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Plastic pollution in our oceans could spread fatal diseases into the food chain through seafood

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Plastic pollution in our oceans could spread fatal diseases into the food chain through farmed seafoods such as mussels and oysters, scientists warn.

Microplastics – plastic particles less than 5 millimetres in diameter – can come from larger plastic products that have broken down, textile fibres, cigarette filters or even beauty products. 

Microplastics that enter our waters may act as a long-distance transport mechanism for human and animal pathogens, such as bacteria, to spread into new areas.

Even these ultra-tiny plastic particles encourage the formation of biofilms – a community of microbes that form a slimy layer on the surface.

Molluscs such as mussels and oysters have a filter feeding system that makes them very susceptible to ingesting these contaminated plastic bits.  

This transfer of pathogens from plastic to marine life could wipe out mollusc populations in some regions, or even pass pathogens to human seafood fans. 

Graphic shows the potential microbe–microplastic interactions occurring at the surface of the plastics. Larger biofilms form on plastics compared with natural particles such as zooplankton. The smooth and comparatively large surfaces of plastics at the microscale are suited to pathogenic communities

Graphic shows the potential microbe–microplastic interactions occurring at the surface of the plastics. Larger biofilms form on plastics compared with natural particles such as zooplankton. The smooth and comparatively large surfaces of plastics at the microscale are suited to pathogenic communities

Researchers say knowledge gaps still exist over how ocean microplastics transport bacteria and viruses once they filter into the ocean, and whether this affects the health of humans and animals.  

‘Microplastic fragments differ markedly from natural floating particles, and there is growing evidence that they represent a potential reservoir of pathogens,’ said Dr Ceri Lewis at the University of Exeter’s Global Systems Institute.

Rice grain-sized plastic found in sardine flesh 

Microplastics have been discovered inside every single sample of seafood bought at a market as part of a scientific study

Researchers cut open oysters, prawns, crabs, squids and sardines and studied them for any sign of microplastics. 

Sardines were found to be the worst affected and had ingested the largest amount of plastic, up to 30mg per serving – the same weight as a grain of rice.  

Microplastics are tiny particles which are less than five millimetres (0.2 inches) in length. 

The health impact of humans ingesting these particles remains a concerning mystery. 

The study was led by the University of Exeter and the University of Queensland.  

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‘Of particular concern are the increasing reports of the presence of numerous pathogens on plastic surfaces in oceans around the world.’

There are millions of tons of plastic reaching the world’s oceans every year and trillions of particles floating on the surface. 

But these plastic particles are known to carry specific combinations of metals, pollutants and pathogens – bacteria, viruses and other microorganisms that cause disease. 

A previous study found antimicrobial-resistant bacteria at concentrations between 100 to 5,000 times higher on microplastic surfaces than in surrounding seawater. 

Antimicrobial resistance (AMR) is when bacteria adapt in response to modern antibacterial medicines and chemicals. 

But the effects of all this on marine animals, aquaculture and humans at the top of the food chain are unknown, Dr Lewis said.   

The new study focuses on implications for the aquaculture industry – the breeding, rearing and harvesting of fish, shellfish and other marine organisms. 

Aquaculture is now the fastest-growing food sector and a good route to bumping up food production globally to feed a growing global population.

The UN Food and Agriculture Organisation has recognised the need for a doubling of seafood production by 2050 to meet global demand, with a 28 million tonne shortfall projected within the next decade.  

Mussels and oysters (pictured) readily take up microplastic particles from their surrounding seawater before they reach the diner's plate

Mussels and oysters (pictured) readily take up microplastic particles from their surrounding seawater before they reach the diner’s plate

In particular, bivalves, a class of molluscs that includes mussels and oysters, ‘arguably offers the primary route to sustainable intensification of production globally’, the experts say. 

Bivalves are filter feeders and are known to take in microplastic particles from seawater.

MICROPLASTIC SOURCES

– Plastic bags

– Bottles

– Fishing nets 

– Cigarette filters

– Car tyres

– Synthetic clothing

According to the UN, there are as many as 51 trillion microplastic particles in the seas, 500 times more than stars in our galaxy. 

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Filter feeders ingest food particles or small organisms by randomly straining them from the surrounding water.

However, this strategy is very susceptible to microplastic uptake.

Lab and field studies have already shown that mussels and oysters readily take up microplastic particles from their surrounding seawater. 

The average microplastic contamination for bivalves ranges from 1.5 to 7.64 particles per individual, but with as high as 178 particles per individual having been recorded.   

Worryingly, strikingly high numbers of one type of marine bacteria called vibrios have have been found in high levels on microplastics, including those from the mid-North Atlantic Ocean.  

“Some vibrio bacteria are known to contribute to disease in bivalves, often causing mass mortality among larvae and in some cases mortality within adult bivalve populations,’ said Dr Craig Baker-Austin at the University of Exeter.   

The team say biofilms that arise rapidly on microplastic surfaces likely play a key role in supporting the unique pathogenic communities on their surface.

The smooth and comparatively large surfaces of plastics at the microscale are suited to pathogenic communities.  

Living within a biofilm is highly beneficial for microbes and can cause them to become more infectious, as reported for the Vibrio cholerae bacteria, with increased levels of metabolic response. 

And previous research has also found a greater richness of microorganisms that have anti-microbial genes on plastics than in seawater.

Microplastics could act as a special location for more effective gene exchange between bacteria, bolstering their resistance human-made antibacterial chemicals. 

