Acid-Resistant Algae Accelerate Biomineralization Driven by Iron-Oxidizing Bacteria in Acid Mine Water through Serving as Electron Shuttles, Iron Ligands, and Seed Crystals

Iron biomineralization driven by chemoautotrophic iron-oxidizing bacteria (e.g., Acidithiobacillus ferrooxidans) facilitates acid mine water remediation but faces considerable technical challenges such as slow Fe(II) oxidation and Fe(III) precipitation. To address these challenges, we used the widely present acid-resistant algae (e.g., Parachlorella kessleri) as a green booster for Fe biomineralization. The assistance of P. kessleri in biomineralization with A. ferrooxidans improved the production of ferric hydroxysulfate minerals (66.2% schwertmannite and 33.8% goethite) in mine water (pH 2.7) with 1.6 and 1.4 times faster rates of Fe(II) oxidation and total soluble Fe precipitation. Mechanistically, algae-secreted extracellular organic matter (EOM), especially CHONS-containing high-molecular-weight (400–650 Da) compounds with low double-bond equivalent (DBE ≤ 10, O/C < 0.2) and high carbon atom (C ≥ 15) (e.g., proteins), acted as electron shuttles with electron accepting and electron donating capacities of 0.9 and 0.7 mmol e–/g C, respectively, that accelerate electron transfer between Fe(II) and A. ferrooxidan to generate more reactive oxygen species (H2O2 and ·OH) for Fe(II) oxidation. Algal EOM could also bond readily with Fe(II) at low pH to form EOM-bound Fe(II). Compared with free Fe(II), EOM-bound Fe(II) was more easily oxidized in the acidic mine water due to its relatively lower Gibbs free energy, higher current intensity, and smaller charge transfer resistance. In Fe(III) precipitation, single spherical algal cells could serve as seed crystals that initiate the heterogeneous nucleation of ferric hydroxysulfate minerals and accelerate their crystallization in mine water by reducing the supersaturation demand. These findings provide new insights into the highly efficient bioremediation of metal-rich acid mine waters with algal-bacterial synergy.