Chelation and uptake of trace metals by microbiota

Upon entering sea water, the important trace metal ions are quickly bound to chelators, molecules that specialize in binding metal ions. Some of these chelators may be inorganic, but the majority are organic molecules produced by marine bacteria, and possibly by other microbes, to scavenge essential trace metals from the environment and facilitate their uptake by microbial cells.

Not much is known about chelators. We don't know how many different kinds of chelators there are. With a few exceptions we don't know what their structure is, or how microbes detect them and take them in. Except for iron chelators we don't know precisely what their function is.

We need to know these things because iron, zinc, and a few other trace metals are required at several steps in the cycling of nitrogen and carbon among their various forms and uses. There is compelling evidence, for example, that in some regions of the world's oceans the scarcity of iron limits the rate of primary production, the transformation of carbon from inorganic forms to organic forms. Primary production is the ultimate source of the organic carbon molecules that form the bulk of the tissue of all living organisms. Primary production is also nature's most effective way of removing the greenhouse gas CO₂ from the atmosphere.

Because its solubility in sea water is very low, iron is exceedingly rare in the open ocean. When iron is scarce marine bacteria (and maybe some microalgae) produce siderophores, small molecules with a high, specific iron affinity. The vast majority of iron chelators are almost certainly siderophores or their break-down products. Siderophore-producing organisms have evolved specialized mechanisms to assist them in harvesting iron, including not only siderophore production but also membrane-bound proteins that help them ingest siderophore-complexed iron.

By effectively increasing the solubility of iron in sea water, siderophores make more iron available for the essential biological processes that require them — but only for organisms that have machinery for dealing with siderophore-complexed iron. Though there are some reports of siderophore production by marine microalgae, most can't ingest siderophore-bound iron. But evolution has endowed them with a different technology for getting the iron they need: special reductase enzymes take apart iron-siderophore complexes outside the cell wall and initiate ingestion of uncomplexed iron. All these processes are poorly understood. Cebic aims to learn more about them.

Chelation and Uptake in detail
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