Defense pathways of Chlamydomonas reinhardtii under silver nanoparticle stress: Extracellular biosorption, internalization and antioxidant genes

Numerous studies have been investigated the toxic effects of silver nanoparticle (Ag-NPs) on algae; however, little attention has been paid to the defense pathways of algae cells to Ag-NPs. In the study, Chlamydomonas reinhardtii (C. reinhardtii) was selected as a model organism to investigate the defense mechanisms to Ag-NPs exposure. The results showed that exopolysaccharide and protein in bound-extracellular polymeric substances significantly increased under Ag-NPs stress. These metal-binding groups including C-O-C (exopolysaccharide), CH3/CH2 (proteins), O-H/N-H (hydroxyl group) and C-H (alkyl groups) played a key role in extracellular biosorption. The internalized or strongly bound Ag (1.90%-17.45% of total contents) was higher than the loosely surface biosorption (0.31%-1.79%). The accumulation of glutathione disulfide (GSSG), together with the decline of reduced glutathione/GSSG (GSH/GSSG) ratio in C. reinhardtii cells, indicated a significant oxidative stress caused by exposure of Ag-NPs. The increasing phytochelatin accompanied with the decreasing GSH level indicated a critical role to intracellular detoxification of Ag. Furthermore, upregulation of antioxidant genes (MSOD, QTOX2, CAT1, GPX2, APX and VTE3) can cope with oxidative stress of Ag-NPs or Ag+. The up-regulation of ascorbate peroxidase (APX) and glutathione peroxidase (GPX2) genes and the reduction in GSH contents showed that the toxicity of Ag-NPs could be mediated by an intracellular ascorbate-GSH defense pathway. These findings can provide valuable information on ecotoxicity of Ag-NPs, potential bioremediation and adaptation capabilities of algal cells to Ag-NPs.