The phenomenon is called primary endosymbiosis, and it occurs when one microbial organism engulfs another, and starts using it like an internal organ. In exchange, the host cell provides nutrients, energy, protection and other benefits to the symbiote, until eventually it can no longer survive on its own and essentially ends up becoming an organ for the host – or what’s known as an organelle in microbial cells.
In the 4-billion-odd-year history of life on Earth, primary endosymbiosis is thought to have only happened twice that we know of, and each time was a massive breakthrough for evolution. The first occurred about 2.2 billion years ago, when an archaea swallowed a bacterium that became the mitochondria. This specialized energy-producing organelle allowed for basically all complex forms of life to evolve. It remains the heralded "powerhouse of the cell" to this day.
The second time happened about 1.6 billion years ago, when some of these more advanced cells absorbed cyanobacteria that could harvest energy from sunlight. These became organelles called chloroplasts, which gave sunlight-harvesting abilities, as well as a fetching green colour, to a group of lifeforms you might have heard of – plants.
And now, scientists have discovered that it’s happening again. A species of algae called Braarudosphaera bigelowii was found to have engulfed a cyanobacterium that lets them do something that algae, and plants in general, can’t normally do – "fixing" nitrogen straight from the air, and combining it with other elements to create more useful compounds.
Altogether, the team says this indicates UCYN-A is a full organelle, which is given the name of nitroplast. It appears that this began to evolve around 100 million years ago, which sounds like an incredibly long time but is a blink of an eye compared to mitochondria and chloroplasts.
https://newatlas.com/biology/life-merger-evolution-symbiosis-organelle/
