Algae engulfs cyanobacterium and gains a plant superpower. Fixing nitrogen straight from the air and combining it with other elements to form more useful compounds. Something similar has happened twice before that we know about. About a billiion years ago a single cell organism engulfed a bacteria that harvested energy from the sun. It evolved into plants. A couple of billion years ago a similar event gave us eukaryote which led to multicellular life.
https://www.popsci.com/science/two-lifeforms-merged-into-one/