One of the simplest organisms lives in complex symbiosis with bacteria

Trichoplax is one of the simplest marine organisms in the world. It lacks a mouth, organs, and is made up of only six cells. But despite being so incredibly simple, scientists from the Max Planck Institute for Marine Microbiology, the University of Hawaii, and North Carolina State University have found that Trichoplax actually lives in a complex symbiosis with a very unusual form of bacteria.

Since the first discovery of bacteria in Trichoplax about 50 years ago, no one has really looked into the relationship between the organism and bacteria in depth. But now, the team of international researchers have assembled the bacteria’s genome and used high-resolution microscopy to see exactly where the bacteria are located.

“Despite being so simple, Trichoplax harbors two very different and highly unusual bacterial symbionts in its cells,” said Harold Gruber-Vodicka from the Max Planck Institute. “Both symbionts are very picky — or cell-specific, as we call it. Each symbiont lives in only one type of host cell.”

The team identified one symbiont, called Grellia, which lives within the endoplasmic reticulum (ER) of Trichoplax. It is the first symbiont known to permanently reside within the organism’s ER, which helps with the production of protein and membrane.

“We reconstructed a detailed three-dimensional model of the ER to show that Grellia lives inside of it, supported by the electron microscopy facility of the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden,” the Institute’s Niko Leisch explained. “Other parasitic bacteria imitate the structure of the ER to trick the hosts into thinking they are not harmful. However, our imaging data clearly showed that Grellia lives inside its host’s ER.”

Grellia, though closely related to parasites, does not harm the Trichoplax, Leisch said. “Although it has genes that would allow it to steal energy from its host, it does not use them.”

The second symbiant to be identified is Ruthmannia, which belongs to a group of bacteria called the Margulisbacteria. It lives within the cells of the Trichoplax and helps the organism digest its algal food.

“Before our study, Margulisbacteria were part of the so-called microbial dark matter — the vast majority of microbial organisms that biologists find through sequencing, but are unable to culture,” said Harald Gruber-Vodicka. “We have never actually seen them, even though their genetic traces were found in aquatic samples all over the globe.”

Gruber-Vodicka and Leisch were able to take the first images of a Margulisbacteria.

“Ruthmannia appears to only eat the fats and other lipids of the algae, and leaves the rest to its host,” Gruber-Vodicka continued. “In return, we think Ruthmannia may provide Trichoplaxwith vitamins and amino acids.”

“In this study, we focused on the symbiotic partners of a single Trichoplax species,” said Nicole Dubilier, Director at the Max Planck Institute for Marine Microbiology. “However, at least 20 more species have been described, and our first results indicate that each host species has its own, very specific set of symbionts. We are excited about taking a closer look at this remarkable diversity and how it evolved. These tiny animals not only look like potato chips, they also pack a crunch when it comes to what’s inside them.”

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By Olivia Harvey, Earth.com Staff Writer

Image Credit: Michael Hadfield / University of Hawaii