Life is good inside your new tougher cortex. You swim around with your two flagellae, eating things smaller than you.

Interestingly, some of the things you eat have got big enough and tough enough to survive inside you, if they’re lucky. Now, it could be to your advantage to have something living inside you, if it isn’t in competition with your current chemistry. If it is, of course, you’ll probably die!

If you don’t get a leg up, with another organism helping you out, then others will, and you’ll be outcompeted and die out. Better choose one.
Quick!

One thing that clearly isn’t in competition with you is anything that uses light energy to makes simple sugars. If you took in one of those, it could share the sugar with you, and you could keep it safe from other predators. Win-win.

Two little species abound, at present, both of which photosynthesisie. There are green/blue bacteria, and rhodophyta, a primitive form of red algae. Either will be enough, it’s just a question of which colour of light you want to be able to use.