Alternatives to oxygen?

Is it possible that the kind of photosynthesis we know on Earth doesn't evolve, but something else instead? There are many other molecules which could provide for the hydrogen needed in photosynthesis, instead of water, and most of them are easier to break up. These have doubtlessly played a role in early bio-evolution on Earth, and at least one of them -- sulfur-hydrogen -- is used by certain bacteria until today. But none of these molecules is in such a plentiful supply as water. On Earth, those making use of water won over. On some other planet, this variety of photosynthesis might never evolve. But on those planets, life will get stuck on a primitive level -- anaerobic digestion just does not put out enough energy to drive complex, mobile, intelligent multicellars.

The only thing which has the chemical potential to rival oxygen is chlorine. This could be a by-product of photosynthesis drawing hydrogen from hydrogen chloride. It could also work with chloride salts; in this case, the hydrogen is actually drawn from water (leaving behind hydroxide), but what is oxodized is chlorine `Chlorine breathers', popular in bad SF, are chemically possible. But are they likely? Chlorides are just as hard to oxidize as water, but they are less abundant, giving advantage to those drawing their hydrogen from water.

Another possibility is that the photosynthesis process draws hydrogen from water, but does not release the oxygen as O2. There are other things that could come out of this. Water could be oxidized to hydrogen peroxide, nitrogen to nitrate, or ferrous oxide to ferric oxide, to give some examples. Hydrogen peroxide would be probably accompanied by molecular oxygen which forms from the decomposition of the peroxide, making oxygen breathing possible. (The planet would, however, still be uninhabitable, with the surface water and atmosphere contaminated by hydrogen peroxide.) Nitrates would be accompanied by nitric oxides. These could be breathed by alien life forms, and support fire, forming a feasible alternative to oxygen.

Thus, we have a few alternatives to our oxygen: chlorine, nitric oxides, and oxygen split off from hydrogen peroxide. However, Earth's oxygen-producing photosynthesis is probably a more likely path than those.