Huh. This is the correct news, but a different article to the one I recall. However, very interesting;

> The first occurred about 2.2 billion years ago, when an archaea swallowed a bacterium that became the mitochondria.

> 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.

> 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.

So, tremendously rare, at least to our knowledge at this time, but not a one-off.

Yes. That's the one. Thank you.