Microalgal Iron-Enriched Polyphosphate Nanoparticles as a Novel Carrier to Deliver Bioavailable Iron

Biogenic polyphosphate nanoparticles from microalgae have shown great promise in mineral nutrition delivery due to their stability, biocompatibility and high bioavailability. In this study, Synechococcus sp. PCC 7002 was cultivated in growth medium containing 116.7 mg/L ferric chloride to produce iron-enriched polyphosphate nanoparticles (FePNPs) with an Fe/P molar ratio greater than 0.65, corresponding to an intracellular iron yield of 2.4 mg/g dry cell weight. Structural characterization using dynamic light scattering, transmission electron microscopy, and scanning electron microscopy revealed that FePNPs were small, negatively charged nanoparticles with a size distribution of 5–10 nm and a ζ-potential of approximately −36.7 ± 7.5 mV. Calcein-fluorescence-quenching assay in polarized Caco-2 cells revealed that iron uptake from FePNPs is divalent-metal-transporter-1 and macropinocytosis-dependent pathways, followed either by transcytosis to the basolateral side or by lysosomal degradation that releases iron into the labile iron pool for subsequent basolateral export via ferroportin. This dual-pathway absorption achieves an average relative bioavailability of 91.5% when compared to conventional FeSO₄. In conclusion, FePNPs represent a novel iron supplement with high bioavailability.