This phenomenon has only been observed twice before: millions of years ago, with the discovery of mitochondria and chloroplasts.
More than two billion years ago, a single-celled primordial bacterium swallowed another bacterium. Through a process called endosymbiosis – a kind of fusion of two organisms – the bacterium grew into an internal ‘organ’. We now know this organelle as the mitochondria. The same thing happened a billion years ago, but with a cyanobacterium, which is how the chloroplast was created. American scientists have now observed this rare phenomenon again.
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Nitrogen-fixing bacteria
More than 25 years ago, scientists from the University of California found the DNA of an unknown bacterium in the Pacific Ocean. The genetic material is believed to belong to a nitrogen-fixing cyanobacterium, which absorbs nitrogen from its environment and converts it into useful nutrients. They then named the otherwise mysterious bacterium ‘OCYN-A’.
Not much later, Japanese researcher Kyoko Hagino found that OCYN-A was present in the cells of a marine algae (Braarudosphaera bigelowii) grew. The sea algae served as a host organism for the bacterium. Ten years later it worked B. bigelowii in the laboratory, after which extensive research into the organisms followed.
Endosymbiosis
The researchers soon discovered that there was endosymbiosis: two organisms from different species that start to live together. This is achieved because a host (in this case the algae) swallows another organism (the cyanobacterium) into its body, as it were.
It is beneficial for both organisms, because they benefit from each other’s metabolism. But the two do not always succeed in achieving a stable relationship, which only happens in rare cases.
But breeding experiments made the researchers suspect that this was such a rare case. The algae and the bacteria grew at the same rate and simultaneously, probably because the two benefited from each other’s nutrients and proteins. The findings were published last month in a trade journal Cell.
Share and discard DNA
This endosymbiosis was typical of the way an organelle behaves in a cell. “And this was exactly the case with mitochondria and chloroplasts, millions of years ago. When they entered a host, they started to grow synchronously with their host,” says research leader Jonathan Zehr in a press release.
But more evidence was needed to be sure that the nitrogen-fixing bacterium had turned into a cell organelle. Of protein analysis they eventually made that discovery: the bacterium could only produce half of the proteins it needed itself, and relied on the algae to provide the rest.
Sjaak Neefjes, professor of chemical immunology at Leiden University, not involved in the research, explains: “Organisms become dependent on each other in endosymbiosis. Mitochondria and chloroplasts once did this by exchanging DNA with their host. This bacterium probably also throws away pieces of its own DNA, but no one knows how this works. That makes it such a special phenomenon.”
These results were enough to state that the UCYN-A bacteria in the algae species B. bigelowii has been transformed into an organelle. Thanks to its ability to fix nitrogen from the air, the organelle is classified as nitroplast. The scientists have published this rare discovery in a professional journal Science described.
Agricultural sector
Only twice before have single-celled organisms on Earth merged in this way to form one organism. Neefjes shares his enthusiasm about the phenomenon: “Evolution allows two organisms to grow into one life form, which then manages to continue living. A beautiful example of how nature works!”
The researchers want to find out whether nitroplasts also occur in other cells, and what beneficial effects they may have.
“It would be interesting to know what is needed to transplant the nitroplast into other bacteria. Then we will hopefully also get an idea of how we can subsequently transplant it to plants,” says Neefjes.
The latter could be beneficial for the agricultural sector. Plants would then absorb nitrogen from the air in a very sustainable way and convert it into other useful nutrients.
Sources: Cell, Science, University of California
Image: Tyler Coale/University of California
