I'll have to look and see if I still have it, but I used to have a one liter Coke bottle with pool water in it, that grew algae continuously for 5+ years, remaining sealed the entire time. It got moved a few years ago, and I haven't seen it. But we've got some cleaning up to do, so it may reappear.

If I find it, and remember, I'll ship it to you, for your microscopic examination.

Ben

PS: If you really want to terrify the folks that see it, grab some water out of a stagnant deck puddle, and show people the live squigglies in the water.