“They found that the isolated bacterium can only produce about half of the proteins it needs, relying on the algal host to provide the rest.”
The alga Braarudosphaera bigelowii swallowed a nitrogen-fixing cyanobacterium called UCYN-A and turned it into an organ. The bacterium now divides in sync with its host and can’t build half its own proteins. Primary endosymbiosis has happened twice before in 4 billion years: mitochondria 2.2 billion years ago and chloroplasts 1.6 billion years ago. Plants still rent their nitrogen from bacteria living next door. This alga moved the bacteria in and took away their independence.