I find that both science fiction writers and exobiologists have very limited imaginations when it comes to creating alien species. There are bacteria on earth that are more different from us than any creature I have encountered in fiction. Consider Geobacter.
Multicellular animals all get their energy by combining hydrocarbons or sugars with oxygen, using reactions that have the form:
Sugar + oxygen -> CO2 + water
But there are bacteria that use metal ions instead of oxygen; they breathe metal. The metabolic reaction of the first species discovered, Geobacter metallireducens, has the form of
Acetate + Ferric iron (3+) -> CO2 +carbonate + water + Ferrous iron (2+)
Over the past 20 years many other kinds have been discovered, including some that metabolize toxic metals like Chromium and Uranium and can be used to remove them from groundwater, since they convert them to less soluble forms.
So whenever I read about people who plan to search for alien life by searching for free oxygen, or who think that the only planets that might support life are the ones with oceans of water, I think, no, the universe is much more strange and diverse than that. Even the earth is much stranger and more diverse than that.
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I suppose the question would be, could a larger, multicellular creature efficiently gain energy that it could use throughout its body by "breathing" a non-gaseous material. Bacteria can use metals they absorb from non-gaseous outside sources because there's very little distance for the ions to travel; but I presume a reason multicellular creatures have evolved to use gasses, or molecules of gasses dissolved in water, is because these can be transferred rapidly throughout a large body. I doubt you could have atmospheric iron. Could liquid-dwelling multicellular creatures absorb and distribute efficiently metallic ions dissolved in the ambient liquid? Perhaps.
Large creatures like us, with solid bodies through which things are moved by fluids, probably could not use this method. Bacteria can work in ways that don't scale up very well. But I can imagine many other kinds of living things, like organized clouds inhabiting dense atmospheres, or organized electrical potentials in crystals, and so on. And I remain certain that if we ever do find extraterrestrial life, it will be something none of us have imagined.
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