11.4.3: Chelation mechanism

Chelation involves formation of a complex between a ligand and a metal cation. Ligands are often organic molecules, and some can be produced and released into natural environments by microorganisms. Microorganisms can secrete chelators to increase the mobility of specific nutrients. If that nutrient is a structural component of a mineral, then removal from the mineral by the chelator can ultimately cause the mineral to dissolve. Chelators can complex many different metals, suggesting that they can play a role in accelerating dissolution of a wide range of minerals (Konhauser, 2007).

As an example, many microorganisms secrete siderophores, which are organic compounds that form strong complexes with ferric iron (Sandy and Butler, 2009). In most natural waters, ferric iron solubility is low, as discussed in Section 12.1.3. Under such conditions, ferric iron commonly exists within an (oxyhydr)oxide mineral rather than a dissolved ion, making it difficult to import into a cell. However, by releasing siderophore into the environment, microorganisms can mobilize the iron and more easily import it. As a consequence, the (oxyhydr)oxide at least partially dissolves.

Page and Huyer (1984) demonstrated that siderophore production directly relates to iron bioavailability and microbial growth. They examined siderophore production in culture experiments containing Azotobacter vinelandii, a nitrogen-fixing microbe. Recall from earlier sections (5.2.1, 7.3) that iron is an essential ingredient needed to make nitrogenase, the enzyme used to fix nitrogen. In their experiments, the types and amounts of siderophores secreted by A. vinelandii depended on the mineralogical forms of iron that were present and the overall iron abundance. Thus, the mineralogy of an environment appears to influence not only the types of chelators being secreted by a microbial community but also the overall significance of chelation weathering.

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