What is that black stuff in the toilet bowl?

I visited my girlfriend’s son at college a couple of weeks ago. Nice kid – academic olympian, but otherwise clueless and maybe a little spoiled (nothing he won’t get over). He lives in an apartment with a couple of other guys. Not surprising I suppose, it was a hygienic nightmare. As is so often the problem, none of them had actually had any “training” as to how to clean their space so I went out and bought some cleaning materials and showed them how to do it, and then made them go to work. The question came up “Josie, what is that black stuff that grows down on the inside edges of the toilet bowl, where the water comes out during a flush?”
It also appears as a ring around the water line sometimes. I know it will show up if you give it time, but I don’t really know what it is. Anybody know?