Upendra Nongthomba, Professor in the Department of Molecular Reproduction, Development and Genetics (MRDG) at the Indian Institute of Science (IISc.), would cherish going to the backwaters of the Krishna Raja Sagar (KRS) dam and having fried fish on the bank of Cauvery river. But in recent times, he has been noticing physical deformities in some of the fish and began to wonder whether it has something to do with the quality of water.
A new study led by Prof. Nongthomba, published in the journal Ecotoxicology and Environmental Safety, reveals that pollutants like microplastics may be causing growth defects in fish, including skeletal deformities, in the Cauvery river.
“Water is essential for everyone, including animals and plants. When it is polluted, it is capable of causing diseases, including cancer,” said Abass Toba Anifowoshe, a PhD student in Prof. Nongthomba’s lab, and the first author of the study.
Prof. Nongthomba’s lab 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, the team analysed the physical and chemical parameters of the water samples. All but one parameter were within the prescribed limits. The exception was dissolved oxygen (DO), whose levels were much lower than they needed to be in samples collected from the slow-flowing and stagnant sites. Water from these sites also had microbes such as Cyclops, Daphnia, Spirogyra, Spirochaeta and E. coli, well-known bio-indicators of water contamination.
Using a technique called Raman spectroscopy, the team detected microplastics – minute pieces of plastic often invisible to the naked eye – and toxic chemicals containing the cyclohexyl functional group (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, they investigated whether pollutants in water could account for the developmental abnormalities seen in wild fish. They treated embryos of the well-known 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 was seen 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 dissolved oxygen (D)), which in turn triggers ROS accumulation in animals like fish.
As millions of people are dependent on Cauvery river water and a recent study from the Netherlands has shown that microplastics can enter the bloodstream of humans, Prof. Nongthomba said, “The concentrations we have reported may not be alarming yet for humans, but long-term effects can’t be ruled out.” he said.
But before they can answer the question conclusively, they need to understand how exactly microplastics enter and affect the host. “This is something which we are trying to address now,” Prof. Nongthomba said.
