Cadmium-Induced Physiological Responses, Biosorption and Bioaccumulation in Scenedesmus obliquus

Cadmium ion (Cd²⁺) is a highly toxic metal in water, even at low concentrations. Microalgae are a promising material for heavy metal remediation. The present study investigated the effects of Cd²⁺ on growth, photosynthesis, antioxidant enzyme activities, cell morphology, and Cd²⁺ adsorption and accumulation capacity of the freshwater green alga Scenedesmus obliquus. Experiments were conducted by exposing S. obliquus to varying concentrations of Cd²⁺ for 96 h, assessing its tolerance and removal capacity towards Cd²⁺. The results showed that higher concentrations of Cd²⁺ (>0.5 mg L^-1) reduced pigment content, inhibited algal growth and electron transfer in photosynthesis, and led to morphological changes such as mitochondrial disappearance and chloroplast deformation. In this process, S. obliquus counteracted Cd²⁺ toxicity by enhancing antioxidant enzyme activities, accumulating starch and high-density granules, and secreting extracellular polymeric substances. When the initial Cd²⁺ concentration was less than or equal to 0.5 mg L^-1, S. obliquus was able to efficiently remove over 95% of Cd²⁺ from the environment through biosorption and bioaccumulation. However, when the initial Cd²⁺ concentration exceeded 0.5 mg L^-1, the removal efficiency decreased slightly to about 70%, with biosorption accounting for more than 60% of this process, emerging as the predominant mechanism for Cd²⁺ removal. Fourier transform infrared correlation spectroscopy analysis indicated that the carboxyl and amino groups of the cell wall were the key factors in removing Cd²⁺. In conclusion, S. obliquus has considerable potential for the remediation of aquatic environments with Cd²⁺, providing algal resources for developing new microalgae-based bioremediation techniques for heavy metals.