This is a considerable evolutionary leap, so much so that we have only seen it three times in the evolution of life.
It is estimated that 8% of human DNA is a legacy of ancient viruses. But there are organisms that steal more than just the DNA of others, organisms that assimilate another in its entirety. A process that we have just observed between algae and bacteria.
Beyond symbiosis. This fusion or assimilation occurred between a newly discovered cyanobacteria (which they called UCYN-A) and the alga Braarudosphaera bigelowii. The bacteria stopped being an independent organism and became an organelle of the alga, a process called primary endosymbiosis.
As far as we know, this is only the third time this has happened in the entire evolutionary history of the planet.
A diffuse border. This “absorption” places the relationship between algae and bacteria one step beyond symbiosis. A diffuse border. To the point that this relationship was not completely unknown until now, it is only now that researchers have realized that they were not looking at two organisms sharing food but rather that they were looking at an algae and something that used to be a bacteria.
Part of what has led scientists to consider that we are dealing with assimilation rather than a simple symbiotic relationship is that the ex-cyanobacteria had lost part of its genes dedicated to photosynthesis and metabolism. That is, now the algae was its primary source of energy.
On the other hand, both organisms contained proteins encoded by the other, in addition to the fact that the bacteria had been “integrated” within the cytoskeleton of the alga. Furthermore, cell division in both organisms was coordinated.
Recent history”. In evolutionary terms, it can be considered that the endosymbiosis between both organisms occurred relatively recently. According to the study carried out by the team responsible for the discovery, the symbiosis relationship would have started about 150 million years ago, while the assimilation about 90 million years ago.
The details of this study were published in two articles in scientific journals. The first in the magazine cell and the second in Science.
From nitrogen to ammonia. And why would an alga want to assimilate a bacteria in this way? The key is nitrogen fixation, that is, the conversion of nitrogen gas into ammonia that living organisms can use.
This chemical reaction is enormously complex since nitrogen gas is very stable. Curiously, it is the least complex life forms, bacteria like UCYN-A, that are the only ones capable of carrying out this transformation. That is why these prokaryotic cells are so relevant for plants, although until now it was symbiotic and not endosymbiotic.
Now Braarudosphaera has become the only complex organism capable of independently carrying out nitrogen transformation. All thanks to its “new organelle”, which its discoverers have called nitroplast.
A story of (endo)symbiosis. It is only the third time this has happened in the billions of years of our planet’s evolutionary history. The third time that we are aware of actually. To put the importance of endosymbiosis in context, just remember what the previous two were.
The first of these occurred about 2,000 million years ago: the appearance of the mitochondria. This cellular organelle is today the energy source of all complex cells. The absorption of mitochondria allowed the evolution of complex organisms allowing cells to have a built-in energy source.
The second only affected one of the kingdoms of nature, but it was decisive in its evolution. This is the absorption of chloroplasts by the first plants. Chloroplasts are, nothing less, the organelles that allow plants to carry out photosynthesis.
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Image | Valentina Loconte/Berkeley Lab / Tyler Coale
