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© Hydra Marine Sciences GmbH | A researcher at the Max Planck Institute for Marine Microbiology taking samples in seagrass beds in the Mediterranean Sea. The measuring device determines the oxygen content in the seafloor.

A nat­ural CO2-sink thanks to sym­bi­otic bac­teria

Like many land plants, seagrasses live in sym­bi­osis with ni­tro­gen-fix­ing bac­teria

Seagrasses cover large swathes of shallow coastal seas, where they provide a vital habitat. They also remove large amounts of carbon dioxide (CO2) from the atmosphere and store it in the ecosystem. However, seagrasses need nutrients to thrive, particularly nitrogen. Up to now, researchers have assumed that the plants take up the nitrogen primarily from the surrounding seawater and sediment. However, in many of the regions where seagrasses are most successful there is little nitrogen to be found. Researchers of the Max Planck Institute for Marine Microbiology in Bremen now show that seagrass in the Mediterranean Sea lives in symbiosis with bacteria that reside in their roots and provide the nitrogen necessary for growth. Such symbioses were previously only known from land plants. The study was published in the journal Nature.

Seagrasses are wide­spread in shal­low coastal re­gions of both tem­per­ate and trop­ical seas, cov­er­ing up to 600,000 square kilo­met­ers, which is roughly the area of France. They form the basis of the en­tire eco­sys­tem, which is home to nu­mer­ous or­gan­isms, some of them en­dangered spe­cies such as turtles, seahorses and manatees, and nurs­ery ground for many eco­nom­ic­ally im­port­ant fish spe­cies. Moreover, seagrasses pro­tect coast­lines from erosion by storm surges and se­quester mil­lions of tons of car­bon di­ox­ide every year, which is stored in the eco­sys­tem as so-called “blue car­bon” for long peri­ods of time.

Lush life despite a lack of nutrients

The hab­itat of many seagrasses is poor in nu­tri­ents, such as ni­tro­gen, for much of the year. Al­though ni­tro­gen is abund­ant in the sea in its ele­mental form (N2), seagrasses can­not use it in this form. How can the plants still thrive? It is thanks to their now dis­covered mi­cro­scopic part­ners: Bac­terial sym­bionts liv­ing within the plants roots that con­vert N2 gas into a form that the plants can use. Wiebke Mohr and her col­leagues from the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy in Bre­men, Ger­many, Hy­dra Mar­ine Sci­ences in Bühl, Ger­many, and the Swiss Wa­ter Re­search In­sti­tute Eawag now de­scribe how this in­tim­ate re­la­tion­ship between seagrass and bac­teria works.

Harmony in the roots

“It was as­sumed that the so-called fixed-ni­tro­gen for the seagrasses comes from bac­teria that live around their roots in the sea­floor,” Mohr ex­plains. “We now show that the re­la­tion­ship is much closer: The bac­teria live inside the roots of the seagrass. This is the first time that such an in­tim­ate sym­bi­osis has been shown in seagrasses. It was pre­vi­ously only known from land plants, es­pe­cially ag­ri­cul­tur­ally im­port­ant spe­cies such as legumes, wheat and sugar cane.” These, too, have sym­bi­otic bac­teria, to which they sup­ply car­bo­hydrates and other nu­tri­ents in re­turn for fixed ni­tro­gen. A very sim­ilar ex­change of meta­bolic products also oc­curs between the seagrass and its sym­biont.

© Wiebke Mohr /Max Planck Institute for Marine Microbiology
© Wiebke Mohr /Max Planck Institute for Marine Microbiology | A part of Fetovaia Bay, in which most samples of this study were retrieved.

