Oh yea?! so where does there exist a natural fusion reactor?!? Man: 1 Nature: 0
(... :P I think the point was that there are— as far as we know, no evolved organisms using fission. Not that there wasn't fission in nature prior to man.)
Those are really just feeding on the reactive chemical species produced by the ionizing radiation from nuclear decays in the rock. Pretty nifty, and I hadn't heard of them before.
The other possible answer to this that I know of are the fungi who might be able to derive metabolic energy from gamma rays--- though last I checked it wasn't entirely certain that's what they're doing.
