Picture by David Fitzgerald

Low concentrations of oxygen dramatically changes the toxicity of a metal pollutant to fish embryos

Low concentrations of oxygen in water systems, hypoxia, results in dramatic changes in how toxic copper is to fish embryos, new research has shown.

Hypoxia is one of the most significant threats to aquatic life worldwide, with strong effects on biodiversity. Low oxygen occurs naturally in water systems, but in recent decades this has been increasing globally due to increased nutrient input into water systems and alterations associated with climate change.

Typically, hypoxic water systems, receiving nutrient-rich effluents, are also affected by other environmental stressors including chemical pollution. Fish populations, therefore, are often affected by hypoxia in combination with other pollutants. Among these is copper, an important contaminant which is present in many UK rivers, lakes and coastal waters.

The researchers set out to investigate the consequences of exposure to chemicals in waters that contain low oxygen using zebrafish embryos as a model system. Surprisingly, they found that low oxygen in the water protected embryos during the first day of development from the toxic effects of copper. This effect was reversed after embryos hatched, with hypoxia significantly increasing copper toxicity during this developmental stage.

These surprising results change our views of how chemicals interact with other stressors in aquatic systems and demonstrate that understanding the impacts of each stressor alone is not sufficient to predict what will happen when they are acting in combination. Much more information is required on how stressors interact to cause effects in living organisms, in order to accurately predict the consequences of pollution in environments already challenged by other stressors.

Jennifer Fitzgerald, a PhD student in Biosciences and lead author of the paper, said: “Our research shows the surprising effects of hypoxia on copper toxicity in zebrafish embryos, which are dependent on the stage of development. We showed that the processes involved in the response to hypoxia in these young embryos served to protect them from the toxic effects of copper, but later in life, when embryos are less able to tolerate hypoxia, combinations of these two stressors resulted in a significant increase in copper toxicity.”

Dr Eduarda Santos, the senior author in this study, said: “The very strong alterations in copper toxicity caused by hypoxia exemplify the importance of considering the concentration of oxygen in the environment when defining the impact of chemical exposures on aquatic organisms. These findings challenge our understanding of the current and future impacts of worldwide oxygen depletion on fish communities already affected by anthropogenic toxicants.”

This project was funded by the Centre for Environment, Fisheries and Aquaculture Science and the University of Exeter.

Hypoxia suppressed copper toxicity during early development in zebrafish embryos in a process mediated by the activation of the HIF signalling pathway is published in the journal Environmental Science & Technology.

Date: 21 April 2016

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