re: Two lifeforms merge in once-in-a-billion-years evolutionary event

They aren’t using the nitrogen for energy. They still use solar energy to convert co2 to sugar.

It’s just that nitrogen is a critical part of many biological compounds but only relatively few organisms can introduce it into a biological system.

What it means by “fixing” is being able to take elemental gaseous nitrogen and put it in something bio available. This happens in the soil by bacteria mostly (sometimes in symbiosis with specific plants). Plants use it, and things that eat plants take it up, and so on.