Detoxification strategy of Microcystis aeruginosa to the toxicity of Cd(II): role of EPS in alleviating toxicity.

Although many studies have found that cadmium (Cd) can be toxic to microalgae, only a few reports focused on the role of extracellular polymeric substances (EPS) in Cd(II) detoxification. The biochemical and physiological endpoints of Microcystis aeruginosa, including the composition and functional groups of soluble EPS (SL-EPS), loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS), were detected to elucidate the toxicity and detoxification mechanisms of Cd(II) for cyanobacteria. Toxicological and physiological assays on M. aeruginosa showed that the 0.25-mg/L Cd(II) resulted in a larger inhibition on growth and Fv/Fm. Nevertheless, Cd(II) significantly induced much higher contents of superoxide dismutase (SOD), intracellular microcystin LR (MC-LR), extracellular MC-LR, and EPS. Scanning electron microscopy with energy dispersive X-ray spectroscopy confirmed that Cd(II) was absorbed into the EPS layer. Fourier transform infrared spectrum analysis revealed that the functional groups bound with Cd(II) of algae biomass, SL-EPS, LB-EPS, and TB-EPS were somewhat different. The C=O/C=N groups of δ-lactam or protein were their prominent functional groups, suggesting that amide or proteins in the EPS played a key role in the adsorption in Cd(II). The concentration of 0.25 mg/L of Cd(II) may change the chemical structure of EPS by altering the production of protein-like substances containing tryptophan. This study indicated that M. aeruginosa could detoxify Cd(II) stress via induction of antioxidant capacity (higher SOD activity and MC synthesis), EPS production, and modification in chemical structure of EPS. [ABSTRACT FROM AUTHOR]