This review examines how metal speciation—the chemical form of metal ions in growth media—critically affects microbial growth, toxicity, and metal bioavailability. Metal ions added as simple salts exist in multiple chemical forms depending on pH, ligand availability, and anion concentrations. Only specific species may be biologically active, making speciation determination essential for accurate microbiological studies. The authors discuss factors controlling metal speciation including complex formation, precipitation, charge effects, and hydrolysis. They explain how complexing ligands reduce metal bioavailability and toxicity, exemplifying with cases where organisms tolerate surprisingly high metal concentrations only because precipitation or complexation renders most metal unavailable. The review demonstrates speciation effects using silver toxicity studies, showing how solubility products and common ion effects can make claims of high metal resistance misleading. Hard/soft acid-base theory and formation constants are presented as frameworks for predicting metal-ligand interactions. The authors emphasize that total metal concentration measurements reveal nothing about actual bioavailable species, and discuss both theoretical predictions and experimental methods for determining speciation, including limitations of current approaches.

Key findings

• Metal bioavailability depends critically on speciation (chemical form), not total concentration; different species of the same metal ion have vastly different biological effects and toxicity
• Precipitation of metal ions as insoluble compounds (phosphates, chlorides, hydroxides) is frequently overlooked in toxicity studies, leading to grossly inflated claims of organism metal resistance
• Complex formation with medium components (organic ligands, chelating agents, phosphates) dramatically reduces metal toxicity and bioavailability compared to simple aqua complexes
• Hard/soft acid-base theory predicts metal-ligand preferences: essential metals are hard, most toxic metals are soft, allowing soft toxicants to competitively displace essential metals
• Formation constants and pH-dependent equilibria can be used to calculate and predict metal speciation, but experimental determination of individual species remains technically challenging and rarely performed in routine microbiological studies