Microplastics in Cauvery may be harming fish: IISc study

NT Correspondent

Pollutants like microplastics in the Cauvery River may be causing growth defects in fish, including skeletal deformities, according to a study by Indian Institute of Science.

A team of IISc scientists, led by Upendra Nongthomba, Professor at the Department of Molecular Reproduction, Development and Genetics (MRDG), conducted the study, which was published in the journal Ecotoxicology and Environmental Safety.

A fish lover, Nongthomba said, “Over the years, I have cherished going to the backwaters of the Krishna Raja Sagara dam and having fried fish on the banks of Cauvery.” But in recent times, he has been noticing physical deformities in some of them. He began to wonder whether it was due to the deteriorating quality of water.

“Water is essential for everyone, including animals and plants. When it is polluted, it may cause various diseases, including cancer,” adds Abass Toba Anifowoshe, a PhD student in Nongthomba’s lab, and the first author of the study.

The team conducted a comprehensive study of pollution at the KRS Dam and its potential effects on fish. They collected water samples from three different locations with varying speeds of water flow – fast-flowing, slow-flowing, and stagnant – since water speed is known to affect the concentration of pollutants.

In the first part of the study, Nongthomba’s team analysed the physical and chemical parameters of the water samples. Except dissolved oxygen (DO), all other parameters fell within the prescribed limits. DO level at Cauvery was much lower than required in samples collected from the slow-flowing and stagnant sites. The water from these sites also had microbes such as Cyclops, Daphnia, Spirogyra, Spirochaeta and E. coli, the wellknown bio-indicators of water contamination.

The researchers went a step further. Using a technique called Raman spectroscopy, they detected microplastics – minute pieces of plastic often invisible to the naked eye – and toxic chemicals containing the cyclohexyl functional group (a functional group refers to atoms in a compound that determine its chemical properties). Microplastics are found in several household and industrial products, and chemicals containing the cyclohexyl group, such as cyclohexyl isocyanate, are commonly used in agriculture and the pharmaceutical industry.

In the second part of the study, Nongthomba’s team investigated whether pollutants in water could account for the developmental abnormalities seen in fish. They treated embryos of the wellknown model organism, zebrafish, with water samples collected from the three sites, and found that those exposed to water from the slow-flowing and stagnant sites experienced skeletal deformities, DNA damage, early cell death, heart damage, and increased mortality. These defects were seen even after microbes were filtered out, suggesting that microplastics and the cyclohexyl functional groups are responsible for the ailments in the fish.

The researchers also found unstable molecules called ROS (Reactive Oxygen Species) in the cells of the fish that developed abnormally. ROS build-up is known to damage DNA and affect animals in ways similar to what Abass and Nongthomba saw in fish treated with water from the slow-flowing and stagnant sites. Other studies have shown that microplastics and chemicals with the cyclohexyl group lead to decreased DO, which in turn triggers ROS accumulation in animals like fish.

A recent study from the Netherlands has shown that microplastics can enter the bloodstream of humans. So, what do the results from Nongthomba’s lab mean for the millions of people who use Cauvery water? “The concentrations we have reported may not be alarming yet for humans, but long-term effects can’t be ruled out,” he said.

But he also admits that before they answer the question conclusively, they need to understand how exactly microplastics enter and affect the host.