What different processes could a living organism use to produce hydrogen (possibly from water) and how do these processes work step-by-step? More specifically, provided that genetic engineering had come this far and the human wished it, could the human body be modified to naturally produce hydrogen and store it and if yes what modifications would have to be made?
I'm just a layman, so go easy on me. >.<
Edit: I failed to mention that the method cannot use sunlight.