MT-like proteins: Potential bio-indicators of Chlorella vulgaris for zinc contamination in water environment

• The growth of C. vulgaris is seriously inhibited by Zn2+ at above 10 μmol l−1.
• Large amounts of Zn in the algal cells distribute in heat stable fractions.
• Zn-metallothionein-like proteins are a potential bio-indicator for Zn pollution.

Chlorella vulgaris has been usually used to monitor the toxicity of Zn in water environment, but the biochemical responses of the algae to Zn2+ at different levels remain unknown to date. In the present study, the growth inhibition, the antioxidant enzymes, the subcellular Zn and the induced Zn-metallothionein-like (Zn-MT-like) proteins in C. vulgaris exposed to 0, 5, 10, 20, 50 and 100 μmol l−1 of Zn2+ were investigated. Results showed that the growth of C. vulgaris was significantly inhibited (p < 0.05) at Zn2+ level above 5 μmol l−1. Compared with the control group, the activities of peroxidase (GSH-Px), superoxide dismutase (SOD) and catalase mostly increased with the rises of Zn2+ concentrations ranging from 5 to 100 μmol l−1, except for GSH-Px with a lower value in the 100 μmol l−1 group compared with that of the 50 μmol l−1 group, and SOD with no significant difference at the 5, 10 and 100 μmol l−1 groups compared with that of the control group. Contrarily, the activities of guaiacol peroxidase significantly decreased (p < 0.05) at Zn2+ concentrations from 5 to 100 μmol l−1 compared with that of the control group. After 72 h exposure, Zn in the algal cells was observed to mainly distribute in the form of heat stable fractions, in which Zn-binding metallothionein-like (MT-like) proteins were largely biosynthesized. In addition, the amounts of Zn-MT-like proteins induced in the algae significantly increased (p < 0.05) with the rises of Zn2+ concentrations at the assay levels. The results indicated that the activities of antioxidant enzymes and the induction of Zn-MT-like proteins in C. vulgaris may play important roles in dealing with the toxicity of Zn2+ to the algal cells, and the induction of Zn-MT-like proteins in C. vulgaris can be used as a potential bio-indicator to monitor the contamination of Zn2+ in water environment.