Minute marine animals live in a sophisticated symbiosis with bacteria

Trichoplax crawling near other organisms among which it typically lives. Credit: Michael Hadfield.

Trichoplax, one of the simplest animals on Earth, is not as simple as it looks, according to researchers from the School of Ocean and Earth Science and Technology (SOEST) at the University of Hawaiʻi at Mānoa, the Max Planck Institute for Marine Microbiology in Bremen, Germany and North Carolina State University.

Trichoplax, together with sponges and jellyfish, belongs to one of the most basal lineages of the animal kingdom. Typically less than a millimeter in diameter, these animals lack a mouth, gut and other organs, and are made up of only six different cell types. Its simplicity makes it a popular model organism for biologists.

Trichoplax lives in a remarkably sophisticated symbiotic relationship with two types of highly unusual bacteria. The first, Grellia incantans, is related to highly parasitic bacteria that cause typhus and Rocky Mountain spotted fever. But intriguingly, Grelliadoes not appear to harm Trichoplax. The second bacterium, Ruthmannia eludens, sits inside the cells Trichoplaxuses to ingest and digest its food. Both symbionts belong to poorly described groups of bacteria that are not well understood.

Trichoplax looks like an irregular flattened balloon. It lives in warm coastal waters around the world, where it grazes on microscopic algae that cover rocks, seaweed and other firm surfaces. Although most aquarists may not know it, Trichoplax can also be found in many saltwater aquaria with corals.

Michael Hadfield, researcher at the UH Mānoa Kewalo Marine Laboratory of the Pacific Biosciences Research Center (PBRC), had long known that Trichoplax could be collected in the lab’s seawater tanks. Hadfield, together with PBRC Director Margaret McFall-Ngai, began the study by asking, “What are the bacteria doing inside some of the most essential cells in Trichoplax’s body?”

Supplying all of the original Trichoplaxfor the study, they invited a collaboration with an international group of scientists at the Max Planck Institute for Marine Microbiology: Harald Gruber-Vodicka, Niko Leisch and Nicole Dubilier.

Together, they have now investigated the bacterial tenants of Trichoplax by sequencing their genomes and using high-resolution microscopy to see where they live.

Said Hadfield, “It was interesting enough that such a simple organism lived in constant association with many bacteria in each of its fiber cells, which serve both as the nerves and muscles of Trichoplax, and even more surprising to discover a second very different bacterium living in the cells that take up and digest Trichoplax’s food.”

The new study was published in Nature Microbiology.

See the full story on the SOEST website.

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Citation:

Harald R. Gruber-Vodicka, Nikolaus Leisch, Manuel Kleiner, Tjorven Hinzke, Manuel Liebeke, Margaret McFall-Ngai, Michael G. Hadfield, Nicole Dubilier: Two intracellular and cell type-specific bacterial symbionts in the placozoan Trichoplax H2. Nature Microbiology. DOI: 10.1038/s41564-019-0475-9