Microplastics enter the waterways through a variety of means and finish suspended in the liquid. They can be transported long distances both in water and via the air, taking them to the furthest corners of the world

Microplastics enter the waterways through a variety of means and finish suspended in the liquid. They can be transported long distances both in water and via the air, taking them to the furthest corners of the world

Researchers also mapped the abundance of sea-surface plastics against areas of intensive aquaculture. 

This revealed areas of high aquaculture production in microplastic hotspots where pathogen transfer could theoretically occur.

One such hotspot is in China, where 57 microplastic particles per individual have been reported in the commercially important Yesso clam.

Despite having a high abundance of microplastic, the Mediterranean has a relatively low number of microplastic particles per mussel. 

The global distribution of microplastic may need to be considered in the future development of aquaculture sites if pathogen transfer is demonstrated to be a risk.

Many studies have suggested that disease transfer from plastic to ingesting organisms may occur, but this has not been demonstrated experimentally. 

‘Understanding the link between microplastics and the risk of transferring pathogens through shellfish is critical to our work to manage and transfer risk for the aquaculture industry,’ said Geir Myre, global head of aquaculture insurance firm AXA XL. 

‘It’s one of many emerging risks we must consider as a result of human impact on the ocean and highlights the connection between ocean risks and public health and safety.’

The study has been published in Trends in Microbiology

WHAT FURTHER RESEARCH IS NEEDED TO ASSESS THE SPREAD AND IMPACT OF MICROPLASTICS?

The World Health Organisation’s 2019 report ‘Microplastics in Drinking Water’ outlined numerous areas for future research that could shed light on how far spread the problem of microplastic pollution is, how it may impact human health and what can be done to stop these particles from entering our water supplies.

How widespread are microplastics?

The following research would clarify the occurrence of microplastics in drinking-water and freshwater sources:

  • More data are needed on the occurrence of microplastics in drinking-water to assess human exposure from drinking-water adequately. 
  • Studies on occurrence of microplastics must use quality-assured methods to determine numbers, shapes, sizes, and composition of the particles found. They should identify whether the microplastics are coming from the freshwater environment or from the abstraction, treatment, distribution or bottling of drinking-water. Initially, this research should focus on drinking-water thought to be most at risk of particulate contamination. 
  • Drinking-water studies would be usefully supplemented by better data on fresh water that enable the freshwater inputs to be quantified and the major sources identified. This may require the development of reliable methods to track origins and identify sources. 
  • A set of standard methods is needed for sampling and analysing microplastics in drinking-water and fresh water. 
  • There is a significant knowledge gap in the understanding of nanoplastics in the aquatic environment. A first step to address this gap is to develop standard methods for sampling and analysing nanoplastics. 

What are the health implications of microplastics?

Although water treatment can be effective in removing particles, there is limited data specific to microplastics. To support human health risk assessment and management options, the following data gaps related to water treatment need to be addressed: 

  • More research is needed to understand the fate of microplastics across different wastewater and drinking-water treatment processes (such as clarification processes and oxidation) under different operational circumstances, including optimal and sub-optimal operation and the influence of particle size, shape and chemical composition on removal efficacy. 
  • There is a need to better understand particle composition pre- and post-water treatment, including in distribution systems. The role of microplastic breakdown and abrasion in water treatment systems, as well as the microplastic contribution from the processes themselves should be considered. 
  • More knowledge is needed to understand the presence and removal of nanoplastic particles in water and wastewater treatment processes once standard methods for nanoplastics are available. 
  • There is a need to better understand the relationships between turbidity (and particle counts) and microplastic concentrations throughout the treatment processes. 
  • Research is needed to understand the significance of the potential return of microplastics to the environment from sludge and other treatment waste streams. 

To better understand microplastic-associated biofilms and their significance, the following research could be carried out:

  • Further studies could be conducted on the factors that influence the composition and potential specificity of microplastic-associated biofilms. 
  • Studies could also consider the factors influencing biofilm formation on plastic surfaces, including microplastics, and how these factors vary for different plastic materials, and what organisms more commonly bind to plastic surfaces in freshwater systems. 
  • Research could be carried out to better understand the capacity of microplastics to transport pathogenic bacteria longer distances downstream, the rate of degradation in freshwater systems and the relative abundance and transport capacity of microplastics compared with other particles.
  • Research could consider the risk of horizontal transfer of antimicrobial resistance genes in plastisphere microorganisms compared to other biofilms, such as those found in WWTPs. 

Can water treatment stop microplastics entering our water supplies?

Although water treatment can be effective in removing particles, there is limited data specific to microplastics. To support human health risk assessment and management options, the following data gaps related to water treatment need to be addressed: 

  • More research is needed to understand the fate of microplastics across different wastewater and drinking-water treatment processes (such as clarification processes and oxidation) under different operational circumstances, including optimal and sub-optimal operation and the influence of particle size, shape and chemical composition on removal efficacy. 
  • There is a need to better understand particle composition pre- and post-water treatment, including in distribution systems. The role of microplastic breakdown and abrasion in water treatment systems, as well as the microplastic contribution from the processes themselves should be considered.
  • More knowledge is needed to understand the presence and removal of nanoplastic particles in water and wastewater treatment processes once standard methods for nanoplastics are available. 
  • There is a need to better understand the relationships between turbidity (and particle counts) and microplastic concentrations throughout the treatment processes. 
  • Research is needed to understand the significance of the potential return of microplastics to the environment from sludge and other treatment waste streams.
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