Bacterial endosymbionts that provide energy enable its unicellular eukaryotic host to breathe nitrates, indicating that unicellular eukaryotes may obtain endosymbionts to supplement or alter the functions of their mitochondrial organelles.
Bremen researchers, along with colleagues at the Max Planck Genome Center in Cologne and their colleagues at the Swiss Aquatic Research Institute Evag, have discovered and provided a unique bacterium that lives inside a unicellular eukaryote. Unlike mitochondria, these so-called endosymbionts receive energy from the respiration of nitrates, not oxygen. “Such a partnership is completely new,” says Jana Miluka, a senior author on the subject. Nature Paper. “This date is unprecedented for a symbiosis that is based on the transfer of respiration and transfer.
In general, in eukaryotes, symbiosis is common. Eukaryotic hosts often coexist with other organisms, such as bacteria. Some bacteria live inside host cells or tissues, and perform certain services such as defense or nutrition. In turn, the host provides a shelter and a suitable place to stay for Symbian Net. Endosymbiosis can also go so far that the bacterium loses its ability to survive on its own outside of its host.
The same was true of the symbiosis discovered by Bremen scientists at Lake Zug in Switzerland. “Our findings open the possibility that simple unicellular eukaryotes, such as protosts, may complement or alter the mitochondrial endosymbionts to their mitochondrial functions,” said John Graf, the study’s first author. “This protist has survived without oxygen by teaming up with an endosymbinate capable of nitrate respiration.” The name of the endosymbiont ‘Candidatus azomax ciliatocola’ reflects this; A ‘nitrogen friend’ who lives in Silite.
The figure is a combination of scanning electron microscope image (SEM, gray) and fluorescence images. Candididus azomax ciliatocola (FISH, visualized by yellow) and bacterial prey in food vacuoles, as well as the giant cell nucleus (stained by DAPI, blue) are visible. The external structure of the cilia is also visible with a weak fluorescent ciliate. Credit: Max Planck Institute for Microbiology, S.Ahmerc amp.
Intimate partnership is always close
So far, it has been thought that eukaryotes survive by fermentation in an oxygen-free environment, as mitochondria need oxygen to produce it. The fermentation process is well documented and is observed in many anaerobic silicates. However, microorganisms cannot draw as much draw ferment from fermentation, and they generally do not grow and divide as fast as their aerobic counterparts.
Graf says, “Our ciliate has found a solution to this. “It contains a bacterium with the ability to breathe nitrate and integrate it into its cells. We estimate that confidence arose at least 200 to 300 million years ago. “Since then, evolution has made this intimate partnership more rural.
Time-shift evolution
The evolution of mitochondria has proceeded in the same way. Jana Milukka explains that m has common roots in all mitochondria. It is believed that a billion years ago when ancestral archaeologists surrounded the bacterium, the two began a very important coexistence: this phenomenon marks the origin of the eukaryotic cell. Over time, the bacterium became increasingly integrated into the cell, gradually reducing its genome. Properties that were no longer needed were lost and only those that benefited the host were retained. Eventually, mitochondria developed, as we know them today. They have their own small genomes as well as cell membranes, and exist in eukaryotes as so-called organelles. In the human body, for example, they are present in almost every cell and supply them – and thus – with energy.
“Our endosymbinate is capable of performing many mitochondrial functions, although it does not divide the normal developing origin into mitochondria,” says Milukka. “It is speculated that the Symbion Net follows the same path as Mitochondria and may eventually become an organelle.”
An opportunity encounter
It is truly amazing that this symbiosis has been unknown for so long. Mitochondria work very well with oxygen – why not be equivalent to nitrates? One possible answer is that no one was aware of this possibility and so no one was looking for it. Studying endosymbiosis is challenging, as most symbiotic microorganisms are not grown in the laboratory. However, recent advances in metagenomic analysis have allowed us to gain a better understanding of the complex interactions between hosts and symbols. When analyzing the metagenome, scientists look at all the genes in the sample. This approach is often used for environmental samples because genes present in the sample cannot be automatically assigned to the organisms present. This means that scientists usually look for specific gene sequences that are relevant to their research question. Metagenomes have millions of different gene sequences and it is quite common that only a small fraction of them are analyzed in detail.
Originally, Bremen scientists were also looking for something else. Research group at the Max-Planck Institute for Microbiology examines microorganisms involved in greenhouse gas methane metabolism. For this, they are studying the deep water levels of Zug Lake. The lake is very closed, which means that there is no ical exchange of water. The deep water levels of Lake Zuk thus have no contact with surface water and are largely isolated. That is why it has no oxygen but is rich in methane and nitrogen compounds, such as nitrate. While exploring the methane munching bacteria with the gene for nitrogen conversion, Graf found a surprisingly small gene sequence that encoded the entire metabolic pathway for nitrate respiration. “We were all stunned by this discovery and I started comparing it DNA With the same gene sequence in the database, ”says Graf. But the only identical DNA was that of symbols living in aphids and other insects. “It simply came to our notice then. How will insects get into this deep water? And why ?, “Graf recalls. The scientists in the research group started playing sports and betting.
Not alone in a dark lake
Eventually, an idea came into vogue: the genome must still be an unknown endosymbiotic. To test this theory, members of the research team conducted several expeditions to Zook Lake Zoom in Switzerland. With the help of local cooperation partner Evag, they collected specimens specifically for organisms to see which contain this unique endosymbiont. In the lab, scientists prepared various eukaryotes with a pipette from water samples. Finally, using the gene marker, it was possible to visualize the endosymbionate and identify its prostate host.
The final tour a year ago was to bring the final certainty. It was a difficult task in the middle of winter. Stormy weather, ganse fog and the pressure of time made the search in the larger lake more difficult due to the first news about coronavirus and possible lockdown. However, scientists were able to obtain many samples from deep water and bring it to Bremen. These samples brought him the final confirmation of his theory. “It’s nice to know they’re there together,” says Jana Milukka. “Usually, these ciliate eat bacteria. But this guy let live and partnered with her. ”
Many new questions
This discovery raises many exciting new questions. Are there similar symbioses that have existed for a very long time and where the endosymbionts have already crossed the boundaries of an organelle? If such symbiotic nitrate exists for respiration, does it also exist for other compounds? How did this symbiosis, which has existed for 200 to 300 million years, end up in a post-ice lake in the Alps just 10,000 years ago? Furthermore: “Now that we know what we’re looking for, we’ve found gene sequences of endosymbinates all over the world,” says Milukka. In France, as well as in Taiwan or in East African lakes that are slightly older than Lake Zug. Is the origin of this symbiosis in one of them? Or did it start at sea? These are the questions that the research group wants to investigate further.
Ref: March 3, 2021, Nature.
DOI: 10.1038 / s41586-021-03297-6
