I'm working on a hard sf novel about humans being forced to survive on a metal-poor planet. They have to take dietary metal supplements at first but eventually a bio-engineer rigs the biochemical pathways from a native species to work in their offspring using bacterial and viral vectors. I'd like a non-metallic element to assist in oxygen gas exchange. An alternative is that they use iron but that it is recycled, somehow, within the body. So, for example, heavy blood loss would be a serious accident since the iron in the blood would be hard to replace. Any ideas would be appreciated.
Specifically, fluorine bound into perfluorocarbon molecules. These substances can dissolve ridiculously high concentrations of oxygen--enough so that they can be used for liquid breathing. Some perfluorocarbons can even carry more oxygen and CO2 than blood can (that's by bulk measure, though, and you definitely don't want to simply replace blood entirely with perfluorocarbons to get that performance--you'd break a lot of other stuff in the process!).
Instead of normal red blood cells, your modified people would have perfluorocarbon vesicles--basically, fancy oil droplets--suspended in their bloodstream.
Of course, there are numerous disadvantages to such a system. For one, fluorine isn't exactly a common element either, so they may be better off sticking with the iron supplement anyway. For another, haemoglobin alters its binding strength according to pH, so it selectively releases oxygen in areas that need it most (i.e., tissues which are producing a lot of metabolic byproducts like CO2 or lactic acid). Oxygen-solvating vesicles would not have that kind of specific response, thus further reducing the efficiency of oxygen delivery.
Everyone loves water for biology. A water molecule can carry an extra oxygen and in doing so becomes hydrogen peroxide. Our bodies generate hydrogen peroxide which circulates around and is induced to give up its oxygen via various enzymatic reactions.
Hydrogen peroxide is generated in numerous biological processes and is implicated as the main transmitter of redox signals. Although a strong oxidant, high activation energy barriers make it unreactive with most biological molecules.
Water is not a metal. It can carry oxygen as the peroxide and as opposed to sulfates or borates, carries it loosely. As the iron containing heme ring is shepherded about by the huge protein hemoglobin, so there would be a hemoglobin analogous protein which would protect one or more molecules of hydrogen peroxide, and mediate the release and reuptake of the oxygen.