A genus of flatworms has evolved such an effective symbiosis with bacteria that it no longer has a lip or anus. Instead, all of Paracetenula’s vigor and nutrients come via the Candidatus bacteria, which weigh just as much as the worm that carries them. The partnership may seem strange to us, but it clearly works for both sides- they’ve been together for a minimum of 500 million years.
We think of swine as being active chasers of food, but representatives of our kingdom are not above having others do the work for them. Corals rely on the vigour to be provided by photosynthesizing algae for much of their food. Even the human rights digestive structure labor because bacteria obtain some nutrients from what we ingest, making it easier for us to suck what we need. Giant tubeworms living around hydrothermal vents live wholly off symbiotic bacteria.
However, when Oliver Jackle studied Paracetenula for his PhD thesis at the Max Plank Institute for Marine Microbiology he found the worms, which constitute organic complexes out of carbon dioxide and hydrogen sulphide, were doing something unlike anything we have watched before.
“In all chemosynthetic symbioses known to date, the host digests the bacteria to retrieve their nutrients, ” Jackle said in a statement. “Other chemosynthetic symbionts additionally use so-called transporter proteins that deliver nutrition to their emcees. In the Paracatenula symbiosis, we did not find either in large quantities. Everything pointed to a different mechanism.”
Jackle has explained the process in Proceedings of the National Academy of Science. The bacteria produce nutrition-rich droplets, which senior writer Dr Harald Gruber-Vodicka compares to fruit. “The bacteria continuously bear fruit, which the worm reaps, ” Gruber-Vodicka said. “In other symbioses, it’s more like gathering a cornfield, their bacteria are altogether mowed down, the worm grasps most of the bacterial cells.”
For other swine, most of what goes in must come out, but Paracetenula appears to play by different rules. It never developed a permutation waste disposal mechanism after abandoning its anus. “Everything the bacteria afford is apparently used by the worm, one direction or another, ” Gruber-Vodicka said.
Meanwhile, Candidatus became its own pieces. Its genome is far smaller than its closest non-symbiotic relatives, having removed all the genes necessary to survive on its own, instead focusing on the development of a organization of nutrient processing and force storage the working papers calls “unparalleled”. For the flatworms, this supplants the force reserves other swine have in specialized cells.
There are multiple species of Paracatenula , and each has a particular species of Candidatus on which it relies.
The discovery required bringing together genomic analysis, electron microscopy and deep knowledge of the most comparable symbiotic relationships. Jackle likewise had to work out how to grow Paracatenula in the laboratory sustainably.