Sleeping with the Fishes — Decay in Wastewater Damages Aquatic Life

Everything from disposed of drugs to hormones and disease-causing bacteria — anything that is rinsed or flushed down the drain — can contaminate wastewater.

A group of researchers from the University of Waterloo in Canada was the first to measure the effects of waste-contaminated water on the metabolism of fish. They found that fish from water contaminated with wastewater effluent breathe faster and require more energy than other fish — energy they could use elsewhere. And they found the bacteria on dead and decaying fish tells a tale of what they've been exposed to.

The research study, led by Paul Craig was presented July 6 at the Society for Experimental Biology Annual Main Meeting in Gothenburg, Sweden.

What's a Necrobiome?

The world (and people, animals, and plants in it) hosts all sorts of microbiomes — communities of microorganisms living naturally in and on our other living things. Bacteria, viruses, and fungi co-exist on our skin, in our gut, and other places to digest food, defend against disease-causing microbes and perform important roles in metabolism.

The organisms that make up a healthy microbiome strike a delicate balance. They work together to do their jobs and maintain a population of 'good' bacteria. Even the often-maligned Propionibacterium acnes bacteria — blamed for acne — has a role in keeping our skin cleared of cellular debris and oil. When one microbe — such as Clostridium difficile in the gut — takes over, the imbalance can turn a happy environment into a devastating infection.

The term necrobiome is used to describe the unique community of bacteria associated with dead and decaying organisms. It yields a wealth of information about the environment the organism lived in, died in, and contributes to the eventual decomposition of the organism. During decomposition, microbes from the environment perform a natural biological degradation. They break down and recycle a body's molecular and cellular components into valuable nutrients, like minerals and carbon, and return them to the environment. The organisms that participate in this process can even help determine investigators determine time of death.

The study by Craig has brought light to how the organisms in the water that fish live in contribute to their necrobiome and help bring about their death.

Effects of the Necrobiome on Fish

Measuring the metabolism of a rainbow darter in a respirometry chamber.

The scientists examined the metabolic performance of rainbow darter fish in clean and effluent-influenced waters from the Grand River watershed, along with the impact of pathogenic bacteria derived from their necrobiome.

Metabolism rate measurements were made on fish using custom-made chambers that measured their breathing. Fish exposed to wastewater had higher metabolic rates, measured by greater oxygen need and enzymes associated with metabolism than fish not exposed to water contaminants. Craig pointed out that more energy required to maintain higher metabolic rates means less energy to find food, evade predators, and attract mates.

The researchers could reverse the effects on the fishes' metabolism by returning them to clean water for a week. They felt the increase in metabolic rate was probably the result of an immune response the fish mounted to try to prevent an infection with disease-causing bacteria in the wastewater — a battle they would likely lose if they were in the wild.

Wastewater from the Grand River site was found to have different kinds of human pathogenic bacteria in it. Clostridium perfrigens, Aeromonas veronii, and Eubacterium tarantellus were all present in the wastewater. They all are known to cause sickness, and the researchers found them in the necrobiome of dead fish in the study.

The necrobiome itself is associated with the decaying fish, however, the type of bacteria that become enriched as the fish decays is attributed to whatever the fish was exposed to while it was alive — such as wastewater or the natural, inherent microbiome that is found in the gut. While wastewater is very low in pathogenic bacteria, a way of identifying and enhancing this is through the necrobiome of organisms exposed to wastewater.

The researchers will have a unique opportunity to compare fish from the same sites after the completion of planned upgrades to wastewater facilities. They are hopeful that the improvements will help reduce levels of pollutants and pathogenic bacteria coming from these facilities into waterways and minimize their impact on fish.

Death and dying are natural processes that scientists continue to dissect for all the information it can give us. Craig's study has just given us insight into how the actions of humans influence those processes in fish exposed to the waste of our daily life.

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Cover image via Kristian Bjornard/Flickr