Enhanced tolerance and resistance characteristics of Scenedesmus obliquus FACHB-12 with K3 carrier in cadmium polluted water

Abstract Microalgae have been recognized as a cost-effective biosorbent for heavy metal pollutants in wastewater, while microalgal biofilms exhibit significant potential in microalgae–water separation post-treatment. In this study, cadmium (II) removal mechanisms of suspended Scenedesmus obliquus FACHB-12 (isolated from southern China) and Scenedesmus obliquus biofilm with K3 carriers were investigated based on their cadmium removal efficiency, pigment content, and antioxidant activity. Scenedesmus obliquus biofilm achieved the highest cadmium (II) removal of 91.27% under 20 mg/L of cadmium stress compared to 87.49% by suspended Scenedesmus obliquus after three days. Under 10 mg/L of cadmium stress, photosynthetic pigments, extracellular polymeric substances, total protein, and antioxidant responses including superoxide dismutase, catalase, and glutathione were measured for both suspended and biofilm forms of Scenedesmus obliquus. During the cultivation process with cadmium solution, temporary inhibition of pigments was observed within the first 24 h and then recovered after two days of exposure to cadmium. Extracellular polymeric substance concentration increased significantly in the biofilm at 34.91 mg/g on the third day of exposure to 10 mg/L of cadmium compared to 22.52 mg/g for suspended Scenedesmus obliquus. Protein levels in both groups increased over time at similar concentrations at the end of the experiment. Antioxidant activity was enhanced during the absorption process, with higher activity observed in the biofilm group. The results suggested that Scenedesmus obliquus biofilm could provide higher cadmium removal efficiency with increased tolerance to cadmium content. This study provides insightful information on the response mechanism of Scenedesmus biofilms under heavy metal oxidative stress.