Extracellular Polymeric Substances Protect Chlorella sp. Against the Cadmium Stress

Extracellular polymeric substances (EPS) are secreted by microalgae and contribute to protecting cells from damage induced by cadmium (Cd) exposure. However, the response mechanism of Chlorella sp. to Cd(II) stress as well as associated changes in the chemical properties (including functional groups and composition) of soluble EPS (SL-EPS), loosely bound EPS (LB-EPS), and tightly bound EPS (TB- EPS) in this microalga, remain unclear. This study aimed to investigate the role of EPS in enabling Chlorella sp. to resist Cd(II) stress. The results demonstrated that Cd(II) stress resulted in a significant inhibition of algal, chlorophyll a (Chl a) contents, and maximum photochemical quantum yield (Fv/Fm) of Chlorella sp., with 7 d EC30 of 6 mg/L. Nevertheless, Cd(II) exposure significantly increased both superoxide dismutase (SOD) activity and EPS content. Fourier transform infrared (FTIR) spectroscopic analysis revealed that differences existed in the functional groups involved in Cd(II) binding across algal cell density, SL-EPS, LB-EPS, and TB-EPS. The carboxyl group was identified as the most prominent functional group and were found to play a crucial role in the adsorption of Cd(II). Additionally, Tryptophan-like protein substance in EPS may be the main component binding with Cd(II) in Chlorella sp. This study indicated that Chlorella sp. resisted Cd(II) stress by increasing SOD activity and EPS content, with protein-like substance containing tryptophan proteins in EPS which could also contribute to protection against Cd stress.