Microplastics in Ocean: A troublemaking traveller

Plastic pollutants are rising alarmingly in aquatic and terrestrial environments. Every year, eight million metric tons of plastics - the equivalent of a garbage truck of plastic waste every minute - are dumped into the ocean. The United Nations Environment Programme recently estimated that the ocean would have more plastics than fish by 2050.

To condense the ongoing research, Ravidas Krishna Naik from the NCPOR and collaborators from the Goa University and Parvatibai Chowgule College of Arts and Science, Goa, reviewed microplastic pollution in the global ocean. They focussed on plastics’ interaction with the marine biosphere and how ballast waters contribute to it.  

Plastic fishing nets are often the culprits behind entangling, injuring and killing marine organisms accidentally. Plastics and their harmful additives contribute to chemical pollution. Also, the particles of plastics provide a surface for the sorption of other harmful chemicals and transport them as carriers.    

Plastics of micro size, those less than 5 millimetres, can enter a wide array of marine life with serious implications. The researchers elaborated studies on how microplastics bioaccumulate through the food web by direct ingestion in marine organisms. Microplastics also lead to adaptive and behavioural changes in marine organisms, impacting feeding, reproductive and photosynthetic ability.

Not limited to the source regions, plastics carry phytoplankton, worms, and bacteria attached to the surface and spread across different ecosystems. This lead to a bigger environmental concern- bioinvasion, where foreign species invade the local ones and sometimes lead to fouling. Among the list, pathogens transported through microplastics are a big threat.

Plastics also serve as a feed for plastic-degrading microorganisms that have emerged in our ecosystems. Plastics are also found inhabiting antibiotic-resistant bacteria, for which medical waste is the major cause. Antibiotic-coated microplastics travelling across the oceans help bacteria to adapt and evolve to be resistant. The researchers found several studies reporting the co-occurrence of metal and antibiotic-resistant microorganisms on the microplastic surface.

But what could have helped bacteria to develop such duel resistance?

The team thought that ballast waters could be one of the pathways. Ballasting and deballasting are regular processes used in cargo ships to maintain the ship’s stability. Ballasting is the pumping-in of seawater in the tanks when the cargo is less. And deballasting - the removal of seawater from the tanks - is done according to the cargo load. However, ballasting at one place and deballasting at another is a way plastics spreads in the ocean.

With this review, the researchers emphasized the unaccounted microplastic pollution through ballast waters. Ballast tanks provide a transient home to microorganisms, especially biofilm-forming ones. Ballast water has been reported to spread globally Vibrio Cholerae- the Cholera-causing pathogen. The team revealed that there are large gaps in understanding microplastic pollution caused by ballast waters and its impacts.

There are certain existing methods to manage ballast waters. One is ballasting in the open sea to prevent picking up polluted waters from the coast. Another one has a water treatment system on board for filtration and disinfection. However, particles may interfere with the disinfection process, and often biofilm-forming microorganisms are resilient to such treatments. Thus, the existing methods look insufficient. So, the team emphasised proper regulations and mitigation strategies to control microplastic pollution through ballast waters.

Taking a step forward, the researchers teamed up with IIT Kharagpur to put forward an idea to regulate the microplastic spread through ballasting and deballasting. They took a model of a ballast water treatment system which filters seawater through a 50-micrometre mesh before filling it in the tanks. After filtration, the water is UV treated to kill microorganisms before storing the water in ballast tanks. While filtering, when the mesh is clogged, the filter is cleaned by backflushing the water, which now contains living and non-living matter greater than 50 micrometres. This backflushed water is discarded through a separate pipeline to avoid interrupting the main ballasting process. 

The researchers thought, ‘why not remove microplastics from the backflushed waters, stopping them from returning to the ocean’. They proposed a three-layered filter containing 500, 300, and 100 micrometres meshes through which the backflushed water should be passed sequentially before discharging. They claim that this simple technique can help remove up to 204 metric tonnes of microplastics per day globally. For this technique, team has filed for a patent.

Even though there is too much anthropogenic involvement in plastic pollution, the main pathways of its spread from the highly polluted low-latitude regions to the polar regions are through atmospheric and oceanic circulations.

 Schematic showing transport and spread of plastics in the ocean through ballast waters (Scenario 1), mitigation via filtration (Scenario 2) and the proposed 3-layered chamber (Scenario 3)

Recently, Shabnam Choudhary and team from the NCPOR, in collaboration with IIT Kanpur, investigated microplastics in the sediments of Kongsfjorden and Krossfjorden, two adjacent fjords in the Arctic. Fjords are technically high-latitude estuaries connected to the ocean at their mouth and glaciers at their head.

The team removed the organic matter from the sediment samples and fractionated different sizes of samples by sieving. They analysed these fractions under a microscope to pick up microplastic particles, noted their colour, and characterised them using Fourier Transform Infrared Spectroscopy. Fibres dominated microplastics in the samples.

The team found polyethylene and polypropylene followed by polyvinylchloride and nitrile, in the sediments. The microplastic particles were majorly of small size ranging from 0.3 to 1 millimetres, which might have resulted from fragmentation of plastic debris during transportation, say the researchers.

The team found mostly white-coloured microplastic particles in the samples followed by blue ones. With spectral analysis, they deduced that most of the white and blue particles were polyethylene, which might have come from carry bags, packaging material, and fishing lines. Polypropylene fibres are mostly found in blue and red colour, and can mainly come from packaging, fishery, and textile industries. Multicoloured particles could have come from ropes, fishing gears and clothing material. For the less abundant black-coloured microplastics, tyres' abrasion and wear and tear could be the source, say the researchers.

Using the distribution of microplastic particles, the researchers evaluated the impact of microplastics on the fjord environment by estimating pollution indices. The team calculated the pollution load index, which is the ratio of the microplastic concentration at each location and the lowest concentration detected in the samples. They also estimated the microplastic impact coefficient as the ratio of the amount of a specific form of microplastics and the total amount of microplastics at a location. From these indices, the researchers found that the microplastics in the Kongsfjorden and Krossfjorden sediments pose a moderate pollution risk.

The team concluded that microplastics might have been transported from lower latitudes via the West Spitsbergen current, which originates from the north Atlantic Ocean. ‘Wind, glacial and sea ice melt, and increased local anthropogenic activities might also have brought microplastics to this region of the Arctic’, say the researchers.

So, even the remote polar regions are not free of this menace. The need of the hour is to expedite the measures to regulate pollution and ultimately develop a better eco-friendly alternative.  This requires the efforts of all - scientists, policymakers, and governing bodies. And also equally important efforts from the people.