Iron accumulation dynamics in Nostoc sp. PCC 7120: extent of biosorption in batch and continuous reactors.

Abstract

Microalgae have recently gained attention for their ability to incorporate iron into organic molecules, potentially enhancing bioavailability and mitigating associated risks, suggesting a promising role in the field of iron supplementation. Among microalgal species, nitrogen-fixing organisms present a relatively high iron demand due to their unique metabolic requirements, positioning them as promising candidates for addressing iron deficiencies. However, information on the dynamics and extent of iron accumulation in diazotrophic cyanobacteria remains limited. Understanding how these organisms regulate iron uptake and storage under different cultivation conditions is essential to evaluate their potential for nutritional and biotechnological applications. This work investigates the iron accumulation dynamics of the diazotrophic species Nostoc sp. PCC 7120 under both batch and continuous systems. The results highlight its ability to adapt and survive across a wide range of iron concentration and demonstrate a strong correlation between iron availability and the internal quota of iron in biomass. High-density batch cultivation revealed that the iron content in Nostoc sp. can span a remarkably wide range, up to tens of thousands of mg Fe kg X -1 , far exceeding typical values reported in the literature. This high accumulation mainly results from consistent external biosorption of the metal onto the cell surface. In continuous steady-state culture, the externally biosorbed fraction was lower but still detectable, suggesting that adsorption substantially contributes to overall iron accumulation. Under strong iron limitation (below 0.5 mg Fe L -1 ), a marked increase in exopolysaccharide production was observed, suggesting that the extracellular matrix plays a functional role in the biosorption of the metal. Nostoc sp. PCC 7120 exhibits an exceptional capacity to tolerate and accumulate iron over a broad concentration range, mediated by both intracellular uptake and extracellular adsorption. These findings reveal that diazotrophic cyanobacteria can effectively modulate their iron management strategies under different regimes and highlight their potential as biofactories for iron-enriched biomass and as model systems for studying biologically driven metal sequestration.