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Unique symbiosis: Bacteria feed worm

Specialized body cells from a marine flatworm harbor bacteria that fix carbon dioxide and deliver all nutrients to its cowardly host

Bremen –

A very unusual form of symbiosis has been discovered by German microbiologists in a worm. The sea flatworm Paracatenula has neither mouth nor gut. Almost half of the body mass consists of bacteria of a special kind that provide it with energy and all vital nutrients, report the researchers in the journal "Proceedings of the National Academy of Sciences (PNAS)". The microbes are closely related to photosynthetically active bacteria and can produce sugars and other organic compounds from carbon dioxide. But as an energy source they do not use the sunlight, but the oxidation of hydrogen sulfide, which is available in the seabed. The symbiotic bacteria release nutrients to their host without being digested themselves. They benefit from the community because they are protected in the body of the worm against predators and competing bacteria.

"The food packages of the symbiont certainly contain lipids and proteins, but probably also sugars, fatty acids, vitamins and other substances used to build biomass and energy," says Harald Gruber-Vodicka of the Max Planck Institute for Marine Microbiology in Bremen. "The fact that a single bacterial species can produce so many different substances and make them available to its host has been seen in every other symbiosis so far." The researchers and his colleagues studied a kind of plankton Paracatenula, the bottom of the Bay of Sant & Andrea lives in front of the Mediterranean island of Elba.

In special body cells, the so-called bacteriocytes, these worms harbor bacteria of a species with the provisional name "Riegeria santandreae". As a result of comparative DNA analyzes, the microbes are closely related to the photosynthesis species Rhodospirillum rubrum. Surprisingly, the Riegeria genome of 1.24 million base pairs was considerably smaller than the genomes of 4.3 to 5 million base pairs of related free-living bacteria. According to the researchers, the genetic material of the symbiotic microbes must have shrunk enormously in an evolutionary process of at least 500 million years without losing the ability to produce all vital substances. During this time, the worms apparently always passed on their bacterial symbionts directly to their offspring.

Only electron micrographs showed that the bacteria pack the nutrients that the worms need into small vesicles that are pinched off the outer membrane and then digested by the host. During digestion, probably no waste materials, because the animals cannot excrete anything. Living together between microbes and animals for better use of food are very common. As a rule, however, the animal host uses the nutrients generated and stored by the symbiont by completely digesting a portion of the microbes. In Paracatenula, the many bacteria that live in the body's own cells form an energy depot that worms use in the same way that other animals use their storage fat.

(Video from Paracatenula sp. Santandrea: https://vimeo.com/290672261)

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