Bacterial-microalgal consortium facilitates simultaneous Ca²⁺, Mg²⁺ and Fe³⁺ immobilization in saline wastewater

Microbially induced carbonate precipitation (MICP) is a promising strategy for high-salinity wastewater treatment, and bacterial-microalgal consortia show potential for enhanced biomineralization under saline stress. However, the regulatory effects of Mg²⁺ and Fe³⁺ on bacterial-microalgal biomineralization systems remain poorly understood. In this study, a consortium of Synechocystis sp. PCC 6803 and Bacillus licheniformis Y1 was constructed to evaluate its physiological responses and biomineralization performance under multi-cation stress. Under the optimal inoculation ratio, the consortium achieved a carbonic anhydrase (CA) activity of 7.6 U/mL and increased the medium pH from 7.0 to 9.1, creating favorable conditions for carbonate mineral formation. Elevated Mg²⁺ strongly inhibited microbial growth, CA activity, and Ca²⁺ precipitation, whereas trace Fe³⁺ partially alleviated these inhibitory effects and increased Ca²⁺ precipitation from 61.5 % to 65.3 % under Mg²⁺ stress. Multi-cation stress stimulated extracellular polymeric substance (EPS) production, including proteins, polysaccharides, and extracellular DNA, which favored metal-ion enrichment and heterogeneous mineral nucleation. Mineralogical analyses revealed a transition from calcite to Mg-rich calcite and aragonite, indicating cation-dependent mineral-phase selection. Microscopic and spectroscopic analyses supported the roles of microbial cells and EPS in ion binding and mineral nucleation, while XPS provided preliminary qualitative evidence of Fe-containing species within the biominerals. These findings indicate that trace Fe³⁺ partially alleviated Mg²⁺-induced inhibition of consortium activity and Ca²⁺ immobilization, providing mechanistic insights into multi-cation biomineralization under saline conditions.