Effects of extracellular polymeric substances on the bioaccumulation of mercury and its toxicity toward the cyanobacterium Microcystis aeruginosa

Abstract

This investigation examines how extracellular polymeric substances (EPSs) and environmental factors affect the bioaccumulation and toxicity of inorganic mercury (+2 oxidation state, Hg(II)) using a culture of Microcystis aeruginosa, which dominates eutrophic reservoir populations. The identified EPSs were classified as carbohydrates and proteins. Evaluation of the bioaccumulation of Hg(II) in cells by multiple regression analysis reveals that the concentration of EPSs in filtrate, the initial concentration of Hg(II) in medium, and the culture age significantly affected the amount of Hg(II) accumulated. Composition profiles revealed that the concentrations of soluble carbohydrates were significantly higher in Hg(II)-accumulated cells than in the control ones. Preliminary results based on scanning electron microscopic (SEM) map investigations suggest that most of the Hg(II) was accumulated in the cytoplasm (intracellular). Additionally, the effective concentrations (EC₅₀) of Hg(II) that inhibit the growth of M. aeruginosa were 38.6 μg L⁻¹ in the logarithmic phase and 17.5 μg L⁻¹ in the stationary phase. As expected, the production of more EPSs in the logarithmic phase typically implies higher EC₅₀ values because EPSs may be regarded as a protective barrier of cells against an external Hg(II) load, enabling them to be less influenced by Hg(II).

Keywords:

• Inorganic mercury
• Microcystis aeruginosa
• extracellular polymeric substances
• bioaccumulation
• toxicity

Acknowledgments

The editorial assistance of Ted Knoy is appreciated.

Additional information

Funding

The authors would like to thank the National Science Council of the ROC, Taiwan, for financially supporting this research under contract no. NSC101-2221-E-241-008-MY1.