Abstract
Trace metals underpin the chemistry of life, serving as indispensable catalytic and structural cofactors across all living organisms. Yet the same properties that make them essential also confer toxicity, requiring tight control over metals intracellular concentrations. Metal homeostasis therefore arises in response to this dual nature from the coordinated activity of transmembrane transporters, metallophores, metal-sensing regulators, and intracellular trafficking systems. Among these components, transmembrane transporters act as the primary gatekeepers that mediate metal flux at the interface between the geochemical environment and physiology. Life has evolved a diverse repertoire of metal transport systems, spanning numerous families and superfamilies that differ in membrane localization, substrate range, energetic mechanisms, and transport directionality. Although each metal is associated with a characteristic set of transporters, strict specificity is uncommon and many transporters display varying degrees of promiscuity. This is the result of shared chemical properties as well as the evolutionary pressures imposed by fluctuating metal availability, and provides insight into the coevolution of life and Earth and informs applications ranging from biogeochemical modeling to biotechnology, bioremediation, metal recovery, and antimicrobial strategies. Here we present a systematic analysis of our knowledge of biologically-relevant metal transport systems.