Browning' is a phenomenon that has received little attention and refers to the brown discolouration of water. It is caused by the release of fulvic acids and humic substances from the decomposition of plant material, facilitated by increasing inputs of organic matter and rising temperatures due to climate change. The effects on aquatic life have now been comprehensively analysed for the first time - with surprising results.
An international team of researchers from the University of South Bohemia in Budweis, the Research Institute for Farm Animal Biology (FBN) in Dummerstorf and the University of Veterinary Medicine in Hanover, Germany, has investigated how rainbow trout respond to increased concentrations of fulvic acid. The results of the study have recently been published in the prestigious 'Journal of Hazardous Materials'.
Molecular defence reactions detected in fish
Young rainbow trout, an important and popular food fish worldwide, were exposed in the laboratory for four weeks to two different concentrations of fulvic acid - comparable to those found in some natural waters today. While the low concentration of 5 mg carbon per litre showed little change, the higher concentration of 50 mg carbon per litre showed clear effects on the trout: Although there was still no visible external or histological damage to the gills, at a molecular level the researchers sounded the alarm.
A total of 34 genes in the gill tissue of the fish reacted to the increased fulvic acid load,' explains Dr Alexander Rebl of the Fish Genetics Working Group at the Research Institute of Farm Animal Biology (FBN) in Dummerstorf. These genes play a central role in detoxification, oxidative stress and immune defence. The activation of the aryl-hydrocarbon receptor signalling pathway, which plays an important role in the recognition and processing of harmful substances, was particularly striking," continues Rebl.
Chemical stress: What browning means for fish
The results show that browning is not just an aesthetic change to our waters. Rather, it triggers a stress response in fish that indicates an active defence against chemical stress. The consequences could be far-reaching: constant stress costs the animals energy, weakens their health in the long term and could make them more susceptible to disease.
We need to take the browning of water bodies seriously as an environmental problem. Even if we don't see any external damage yet, our results clearly show that fish already react to low levels of pollution,' says Dr Thora Lieke, a researcher at the University of South Bohemia. Now we need to look more closely at the long-term effects on fish populations and the whole aquatic ecosystem,' she continues.
Important basis for environmental protection measures
The results of the study provide an important basis for better assessing the condition of our freshwater systems and developing timely protection measures. Future studies must increasingly aim to better understand the long-term effects of browning on aquatic life and thus protect aquatic ecosystems in the long term. A comprehensive understanding of the effects of climate change and human impacts on our water bodies is essential. This will not only benefit animals and the environment, but also consumers, as healthy waters are the basis for a sustainable and safe food supply. Measures to combat browning therefore also ensure the long-term quality of popular food fish such as rainbow trout.
Image 1: 'The rainbow trout is not only a popular food fish, but also an important indicator of water health. Their response to fulvic acid provides clues to the hidden dangers of browning' © Tom Goldammer/FBN
Image 2: 'Molecular analyses of fish samples provide important insights into the stress responses of rainbow trout. Dr Alexander Rebl (FBN, left) and Dr Thora Lieke (University of South Bohemia) working together in the laboratory. © Pavel Šablatura/University of South Bohemia
Photo 3: Scientist Dr Thora Lieke explaining to Dr Alexander Rebl the experimental conditions under which the trout were exposed to different concentrations of fulvic acid. © Pavel Šablatura/University of South Bohemia
About the FBN:
The Research Institute for Farm Animal Biology (FBN), based in Dummerstorf near Rostock in the federal state of Mecklenburg-Western Pomerania (formerly the Research Institute for the Biology of Farm Animals), was founded in 1993 and is dedicated to application-oriented basic research in the field of farm animal biology. Around 300 employees carry out interdisciplinary research and work in the four focus areas: Individualising farm animal husbandry, farming animals in sustainable resource cycles, coping with critical life phases of farm animals and promoting diversity in animal farming. The aim is to research responsible animal farming as an indispensable component of sustainable agriculture. The animal is the focus of scientific research, from the genome to metabolism and behaviour.
Press and public relations
Jan Ostermayer
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