Cyanobacterial extracellular polymeric substances empowered biological aqua crust formation via selective mineral adsorption for sustainable metal(loid) bioremediation.

A newly discovered biological aqua crust (biogenic aqua crust-BAC) in mine drainage effectively sequesters both heavy metals (e.g. Cd) and metalloids (e.g. As), yet its formation mechanisms remain unclear. Here, batch microcosm experiments were conducted with the cyanobacteria Limnothrix planctonica and two key minerals (goethite and kaolinite) of different particle sizes to investigate BAC development. Results revealed that extracellular polymeric substances (EPS) adsorbed to mineral surfaces primarily via chemical bonding, with proteins exhibiting nearly twice the adsorption frequency of polysaccharides (74.2% vs. 39.0%). Notably, small-sized goethite successfully formed the embryonic structure of BAC with L. planctonica. Goethite preferentially adsorbed proteins through coordination and hydrogen bonding, showing 1.9-2.1 times higher efficiency than kaolinite. In contrast, kaolinite favored polysaccharide adsorption via hydrogen bonding, with 1.6-2.1 times greater capacity than goethite. These findings elucidate the mineral-specific selective adsorption of EPS as a key mechanism driving BAC formation and structure development. This study provides mechanistic insights into EPS-mineral interactions that underpin BAC formation, laying a theoretical foundation for optimizing nature-based strategies in mining-impacted waters.