Deep-Sea Mussels with Highly Toxic Tenants

Science 08.10.2015

Deep-sea mussels with highly toxic tenants

Bacteria inhabiting deep-sea hot vents as symbiotic tenants of mussels are
equipped with a whole arsenal of toxins – more than any known pathogen.
Find out about this discovery, and why it is rather healthy than harmful
for the mussels, in a new publication by MPI scientists in the open-access
journal eLife

Imagine you have a tenant living in your house. They’re keeping your
fridge topped up. But in addition to this, they’re producing all kinds of
toxic substances. More harm than good? Not necessarily; it all depends
what you’re using the toxins for.

Deep-sea hot vents are one of the most unusual habitats on Earth: At first
sight they appear hostile and uninviting, but in fact, they are teeming
oases of life. Likewise, their unique inhabitants are always surprising
us. Find out how toxic tenants can also be beneficial in a new publication
by an international research team led by Jillian Petersen from the Max
Planck Institute (MPI) for Marine Microbiology, published in the open-
access journal eLife.

Mussels of the genus Bathymodiolus, related to the well-known blue mussel,
are among the most dominant inhabitants of hot vents in the deep ocean. In
their gills, they house so-called chemoautrotrophic symbionts. These
symbionts include sulfur-oxidizing bacteria, which convert substances
normally not used by the mussels into tasty sugars.

Jillian Petersen and her colleagues have now taken a closer look at the
genes that some of the symbiotic tenants of deep-sea mussels contain in
their genomes. To their surprise, what they found was a vast array of
hazardous substances. The symbiotic bacteria command an arsenal of genes
that are responsible for the production of toxins. The number of toxins is
impressive: With up to 60 toxins, the microorganism’s arsenal is better
stocked than many nasty germs such as those that cause pest and cholera.
However, down in the deep sea, the bacteria leave their host unharmed. In
fact, they promote the health of their mussel hosts. How is this possible?

“We suspect that they bacteria have tamed these toxins”, explains
Petersen. “Thus, they can now take advantage of them for the benefit their
host.” Two kinds of beneficial effects of the toxins are possible: On the
one hand, they might help mussels and bacteria to find and to recognize
each other, essential steps to establishing a successful symbiosis. On the
other hand, the toxins may help the mussel to defend itself against

“Symbioses are usually assumed to have only one benefit – the symbionts
either help the host to feed or to defend itself. Our study shows that the
partnership of Bathymodiolus and the sulfur-oxidizing bacteria seems to
provide both: defence and food. That is very unusual”, emphasizes Lizbeth
Sayavedra, who conducted the research as part of her doctoral thesis. The
tenant not only fills the fridge, it also keeps the burglars out.

In the next steps, Petersen now wants to investigate the details of this
defence mechanism. The research team has developed a method proving that
at least one of the toxins is exported to the mussel tissue. “Our results
give fresh impetus to the research on the role of parasites and pathogens
in the deep sea”, says Petersen, who has recently established an
independent research group at the University of Vienna.

“The Bathymodiolus symbionts produce more of these supposedly harmful
substances than any known pathogen”, adds Liz Sayavedra. “Who knows –
maybe one day we’ll discover that some of the genes that are currently
annotated as toxins may have first evolved through such beneficial

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Picture: Bathymodiolus mussels at the Menez Gwen hydrothermal vent off the Azores, pictured during Meteor cruise M82/3. MARUM, University of Bremen/Germany.

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