High-affinity binding of mercury ions by R-phycocyanin from Porphyra haitanensis enables their sensitive detection...

Water pollution is one of the major environmental challenges of the 21st century. Heavy metals are among the most critical water pollutants, posing a significant global environmental concern. Red macroalgae exhibit substantial potential for biosorption of heavy metal ions from aqueous solution. They are rich in phycobiliproteins, pigments with covalently attached tetrapyrrole chromophores that have remarkable potential to bind various metal ions. This study investigated the potential of R-phycocyanin, a highly abundant phycobiliprotein in the red macroalga Porphyra haitanensis, to bind Pb²⁺, Cd²⁺, and Hg²⁺ ions. Fluorescence quenching of R-phycocyanin's phycocyanobilin chromophore by selected heavy metal ions revealed moderate to high binding affinities: 1.05 × 10⁵, 1.99 × 10⁵ and 1.76 × 10⁷ M^-1 for Pb²⁺, Cd²⁺, and Hg²⁺, respectively. The presence of Hg²⁺ substantially influences the absorption spectrum of R-phycocyanin, decreasing the absorption intensity of the phycocyanobilin chromophore by 75%. On the other hand, selected heavy metal ions did not significantly affect the secondary structure of R-phycocyanin. The thermal stability of R-phycocyanin was strongly affected by the presence of Pb²⁺, Cd²⁺, and Hg²⁺, with Hg²⁺ being the most detrimental, decreasing the melting point of R-phycocyanin by 14 °C. R-phycocyanin was immobilised in an alginate gel, and this system detected Hg²⁺ in spiked drinking water at 2 ppb. 0.35 mg of immobilised R-phycocyaninreduced Hg²⁺ levels in drinking water by approximately 25%, over 2 h incubation. These findings highlight the strong potential of R-phycocyanin for dual functionality: the sensitive detection and removal of Hg²⁺ from contaminated water systems.