Removing Pb²⁺ ions from aqueous solution with freshwater cyanobacteria: Combined adsorption and intracellular...

This study investigated the removal of lead ions (Pb²⁺) from aqueous solutions by two freshwater cyanobacteria with distinct cellular inclusions: Synechococcus elongatus FACHB-410, which is rich in polyphosphate (polyP), and Cyanothece sp. PCC 7425, containing both polyP and amorphous calcium carbonate (iACC). Both strains were exposed to Pb²⁺ concentrations ranging from 1 to 50 mg/L and demonstrated high removal efficiencies exceeding 92%. The removal mechanism involved not only adsorption onto cell surfaces but also significant intracellular precipitation. The proportion of intracellular precipitation to total removal was 13.32-36.05% for strain FACHB-410 and 38.07-78.91% for strain PCC 7425. Isotopic analysis revealed fractionation of ²⁰⁸Pb/²⁰⁶Pb during removal, especially in the early incubation phase, suggesting preferential enrichment of lighter Pb isotopes. Surface adsorption was primarily mediated by carboxyl groups. The cellular sequestration of Pb²⁺ was accompanied by Ca²⁺ release, indicating that Pb²⁺ uptake occurred via exchange with intracellular light metal ions. Within the cells, Pb was primarily biomineralized into amorphous Pb-Ca phosphate nano-granules. This mineralization likely depended on phosphate (PO₄^3-) derived from polyP hydrolysis via polyphosphatase (Ppx). Although Pb was the dominant cation in these granules, Ca also played a substantial role. PCC 7425 released more Ca²⁺ and formed Pb-Ca phosphates via iACC dissolution, while FACHB-410 appeared to rely more on cytosolic calcium sources. These findings reveal that beyond adsorption, the studied freshwater cyanobacteria employ a novel Pb detoxification strategy involving polyP-mediated intracellular precipitation. This highlights their potential for sustainable bioremediation of lead-contaminated water systems.