I don't know if you have ever wondered how complex life on Earth evolved?
In the past, biologists generally believed that life from simple to complex was a matter of course and inevitable, but in reality, life on Earth has gone through a very long period of simple life stages – about half of the Earth's age.
On the other hand, there is a huge chasm between simple life and complex life forms on Earth today, and there are no "intermediates" at all, which is not like a gradual transformation from simple to complex.
If life had been a gradual process from simple to complex, there would still be many life forms that have crossed the chasm living on Earth today.
Therefore, life on earth from simple to complex should be created by a qualitative leap, from simple to complex, we can now know that the essence of this leap is that life has found mitochondria, which makes simple prokaryotic cells become more complex eukaryotic cells, and then there are complex life forms.
Figure: Schematic diagram of mitochondria
Once cells have mitochondria, they can overcome the basic barrier that prevents prokaryotes like bacteria, archaea and others from growing – a barrier about energy use.
ATP, the cell's universal energy currency, is manufactured on the cell membrane, and as the cell grows larger, its surface area to volume ratio decreases, resulting in a relatively smaller membrane available.
In other words, as prokaryotic cells become larger and larger, their energy needs quickly outstrip their supply, making them unable to sustain themselves.
Whereas eukaryotic cells with mitochondria can overcome this problem by adding more mitochondria, and it is very easy to do because the mitochondria themselves have the ability to replicate themselves.
Now all complex life on Earth comes from a common ancestor - a prokaryotic cell that acquired mitochondria, because it has the blessing of mitochondria, which can be evolutionarily free, and they can accumulate larger and more complex genomes, thus making complex life possible.
Figure: The structure of chloroplasts and photosynthetic cyanobacteria is basically the same
Another qualitative leap in the history of life on Earth was the acquisition of chloroplasts about 1 billion years ago, and this change in eukaryotic cells laid the foundation for the birth of plants later.
There are several different hypotheses about how terrestrial organisms acquire mitochondria and chloroplasts, but for now, the most accepted is the endosymbiosis hypothesis.
The hypothesis is that eukaryotic cells live in mutually beneficial symbiosis with eukaryotic cells by devouring bacteria that possess special abilities in the eukaryotic cell's body.
But over time, the symbiosis grew so close that those bacteria were completely transformed into a part of the eukaryotic cell — called organelles, which perform specific cellular functions.
In addition to mitochondria and chloroplasts, there is another known example of organism getting organelles by this (eukaryotic engulfing bacteria) way, known as chromosomes.
This example is relatively little known, but it is precisely because of its existence that it makes it possible for cephalopods such as squid and octopus to change color.
However, whether mitochondria, chloroplasts, or pigments, they have all been judged to be organelles rather than individual bacteria.
Recently, two seminal articles published in the journal Cell and Science, respectively, revealed an organelle that is being born and is produced by eukaryotic cells engulfing prokaryotic bacteria in symbiosis for the fourth time known in the history of life on Earth.
This organelle, now named nitroplast, gives eukaryotes the ability to fix nitrogen – converting atmospheric nitrogen molecules into nitrogen-containing compounds needed for life.
Prior to the discovery of nitrogen, eukaryotes were thought to have no ability to fix nitrogen, so nitrogen fertilization was essential if our food crops were to increase yields.
You might say, don't soybeans have the ability to fix nitrogen?
Pictured: Roots of soybeans
If you have seen the roots of soybeans, you will know that the roots of soybeans have patches of nodulous tissue, which is caused by the nitrogen-fixing bacteria that live in symbiosis with soybeans.
Life on Earth found its way to fix nitrogen much earlier than we thought – nitrogen-fixing bacteria may have appeared even earlier than the earliest photosynthetic bacteria, and nitrogenase is believed to have been around since at least 2.2 billion to 1.5 billion years ago.
Although bacteria learn to fix nitrogen very early, eukaryotes never get this ability, and before the nitrosome, biologists thought that all nitrogen-fixing eukaryotes were due to symbiosis.
Prior to the publication of these two articles, scientists also thought that the "nitrosome" was just a symbiotic bacterium within algae.
The one marked in black is the nitro body, photo: Tyler Coale
What bacteria do nitro bodies come from?
In 1998, a team led by Professor Jonathan Zehr at the University of California, Santa Cruz, discovered a new species of cyanobacteria with nitrogen-fixing capacity in the Pacific Ocean.
The team named the new nitrogen-fixing cyanobacteria UCYN-A, which is the predecessor of the nitrosome, or "wild model".
Almost around the same time as the discovery of UCYN-A, Kyoko Hagino, a paleontologist at Kochi University in Japan, began actively experimenting in the lab to cultivate a nitrogen-fixing algae called raarudosphaera bigelowii, which was eventually proven to be the host organism of UCYN-A.
After more than a decade of hard work, Kyoko Hagino succeeded in cultivating this algae containing UCYN-A in the laboratory, which helped to study the extraordinary symbiotic relationship between the two later.
伴随宿主细胞分裂而分裂的硝基体,图源:Valentina Loconte
Why is UCYN-A already an organelle?
An organelle is also defined, and it needs to meet at least two criteria: it must be inherited through cell division and dependent on proteins provided by the host cell.
The article published in Cell reveals that UCYN-A and its host algae cells grow in sync and are controlled by nutrient exchange, which is very much in line with organelle standards.
The article published in Science revealed that UCYN-A obtains its protein from host algae cells, suggesting that UCYN-A has abandoned some of its own cellular mechanisms in favor of relying on the host to function.
This meets the second criterion, because bacteria start to discard their DNA and turn to dependence on the mother cell, which is what happens to the organelles.
All these findings confirm that UCYN-A in raarudosphaera bigelowii has turned into an organelle (nitrosome) and not a separate bacterium.
Figure: Archaea
At last
Compared with the billions of years of existence of mitochondria and chloroplasts, nitrosomes are very "young" organelles, and they may have only begun to evolve gradually in eukaryotic cells in the last 100 million years.
No one knows if nitrosomes will have as profound an impact on the evolution of life on Earth as mitochondria, chloroplasts, and pigments, but one thing is clear: nitrosomes will certainly not be the last.
There must have been many more bacteria after and before it that were or had been transformed by eukaryotic cells – some may be older than mitochondria, but they were not so influential that they were undetected.
Original report: https://www.iflscience.com/the-once-in-an-eon-event-that-gave-earth-plants-has-happened-again-73878
Literature:
1. Hattapus://doi.org/10.1126/Science.ADK1075
2. Hattapus://doi.org/10.1016/j.cell.2024.02.016