News Release

Go hard or go home: Scientists show reduced heavy metal toxicity in goldfish using hard water

New study investigates how water hardness affects zinc accumulation and oxidative stress in goldfish

Peer-Reviewed Publication

National Korea Maritime and Ocean University

Controlling water hardness can effectively reduce zinc absorption and toxicity in goldfish

image: Researchers find that increasing the calcium ion concentration in water contaminated with zinc can result in reduced zinc accumulation, toxicity, and stress in the bodies of goldfish exposed to the heavy metal. view more 

Credit: National Korea Maritime & Ocean University

When industrial effluents containing high levels of heavy metals are discharged into fish bodies, they pose a serious threat to aquatic ecosystems. One such heavy metal, zinc, is required by organisms in miniscule amounts, but if it accumulates in higher concentrations, zinc can trigger oxidative stress in the fish body. This causes metabolic, physiological, and cellular damage, including protein degeneration and eventually, cell death. Moreover, humans who consume this fish are also exposed to a high level of zinc, resulting in similarly dangerous consequences for the human body.

While the ideal solution for this problem would be to control the release of effluents into water bodies, there is another way. Maintaining ideal water quality parameters, like hardness (i.e., the level of certain positively charged ions—like calcium and magnesium ions—in water), can inhibit the absorption of heavy metals in fish, and by extension, reduce our exposure to these contaminants.

Evidencing this in a new study published in Volume 11, Issue 5 of Antioxidants on 5 April 2022, scientists from the Korea Maritime and Ocean University (KMOU) describe how zinc absorption and toxic stress in goldfish changes at different concentrations of zinc and water hardness. “Compared to seawater, freshwater has a lower concentration of ions, which increases its risk of heavy metal contamination. Therefore, we wanted to explore the possibility of reducing this toxicity in fish by regulating water hardness,” explains Dr. Cheol Young Choi—Professor at KMOU and lead scientist for this study.

Dr. Choi’s team assessed key antioxidant markers and total antioxidant capacity, which indicate the degree of oxidative stress in the body. The team also conducted terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assays to determine the severity of apoptosis in the liver, where the antioxidant response is most acute. They studied the oxidative stress progression in goldfish exposed to water having increasing zinc concentrations (plus a control group without any zinc exposure) over a period of 14 days. The team also adjusted the hardness of this water with calcium carbonate.

The results showed a significant difference in zinc accumulation and toxicity in fish who had been exposed to different zinc concentrations and water hardness levels. The fish exposed to the highest concentration of zinc showed the most accumulation and oxidative stress over 14 days. However, for the same concentrations and time, zinc accumulation and stress markers were significantly lower among the fish who were kept in hard, or very hard water. Similarly, the TUNEL assays showed more cellular degeneration and death in zinc-exposed fishes, which reduced with increasing water hardness.

This study demonstrates the effectiveness of higher water hardness in reducing the oxidative stress and bioaccumulation of zinc in fishes. “Bioaccumulated heavy metals are hazardous—not only to fish, but also to humans who later consume these fish. Our study proposes an eco-friendly solution to this problem. Hopefully this will pave the way for further research exploring the contribution of other water quality parameters, like temperature and pH, in reducing heavy metal accumulation in aquatic organisms.”, says Dr. Choi.






Authors: Cheol Young Choi1, Zhongze Li1, Jin Ah Song2, and Young-Su Park3


1Division of Marine BioScience, Korea Maritime and Ocean University, Busan

2Marine Bio-Resources Research Unit, Korea Institute of Ocean Science and Technology, Busan  

3Department of Nursing, Catholic University of Pusan, Busan


About National Korea Maritime & Ocean University 

South Korea’s most prestigious university for maritime studies, transportation science and engineering, the National Korea Maritime & Ocean University is located on an island in Busan. The university was established in 1945 and since then has merged with other universities to currently being the only post-secondary institution that specializes in maritime sciences and engineering. It has four colleges that offer both undergraduate and graduate courses.  



About the author

Dr. Cheol Young Choi is a Professor at the Division of Marine Bioscience at Korea Maritime and Ocean University. Dr. Choi completed his PhD from Tokyo University of Marine Science and Technology, and postdoctoral training at Calgary University and National Institutes of Health (NIH), before joining the Korea Maritime and Ocean University.

His group is currently working on endocrine regulatory mechanisms of cleaner shrimp using the wavelength of LED lights, endocrine regulatory mechanisms of the gonadotropin-inhibitory hormone in protandrous cinnamon clownfish, and biomarker development for marine environmental risk assessment by analyzing the expression of oxidative stress-related genes.

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