pec­ted find­ing chal­lenges pre­vi­ous con­cepts that sym­bionts are ac­quired loc­ally. It sug­gests that lu­cinid sym­bionts are much more mo­bile”, Os­vatic ad­ded. The re­mark­able flex­ib­il­ity in this part­ner­ship is ad­vant­age­ous to both host and sym­biont, as it in­creases the like­li­hood of loc­at­ing a com­pat­ible part­ner across di­verse hab­it­ats all over the globe. Prior to this study, lu­cinid re­search has mainly been car­ried out in eas­ily ac­cess­ible loc­a­tions. Now for the first time the team around Wilkins and Os­vatic presents an ex­pan­ded, global data­set that has led to and will con­tinue to fa­cil­it­ate new dis­cov­er­ies and show how dis­tant hab­it­ats might be con­nec­ted.

Scientists collaborating to find collaborating organisms

Just like the re­la­tion­ships between sym­bionts and lu­cinid clams, this dis­cov­ery would not have been pos­sible without the sci­ent­ists reach­ing out and form­ing col­lab­or­a­tions across the world. “Our con­tacts (and now friends) across the world have given us ac­cess to an un­pre­ced­en­ted di­versity of lucinds, both dir­ect from the beaches and from mu­seums across the world”, said Be­ne­dict Yuen from the Uni­versity of Vi­enna, senior au­thor of the pa­per. “We were given ac­cess to a wide vari­ety of lu­cinid samples at the Nat­ural His­tory Mu­seum in Lon­don through John Taylor. Samples were also col­lec­ted per­son­ally by our team and col­lab­or­at­ors Mat­thieu Leray in Panama, Yolanda Ca­macho in Costa Rica, Olivier Gros in Guade­loupe and Jan A. van Gils in Maur­it­ania.”

This is where the investigated clams like to live: Lucinid habitat characterized by sandy patches within seagrass (Thalassia testudinum) beds in Bocas del Toro, Panama. (© Laetitia Wilkins)
This is where the investigated clams like to live: Lucinid habitat characterized by sandy patches within seagrass (Thalassia testudinum) beds in Bocas del Toro, Panama. (© Laetitia Wilkins)

Also discovered: Two new species in cosy togetherness

Moreover, the ex­tens­ive data col­lec­tion of Wilkins, Os­vatic and their team res­ul­ted in the dis­cov­ery of two new lu­cinid sym­bionts, which have now been de­scribed and named after Miriam Weber and Chris­tian Lott, both former re­search­ers from the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy in Bre­men. These sym­bionts – now known as Thiodioazotropha weberae and lotti – are found in the clam Loripes orbiculatus on the Italian is­land of Elba, where the sym­bionts peace­fully co-ex­ist in the gills of the same host. “Be­fore ge­n­omic ana­lyses were used, it was as­sumed that each clam hosts only one spe­cies of sym­bionts”, Wilkins ex­plained. “However, many clams on Elba har­bor two sym­biont spe­cies. Miriam and Chris­tian dis­covered this clam pop­u­la­tion in the bay of Fetovaia and it is thanks to them that we could amass a very power­ful data­set on this sym­bi­osis.”


Next, the re­search­ers want to find out how the sym­bionts travel. “They leave their bi­valve home to tra­verse the globe”, ad­ded co-senior au­thor Jill­ian Petersen. “Both be­ne­fi­cial sym­bionts such as Candidatus T. taylori but also patho­gens can dis­perse in the en­vir­on­ment, but we usu­ally don’t know how.”

Ori­ginal pub­lic­a­tion:

Par­ti­cip­at­ing in­sti­tu­tions:

Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy, Ger­many

Uni­versity of Vi­enna, Aus­tria

Uni­versity of Cali­for­nia, Davis, USA

Smith­so­nian Trop­ical Re­search In­sti­tute, Re­pub­lic of Panamá

Uni­ver­sidad de Costa Rica, Costa Rica

Uni­versité des An­ti­lles, Guade­loupe

Royal Neth­er­lands In­sti­tute for Sea Re­search, The Neth­er­lands

Please dir­ect your quer­ies to:

Max Planck Research Group Leader

Dr. Laetitia Wilkins

MPI for Marine Microbiology
Celsiusstr. 1
D-28359 Bremen
Germany

Room: 

2506

Phone: 

+49 421 2028-8750

Dr. Laetitia Wilkins

Head of Press & Communications

Dr. Fanni Aspetsberger

MPI for Marine Microbiology
Celsiusstr. 1
D-28359 Bremen
Germany

Room: 

1345

Phone: 

+49 421 2028-9470

Dr. Fanni Aspetsberger

More pic­tures:

Laetitia Wilkins and Benedict Yuen in Cahuita, Costa Rica, dissecting freshly collected clams. (© Laetitia Wilkins)
Laetitia Wilkins and Benedict Yuen in Cahuita, Costa Rica, dissecting freshly collected clams. (© Laetitia Wilkins)
Laetitia Wilkins and Benedict Yuen sampling clams in Bocas del Toro, Panama in a Mangrove habitat. (© Jonathan Eisen)
Laetitia Wilkins and Benedict Yuen sampling clams in Bocas del Toro, Panama in a Mangrove habitat. (© Jonathan Eisen)
John Taylor at the London at the Natural History Museum is showing Jay Osvatic a shell of Meganodontia acetabulum, the largest living lucinid. (© Benedict Yuen)
John Taylor at the London at the Natural History Museum is showing Jay Osvatic a shell of Meganodontia acetabulum, the largest living lucinid. (© Benedict Yuen)
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