Many cooks spoil the broth? Deep-sea mussels follow a different principle. They form symbioses with an unexpectedly large number of bacterial strains. Researchers at the Max Planck Institute for Marine Microbiology in Bremen and the University of Vienna suspect that what at first glance seems to contradict previous assumptions in evolutionary biology could actually be a widespread principle.

Bacteria provide nutrients

On several research trips to black smokers, hydrothermal vents of the deep sea, the scientists have collected Bathymodiolus mussels, which are distant relatives of the blue mussels. The biologists then analyzed which bacteria live in the gills of the mussels and sequenced their genomes. These microorganisms live in symbiosis with the mussels: While the bacteria, for example, convert substances that the mussels cannot use into valuable nutrients for them, the mussels offer the bacteria a safe habitat within the nutrient-rich hot springs.

Pre­pared for all even­tu­al­it­ies

The researchers had expected to find one or two symbiotic strains in the gills. In the journal "Nature Microbiology", however, they reported that up to 16 symbiotic bacterial strains live in the gills of each mussel. They ful­fill dif­fer­ent func­tions, help with dif­fer­ent meta­bolic con­ver­sions and have dif­fer­ent abil­it­ies. “Dif­fer­ent sym­bionts can, for ex­ample, use dif­fer­ent sub­stances and en­ergy sources from the sur­round­ing wa­ter to feed the mus­sel,” ex­plains Max Planck researcher Rebecca An­sorge. Oth­ers are par­tic­u­larly res­ist­ant to vir­uses or para­sites. Depending on the environmental conditions, one strain can dominate the other. The mussel can therefore adapt quickly, even if the conditions in the black smoker change or the mussel changes its location.

Special form of symbiosis

The large number of symbionts surprised the researchers above all because symbiotic bacterial strains usually compete for the nutrients supplied by the partner. However, since the mussels apparently only provide the habitat and the bacteria feed on the nutrient-rich surrounding water, the different strains can coexist, even if the conditions for individual strains are less favorable.

"Next, we want to in­vest­ig­ate whether this di­versity also ex­ists in other deep-sea sym­bi­oses, for ex­ample in sponges or clams," explains Ansorge. "We also want to ex­am­ine if our ob­ser­va­tions are typ­ical for sym­bi­oses or if they also oc­cur in closely re­lated free-liv­ing bac­teria, which are very com­mon in the oceans."

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