Arsenic (As) contamination of groundwater and its health impacts have become a global-environmental and health threatening issue due to access of toxic As into the drinking water through naturally leaching from rocks, mining and other industrial processes. This carcinogenic metal(loid) mostly exist in the environment as toxic inorganic arsenate (As^V) and arsenite (As^III). Therefore, remediation of As pollution in water has been studied intensively.

Microalgae widely exist in aquatic environments and their extracellular polymeric substances (EPS) contribute to dissolved organic matter (DOM). These macromolecules encompass many functional groups such as carboxyl, phosphoryl, hydroxyl and sulfhydryl, which may affect the mobility and transformation of heavy metal(loid)s through various interactions. However, more investigations are still needed to reveal how EPS influences the As accumulation and speciation in microalgae. In this study, we hypothesized that EPS could modify the As metabolism in the microalgal cells. In order to verify this hypothesis, the effect of As on the EPS synthesis of one cyanobacteria, Synechocystis PCC6803 (PCC) and one green microalga, Chlamydomonas reinhardtii was investigated. In addition, we determined the As adsorption, absorption and speciation in the two strains with and without EPS upon different As treatments (Synechocystis PCC6803: 0-500 µM As^III and As^V; C. reinhardtii: 0-10 mg L^-1 As^III and 0-200 µg L^-1 As^V). Results showed that the EPS yield of Synechocystis PCC6803 was significantly increased with addition of As in the medium and the amount was significantly higher in response to As^(III) than As^(V) treatment. The cyanobacterial cells with EPS accumulated up to 8 % and 30 % more As^(III) and As^(V) than the EPS-removed cells, along with absorption capacity of 596.57 µg g^-1 DW of As^(III) and 751.17 µg g^-1 DW of As^(V) respectively. Adsorption and oxidation/reduction of As^(III) and As^(V) by the PCC6803 cells were also significantly higher in the presence of EPS. In comparison, there were no significant impact of EPS on the concentration of As in the C. reinhardtii cells. The synthesis of EPS was not affected by the As treatments either. However, after the removal of EPS, As adsorption on the cell surface increased and much less As^V was reduced to As^III. Fourier transform infrared (FT-IR) analyses revealed that the EPS of both strains was enriched with functional groups such as C-O-C, NH, and OH, suggesting that the EPS could contribute to accumulation and redox transformation of As in the microalgal cells and they have potential to be applied in the As bioremediation in aquatic environments. Nevertheless, the roles of individual components of EPS (e.g., polysaccharides, proteins, etc.) in the As accumulation and transformation and mechanism for the EPS synthesis in the microalgae under As stress still require further studies.