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Organisms acquire metals from the environment by releasing small molecules that solubilize and promote their specific uptake. The best known example of this nutrient uptake strategy is the exudation of siderophores, which are a structurally-diverse class of molecules that are traditionally viewed as being integral to iron uptake. Siderophores have been proposed to act through a variety of processes, but their effectiveness can be mitigated by a variety of chemical and physical processes of both biotic and abiotic origin. Processes that occur at the surface of minerals can degrade or sequester siderophores, preventing them from fulfilling their function of returning metals to the organism. In addition, biotic processes including enzymatic degradation of the siderophore and piracy of the metal or of the siderophore complex also disrupt iron uptake. Some organisms have adapted their nutrient acquisition strategies to address these potential pitfalls, producing multiple siderophores and other exudates that take advantage of varying kinetic and thermodynamic factors to allow the continued uptake of metals. A complete understanding of the factors that contribute to metal uptake in nature will require a concerted effort to study processes identified in laboratory systems in the context of more complicated environmental systems
