Fully regenerative life support without plants?

A combination of sabatier, water electrolysis and methane pyrolysis lets you have a life support system the only output of which is pyrolytic carbon. This closes the cycle on everything but carbon itself, feeding the carbon back to the crew will require serious reprocessing (unless they need those carbon tablets for digestion problems ). In fact, except for the pyrolysis step, this is what's currently running on the ISS. The ISS has a mostly closed oxygen and water loop.

They're even proposing it for terrestial applications, and if methane (which is what you get out of a sabatier reactor) wasn't so cheap, it'd be a good way of dealing with atmospheric CO2 while providing a convenient source of fuel. Actual energy would have to come from somewhere else, but nuclear power could take care of that easily enough.