Molecular complexation properties of Cd²⁺ by algal organic matter from Scenedesmus obliquus

A detailed understanding the metals binding with algal organic matter (AOM) is essential to gain a deeper insight into the toxicity and migration of metals in algae cell. However, the molecular complexation mechanism of the metals binding with AOM remains unclear. In this study, cadmium ion (Cd²⁺) binding properties of AOMs from Scenedesmus obliquus, which included extracellular organic matter (EOM) and intracellular organic matter (IOM), were screened. When Cd²⁺ < 0.5 mg/L, the accumulation of Cd²⁺ could reach 40%, while Cd²⁺ > 0.5 mg/L, the accumulation of Cd²⁺ was only about 10%. EOM decreased gradually (from 8.51 to 3.98 mg/L), while IOM increased gradually (from 9.62 to 21.00 mg/L). The spectral characteristics revealed that IOM was richer in peptides/proteins and had more hydrophilic than EOM. Both EOM and IOM contained three protein-like components (containing tryptophan and tyrosine) and one humic-like component, and their contents in IOM were higher than that in EOM. The tryptophan protein-like substances changed greatly during Cd²⁺ binding, and that the tryptophan protein-like substances complexed to Cd²⁺ before tyrosine protein-like substances in IOM was identified. Moreover, the functional groups of N-H, O-H, and CO in AOM played an important role, and the N-H group was priority to interacts with Cd²⁺ in the complexing process. More functional groups (such as C-O and C-N) were involved in the metals complexing in EOM than in IOM. It could be concluded that Cd²⁺ stress promoted the secretion of AOM in Scenedesmus obliquus, and proteins in AOM could complex Cd²⁺ and alleviate its toxicity to algal cell. These findings provided deep insights into the interaction mechanism of AOM with Cd²⁺ in aquatic environments.