A symbiotic partner that can use nitrate instead of oxygen: Researchers have found bacteria in ciliate animals from Lake Zug in Switzerland that provide their hosts with energy in astonishing ways. The endosymbionts complement the function of the mitochondria of their hosts – there is a direct transfer of energy. This is a previously unknown form of symbiotic communities, say the scientists.
Win-win communities through give and take: In the course of evolution, many forms of symbiotic exchange have emerged between different living beings. The “most intimate” version is the so-called endosymbiosis. These are communities between a host and partners who live in it – usually special bacteria or algae. The endosymbionts can do different tasks for their partners: They prepare his food, for example, or provide him with essential substances. A well-known example are the unicellular algae that live in coral polyps and supply them with carbohydrates from photosynthesis. In return, the host offers its small partners favorable living conditions – in some cases the endosymbionts can no longer live independently.
An amazing team lives in Lake Zug
The previously unknown version of an endosymbiosis was discovered by chance by Jon Graf’s researchers from the Max Planck Institute for Marine Microbiology: Using genetic methods, they actually looked for methane bacteria in the oxygen-free water layers in the depths of Lake Zug. But instead of the traces of methane eaters, they came across a gene sequence that encoded the entire metabolic pathway for nitrate breathing. “We were very surprised and first looked for possible explanations,” says Graf. Eventually the theory remained that the identified genome must belong to an unknown symbiont because of its small size. The researchers searched specifically for symbiont and host in the deep water of Lake Zug. They eventually identified a bacterium that lives in an eyelash animal using genetic markers. These hosts are single-celled organisms, but they belong to the more highly developed organisms (eukaryotes).
As the researchers explain, it was previously assumed that eukaryotes, like ciliates, obtain their energy through fermentation in oxygen-free environments. However, fermentation is energetically unfavorable: The microorganisms draw comparatively little energy from this metabolic pathway and are therefore often sluggish and grow slowly. “Our eyelash animal apparently found a solution for this,” says Graf. “It has taken up a bacterium with the ability to breathe nitrate and has integrated it into its organism.” His colleague Jana Milucka continues: “Such a community is completely new. This endosymbiont and its host represent a previously unknown example of a symbiosis that is based on the direct transfer of energy and not on nutrition. ”The studies show that the bacteria do not supply the ciliate animals with carbohydrates, but directly with the energy carrier ATP .
The symbiote acts like an organelle
Usually this is the function of the cell power plants of the eukaryotes – the mitochondria. “Our discovery now shows the possibility that unicellular eukaryotes contain energy-supplying endosymbionts in order to supplement or replace the functions of their mitochondria,” says Graf. “The endosymbiont, whom we have called ‘Candidatus Azoamicus ciliaticola’, can breathe the nitrogen compound nitrate and use it for energy production.” The term “Azoamicus” also describes this in a similar way: “Friendship based on nitrogen”.
An interesting aspect of the discovery is the reference to the so-called endosymbiotic theory in connection with the evolution of the eukaryotes. Because it is assumed that the development of the mitochondria goes back to an endosymbiosis. According to this, a primordial microbe ingested a bacterium more than a billion years ago and this resulted in a symbiosis with far-reaching consequences: It was the origin of the eukaryotic cells. In the course of evolution, according to the endosymbiotic theory, the ingested bacterium was more and more integrated into the cell, reducing its own genome until only those properties remained that were useful for the host. As a result, these bacteria eventually developed into organelles. To this day, the mitochondria still have their own small genome and membrane. “It is not unlikely that our symbiote will take the same path as the mitochondria and eventually become an organelle,” says Milucka.
The scientists now want to stay on the ball because their discovery has raised further exciting questions. Possibly there are many previously undiscovered symbioses of a comparable kind – and perhaps even those in which the endosymbiont has already crossed the border to an organelle. The researchers also want to investigate the origin of the symbiosis. Because, according to the genetic evidence, it was formed 200 to 300 million years ago. So it can hardly have developed in the alpine lake, which has only existed for about 10,000 years. The team is now on the trail of this and other puzzles.
Source: Max Planck Institute for Marine Microbiology, specialist article: Nature, doi: 10.1038 / s41586-021-03297-6