The bac­teria that live in the seagrass roots are a new dis­cov­ery. Mohr and her team named them Celerinatantimonas neptuna, after their host, the nep­tune grass (Posidonia). Re­l­at­ives of C. neptuna have pre­vi­ously been found in as­so­ci­ation with sea­weeds. “When the seagrasses moved from land to sea about 100 mil­lion years ago, they prob­ably ad­op­ted the bac­teria from the sea­weeds,” Mohr spec­u­lates. “They vir­tu­ally copied the sys­tem that was highly suc­cess­ful on land and then, in or­der to sur­vive in the nu­tri­ent-poor sea­wa­ter, ac­quired a mar­ine sym­biont.” The cur­rent study looked at seagrasses of the genus Posidonia in the Medi­ter­ranean Sea. However, such sym­bi­oses may also oc­cur else­where. “Ge­netic ana­lyses sug­gest that sim­ilar sym­bi­oses also ex­ist on trop­ical seagrasses and in salt marshes,” says Mohr. “This way, these flower­ing plants man­age to col­on­ize a wide vari­ety of seem­ingly nu­tri­ent-poor hab­it­ats, both in the wa­ter and on land.”

Going with the seasons

As the sea­sons change, the amount of nu­tri­ents present in coastal wa­ter var­ies. In winter and spring, the nu­tri­ents present in the wa­ter and sed­i­ment seem suf­fi­cient for the seagrasses. “At that time, we do find scattered sym­bionts in the roots of the plants, but they are prob­ably not very act­ive,” says Mohr. In sum­mer, when sun­light in­creases and more and more al­gae grow and con­sume the few avail­able nu­tri­ents, ni­tro­gen quickly be­comes scarce. Then the sym­bionts take over. They dir­ectly sup­ply the seagrasses with the ni­tro­gen they need. This is how seagrasses can reach their largest growth in sum­mer, when nu­tri­ents are most scarce in the en­vir­on­ment.

© Daniela Tienken/Soeren Ahmerkamp /Max Planck Institute for Marine Microbiology
© Daniela Tienken/Soeren Ahmerkamp /Max Planck Institute for Marine Microbiology | The symbiosis under the microscope: On the left a cross-section through a seagrass root, on the right a fluorescence image of the bacteria (in pink) inside the seagrass root.

Many different methods for a clear picture

The present study bridges the en­tire eco­sys­tem, from seagrass pro­ductiv­ity to the sym­bionts that live in their roots and ul­ti­mately fuel the sys­tem. To ac­com­plish this, the re­search­ers used a vari­ety of dif­fer­ent meth­ods to un­der­stand the sym­bi­osis as fully as pos­sible: Oxy­gen meas­ure­ments car­ried out in the wa­ters of the Medi­ter­ranean Sea re­vealed the pro­ductiv­ity of the seagrass meadow. Mi­cro­scopy tech­niques, in which in­di­vidual bac­terial spe­cies are color-labeled (known as FISH), helped to visu­al­ize the bac­teria in and between the root cells of the seagrass. In the NanoSIMS, a state-of-the-art mass spec­tro­meter, they showed the activ­ity of the in­di­vidual bac­teria. Ge­n­omic and tran­scrip­tomic ana­lyses re­vealed which genes are prob­ably par­tic­u­larly im­port­ant for the in­ter­ac­tion and that these path­ways are heav­ily used. As a res­ult, the re­search­ers suc­ceeded in provid­ing a sound and de­tailed de­scrip­tion of this amaz­ing col­lab­or­a­tion. “Our next step is to study these new bac­teria in more de­tail,” says Mohr. “We want to isol­ate them in the labor­at­ory to fur­ther in­vest­ig­ate how the sym­bi­osis works and how it de­veloped. It will cer­tainly also be ex­cit­ing to search for com­par­able sys­tems in other re­gions and hab­it­ats.”

  • Wiebke Mohr, Nad­ine Lehnen, So­eren Ah­merkamp, Han­nah K. Marchant, Jon S. Graf, Bernhard Tschitschko, Pelin Yil­maz, Sten Littmann, Har­ald Gruber-Vodicka, Nikolaus Leisch, Miriam Weber, Chris­tian Lott, Carsten J. Schubert, Jana Milucka, Mar­cel M. M. Kuypers (2021): Ter­restrial-type ni­tro­gen-fix­ing sym­bi­osis between seagrass and a mar­ine bac­terium. Nature (2021) DOI: 10.1038/s41586-021-04063-4

Max Planck Institute for Marine Microbiology 2021

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