"It could be any of those [sodium, or other ion gradients]. The fact of life on earth is that it tends to use proton gradients, and we know particular environments that do use proton gradients, and the reason I think protons is because , which is to say the proton concentration, can modulate the reactivity of both and . Now sodium concentrations wouldn't do that, but protons, if you've got gas in alkaline fluids, hydrothermal fluids... what you've got coming out of these s, hydrogen is more reactive in alkaline conditions. It really doesn't want to push its electrons onto something else, but if it's in alkaline conditions it pushes its electrons onto something else, and the protons are left behind and they will react immediately with the hydroxide ions to form water, which is thermodynamically very favored, and so it's far more likely to push its electrons onto CO2 if it's in alkaline solution."