One of the surprises of modern biochemistry is the discovery of the incredibly sophisticated devices used by microorganisms to survive in inhospitable environments. It seems anthropomorphic to refer to these survival mechnisms as strategies, yet they so resemble some of the most clever kinds of schemes we can devise that it is hard to find a more appropriate word. Indeed, when the study of life teaches new useful methods, we utilize the knowledge of bionics, the science of designing systems or instruments modeled after organisms.
An example of a cellular strategy is the mining of iron by bacteria. The element is one of the most plentiful on earth, consisting of about 5 percent of rocks. Because of the high atmospheric concentration of oxygen, most iron exists as very insoluble ferric oxides and hydroxides. Therefore, even given the substantial total quantity of the metal, the concentration in soluble form needed by organisms is extremely low, somewhere around one part per hundred million in the oceans. Nevertheless, iron is an essential element for most cellular processes, and a given bacterium can survive only if it somehow manages to extract such atoms from its environment.
Living systems have a subtle, low-temperature, atom-by-atom method of solving the problem. The bacterium synthesizes and excretes into its surroundings molecules that have an extremely high affinity for binding iron at a very specific site. Such compounds are called chelators (from the Greek word for claw) because of the strength with which they scavenge every available free iron atom.
This shifts the equilibrium and leads to the solubilization of the plentiful oxides. The next step in the process is carried out by a transport system in the cell membrane that pumps the complexed metal back into the cell. Within the cytoplasm the chelator retains the iron so that it is unavailable for biochemical reactions. Two different schemes are used to pry the iron loose from the molecular claw. In one of these, the enzymes digest the binding structure. This is a high price to pay, for the cell must synthesize replacement molecules to get more iron. In the other method, the iron is chemically reduced to the ferrous form, which is then released. In either case, the metal atom is free within the cell where it is quickly utilized in making various Ferro proteins.