- A study has found microplastics and colorants (dyes and pigments), probably of human origin, inside Asian green mussels collected from Kasimedu harbour in Chennai.
- The microplastics were identified mainly as polystyrene polymers of the size 30 micrometers and thread-like fibres (ranging from 5 to 25 micrometers) while the colourants ranged from 62 to 103 micrometers and were orange, green, dark red and light blue.
- It is likely the microplastics have been ingested by the filter-feeding mussels because they are of the same size range as their food particles.
- More research is needed to determine the effects of microplastics on the mussels and their rate of transfer in the food web.
Microscopic plastic particles and colourants have been detected inside the commercially important Asian green mussel Perna viridis from a fishing harbour in Chennai for the first time in a preliminary study.
“Although, several research articles have reported the presence of microplastics even in market bivalves/fishes of natural coastal waters in Europe and Japan, the present results are surprising as this has been reported for the first time from India,” said S.A. Naidu, a project scientist at Chennai’s National Centre for Coastal Research (NCCR), Ministry of Earth Sciences and author of the paper. “Chennai harbour region is influenced by industrial, domestic and other land-based sources of microplastics.”
The sheer scale of plastic pollution in the oceans is staggering: scientists estimate over 5.25 trillion pieces of plastic inundating the oceans. But while large plastic debris floating in the seas and oceans—that are increasingly being found in alarming quantities inside marine life such as sea turtles and whales, for example—can be spotted easily and scooped up, tiny bits of plastic, which are invisible to the naked eye, remain a pressing concern.
Plastic waste dumped on land ends up in the ocean through river discharge as well as domestic and industrial waste. In the sea, ship spillages, fishing trawlers, and coastal gas platforms also leave behind a trail of plastic debris.
Over time, much of the debris undergoes fragmentation and degradation into tiny particles, known as microplastics (less than 5 mm), by physical forces such as the waves and currents, ultraviolet radiation and microbial breakdown. Microplastics can remain afloat or sink to the seafloor and accumulate in sediments depending on their density. They can be ingested by both suspension and filter-feeders.
As part of the “Marine Litters and Microplastics” programme of the NCCR, Naidu collected five mussels from three locations in Kasimedu fishing harbour of Chennai coast, which receives plastic debris through various sources such as fishing boats, industrial and domestic sewage, and discharge from rivers.
Microplastics probably ingested by filter-feeding
After carefully removing the soft tissues of the mussels, he examined them directly under the microscope. He also treated the soft tissues with acid to digest them and isolate the microplastics. Using Raman spectroscopy, a method that determines the type of plastic from their characteristic spectral bands, Naidu deduced that the microplastics in the soft tissues were mainly polystyrene polymers around 30 micrometers in size. He also observed thread-like fibres ranging from 5 to 25 micrometers.
Some coloured particles did not match any kind of known plastics in the Raman spectral library, suggesting they could be colourants—dyes and pigments. These microscopic colourant particles ranged from around 62 to 103 micrometers and were found in orange, green, dark red and light blue colours.
Naidu believes the colourants are probably of human origin. They could have come from textiles, synthetic paints, paper printing, food, ship, idol-making and cosmetics, among other industries. A study estimated that over 10,000 different dyes and pigments are used industrially and in 2010, India produced around 200,000 tonnes of dyes, according to one estimate. Some of these dye effluents are discharged into the sea.
It is likely that the microplastics and colourants have been ingested, believes Naidu, because mussels are filter feeders. A standard mussel filters on average 24 liters of water per day. “When the organisms sieve water through the inhalant siphon across the complexly folded gills, planktons and other food particles are retained but along with them, non-food microplastics which are microscopic-sized and unwanted are also retained inadvertently,” he explained.
After the Chennai floods in November 2015, a team of scientists observed a threefold increase in microplastic pellets along the coast of Chennai, providing evidence for the transport of a large quantity of microplastics from land into the sea through the Cooum and Adyar rivers.
Tracy Mincer, assistant professor at Florida Atlantic University, is not surprised by the presence of microplastics in the Asian green mussel in this study. “The plastic particles were in the size range that is typically found in marine aggregates,” he said. In fact, he pointed out that his team also found the same patterns in a 2018 study conducted on the widely consumed blue mussel Mytilus edulis collected from the U.S. northeast coast. “The diversity of plastic materials was similar to our findings,” he noted.
“With our study, we looked at not only presence but clearance rates of plastic debris and found that for the most part, plastic tends to be expelled in pseudofeces and feces of Mytilus edulis,” added Mincer.
Does the plastic affect mussels and humans?
“As for the impact these plastics may have on benthic animals [organisms living near the sea bed], it seems likely that these plastic particles are putting a burden on certain suspension and filter-feeding organisms but more work is needed to understand the risks,” said Mincer. For example, “are the plastics being incorporated into animal tissue over time? And if so, do these plastic particles biconcentrate up the food chain?”
“The bivalve mussels of the fishing harbour waters are not generally fit for human consumption anywhere,” said Naidu, adding that the study was mainly conducted to see if microplastics were found in the soft tissues of the mussels.
“In future, it is proposed to take up the study of mussels in different environments along the Indian coast for assessing the status and comparison between polluted and unpolluted areas,” said NCCR Director M.V. Ramana Murthy and Pravakar Mishra, Project Leader of the “Marine Litters and Microplastics” program. “Studies on the accumulation rates and residence time of microplastics in organisms are needed to understand the transfer rates of microplastics across the food web.”
Recognizing the severity and dangers of plastic pollution, the state of Tamil Nadu had banned the sale of plastic bags and cutlery starting January this year, following similar bans in the states of Himachal Pradesh, Goa, and Maharashtra.
Naidu, S.A. (2019). Preliminary study and first evidence of presence of microplastics and colorants in green mussel, Perna viridis (Linnaeus, 1758), from southeast coast of India. Marine Pollution Bulletin, 140, 416–422.
Banner image: Microplastics filtered by mussels. Photo courtesy S.A. Naidu/NCCR-MoES Chennai.