A protist harbors both green algae and purple bacteria symbionts

M.Over a century ago, a German school teacher named Alfred Kahl made a career change and became a student of the protozoologist Eduard Reichenow. In 1926, when he was nearly 50 years old, Kahl published a paper identifying, classifying, and describing hundreds of new species of protists. One of these, Pseudoblepharisma Tenue, is a ciliate found in the Simmelrieder Heide, about eight hectares of wetland in southern Germany. In his manuscript, Kahl noted about pink bacteria and green algae that live in the body P. tenue, but so far no one has followed up on this peculiar observation.

In a study in study published on June 11th Scientific advances Researchers confirm that P. tenue has two types of symbionts – a green alga and a purple bacterium – an unusual feature in a eukaryote.

“Usually symbionts are a more specialized component, and you may get one and not the other,” says Megan Sørensen, a microbiologist at Stockholm University and Uppsala University who was not involved in the study. “The collaboration of these three species – the host, the purple bacteria, and the green alga – reveals a complex level of metabolic integration that is really fascinating.”

While this particular combination of organisms is unique, symbiotic interactions “are very common in any type of aquatic ecosystem,” says Matthew Johnson, a protist ecologist at the Woods Hole Oceanographic Institute in Massachusetts who did not participate in the study. These symbiotic associations are one way to expand the host’s niche and survive in difficult environments, he adds.

As a PhD student at Dalhousie University in Canada, Sergio Muñoz-Gómez, now a postdoc at Paris-Saclay University in France, focused on mitochondria. Researchers have suggested that the mitochondrial ancestor might have been purple bacteria, he explains, but there was only one study from 1993 that looked at purple bacteria in eukaryotes. He wanted to study this organism, but the then postdoc Sebastian Hess told him about another organism, P. Tenue.

See “The bacterial origins of the mitochondria turned upside down”

Hess knew P. tenue because as a student at the University of Bonn in 2006 he had seen a book with color photos and scanning electron microscope images of microscopic organisms found in the Simmelreider Heide by the independent microscopist Martin Kreutz and the organism biologist Wilhelm Foissner, then a biologist at the University of Salzburg in Austria. “I always thought, ‘Well, that’s interesting because it’s pink,’” says Hess. “I showed him pictures of this organism and we both agreed that it was very interesting and deserves a study.”

Bring after varying degrees of success P. tenue Rehearsals at Dalhousie University, Hess – meanwhile group leader at the University of Cologne – invited Muñoz-Gómez to come to Germany and examine the ciliates where they did not have to navigate long distances. Kreutz collected the protists in southern Germany and sent them to Cologne every few weeks. By learning P. tenue In the laboratory, the researchers confirmed that it is about 180 microns long – about twice the size of a human hair – and is able to swim quickly and phagocytize other microorganisms.

Clock Pseudoblepharisma Tenue swim in its natural habitat.

Hess laboratory

The authors were unable to identify cultures from P. tenue, So they sequenced wild-caught samples to determine the identity of their symbionts. The green algae found in it P. tenue is most closely related to Chlorella sp. K10, an endosymbiont from another freshwater eukaryote, Hydra viridissima Strain K10. P. tenue‘s purple bacterium is most closely related to members of the genus Thiodictyon, which uses bacteriochlorophyll as a photosynthetic pigment and also metabolizes sulfur to use it as an electron donor for photosynthesis. They found that the purple bacterium was inside P. tenue lacks the genes required to metabolize sulfur, meaning it likely depends on the ciliate host but has retained genes for photosynthesis.

Green algae are experts in photosynthesis in oxygen-rich environments, while purple bacteria excel in photosynthesis in low-oxygen conditions. Having both symbiotes can help P. tenue thrive in the oxygen-variable sediments of muddy ponds. “We have some very plausible hypotheses based on genomic data [and] on microscopic observations of how the ciliate likes to live or which environment it prefers, but we want to gain more experimental data on the type of molecules that the symbionts exchange between them, ”explains Muñoz-Gómez.

Testing like P. tenues metabolism and symbiosis affecting the choice of environment would be another way of tracking this, says Sørensenen The scientist. “Actually being able to grow them in the laboratory and therefore do rigorous testing under various conditions [or] Having more environmental samples to test different environmental conditions within the samples would be different approaches, ”she explains.

The nature of the relationship between the three organisms remains an open question. “We look at organisms that live together and automatically assume that it is beneficial,” says Sørensen, “but there are also exploitative relationships. Something they have no idea about [about] Here is who benefits from it and whether there is exploitation. “

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