Silicate minerals represent an important reservoir of essential nutrients at Earth's surface. Due to the slow kinetics of primary silicate mineral dissolution and the potential for nutrient sequestration by secondary mineral precipitation, the bioavailability of many silicate-bound nutrients may be limited by the ability of microorganisms to actively scavenge these nutrients via organic ligand production. In this study, the effectiveness of ligand production as a means to scavenge Fe from Fe-silicates is addressed through targeted laboratory experiments using olivine as a model mineral.
Preliminary results show that microbial Fe-binding ligands (i.e. siderophores) can accelerate olivine dissolution rates stoichiometrically by almost an order of magnitude in experiments buffered at circumneutral pH. In addition to higher reaction rates, organic Fe-binding ligands fostered the accumulation of dissolved Fe in solution, which was below detection in the abiotic experiments due to the precipitation of secondary Fe minerals in the presence of O2. Accelerated olivine dissolution rates in the presence of microbial Fe-binding ligands is somewhat unexpected because these ligands are known to be highly selective towards Fe3+ whereas olivine dominantly contains Fe2+. Spectrophotometric analysis of the ligand complexes produced during reaction with olivine reveals the dominance for Fe3+ -ligand complexes in solution.