Investigation of extracellular polymeric substances (EPS) from cyanobacteria/blue green algae and their interaction with heavy metal ions: Insights from excitation-emission matrix fluorescence spectroscopy

Extracellular polymeric substances (EPS) are the giant polymers composed of polysaccharides, proteins, nucleic and humic acid with remarkable applications. This study was aimed to evaluate the binding efficacies of two EPS extracted from cyanobacterial strains (Microsystis aeruginosa and Microsystis ichthyoblabe) with four heavy metal ions (lead, cadmium, copper, and zinc) using excitation-emission matrix (EEM) spectroscopy with the main focus on the fluorescence properties of protein present in each EPS. Characterization of EPS showed that they were composed of > 90% of organic contents with higher carbohydrates (59.54 and 60.72%) than proteins (26.49% and 30.2%) and lower humic acid contents (8.29% and 6.96%). The amino acids profile revealed tryptophan at maximum (6.28% in EPS-A and 4.26% in EPS-I) followed by tyrosine (5.05% in EPS-A and 3.47% in EPS-I), glutamic acid (4.14% in EPS-A and 2.74% in EPS-I), leucine (3.92% in EPS-A and 2.82% in EPS-I) and phenylalanine (3.14% in EPS-A and 1.97% in EPS-I). The percentages of amino acids recorded were higher in EPS-A than EPS-I. A fluorescence peak at 280/350 nm (Ex/Em) was identified in the fluorescence spectrum for each EPS using EEM spectroscopy. Quenching experiments showed gradual decrease in EPS fluorescence intensity with increased metal concentration due to EPS-metal association. The Stern-Volmer plot displayed static quenching mechanism of metals in deactivating EPS fluorophores. Pb(II) and Cd(II) displayed superior quenching at 50 mg/L whereas Zn(II) and Cu(II) indicated lower quenching at 100 mg/L. The binding potencies of EPS from cyanobacteria with metal ions is a potential green biotechnological approach for toxic metals remediation from aqueous medium.