Characterization of extracellular polysaccharide/protein contents during the adsorption of Cd(II) by Synechocystis sp PCC6803

Cyanobacteria have been proven to be cheaper and more effective for the removal of metallic elements in aqueous solutions. In this study, the living cyanobacteria Synechocystis sp. PCC6803 was used to adsorb Cd(II) and its extracellular polymeric substances (EPS) were investigated in the adsorption process. The initial stage of adsorption of Cd(II) was a rapid process, and then increase slowly accompanied with the increases of biomass. The final adsorption percentage could achieve 86% when the Cd(II) concentration was 0.5 mg/L. It proved that Synechocystis sp. PCC6803 has a good adsorption capacity for heavy metal ions. EPS was extracted to investigate the secretion of which was dynamic and the maximum extracellular polysaccharides and proteins were 134.2 and 100.9 mg/g, respectively. Furthermore, the real-time PCR (RT-PCR) results of genes (slr0977 and exoD) involved in EPS synthesis and secretion indicated that the EPS production was firstly increased and then decreased slightly. Transmission electron microscope (TEM) observation revealed that heavy metal ions were absorbed into EPS layer. Fourier transform infrared spectrum (FT-IR) analysis showed that EPS was rich in functional groups which could combine with heavy metal ions, such as -OH and -NH groups. All the results obtained show that the secretion of EPS by cyanobacteria was one of the ways to resist heavy metal stress. And it shows a trend of rising first and then decreasing, the change regulation of which was consistent with adsorptive behavior.