Plastic pollution has garnered media attention - and public worry. But microplastics - tiny particles of less than five millimeters - are now widely abundant in our ecosystem. They may pose direct threats to human health and ecosystem functioning. Experts at the University of Birmingham are investigating the scale of microplastic pollution and the potential impacts they may cause now, and in the future.

Concerns about plastic pollution have sparked high profile campaigns worldwide, and emotive images of marine animals ensnared in plastic waste fill the media. However, these ‘macroplastics’ are only one part of the story. The vast majority of this plastic waste is carried out to the oceans via multiple sources, primarily rivers and waterways. 

Microplastics - particles less than 5mm in diameter - are increasingly abundant in the ecosystem too. Some, known as primary microplastics, have been manufactured to be small particles, such as cosmetic 'beads'. Secondary microplastics are those resulting from from the breakdown of larger fragments of plastics, through the mechanical abrasive effects of being washed down a river or battered by ocean waves, or through washing machines, such as microfibres in synthetic fleece clothing.

A not-so-micro problem

The sources of microplastics are manifold, including waste-water inputs, runoff from roads and sealed surfaces or inappropriate waste management. While waste-water treatment can reduce the amount of plastics, a large number of particles continues to end up in rivers around the world. Further, biosolids as residuals of waste water treatment processes may be used in agriculture as fertilizers, meaning that microplastics can find their way into soils and potentially valuable groundwater resources via other pathways that are as yet not well understood.

Microplastics may pose greater risks to human and environmental health than the more visible macroplastics, because they can be ingested, inhaled or absorbed by organisms and potentially move through the food chain. Potential health consequences of this are as yet poorly understood. Some plastic additives such as bisphenoyl-A (BPA) have been recognised as potentially harmful to the endocrine system and are largely phased out of food packaging. They can, however, still reach and harm the environment through the breakdown of the plethora of other plastic products.

From river to ocean: a transformation

There are two important implications for research. Firstly, we need to better understand the mechanisms by which plastics are transported through our rivers to the oceans, their change and potential decay along these pathways and their accumulation in rivers, sediments, soils and organisms. Second, research is needed into the effects of such plastics on the affected ecosystems, including organisms at all levels of the food chain. Academics at the University of Birmingham are seeking to address these concerns in a series of interdisciplinary research projects. 

How plastics are transported, accumulated and break down in freshwaters is a focus area of Professor Stefan Krause, a hydrologist with an interest in biogeochemical cycling, contaminant transport and ecohydrological feedbacks in complex landscapes. His work examines the fate, transport and environmental impacts of microplastics in rivers. 

“Rivers are not just transporters of plastic but are transformers of plastic,” he explains. Krause’s work on river flow dynamics and particularly exchange fluxes between rivers and their streambed sediments shows how plastics behave and “age” as they are transported. Understanding of these processes enables predictions about their accumulation and degradation pathways as well as potential impacts on exposed organisms. For example, a microplastics survey of the River Tame and its tributaries conducted by an interdisciplinary team of students and researchers at the University of Birmingham proved the impossibility of making simple predictions about microplastic concentrations in urban river environments based only on the presence of waste water treatment works or local population density. Hydrodynamic factors - such as alterations in flow velocities resulting from variability in the shape of waterways and in particular the presence of lakes for example - can result in increased deposition of microplastic particles together with other sediments.

Professor Krause explains that the environmental conditions in which plastics are transported or accumulated determine how they are altered and potentially broken down. Low density plastics such as polystyrene or polyethylene floating at the surface of the river will be exposed to sun light, causing chemical and physical changes that bypass those higher density plastics that are suspended in the river or even have found their way into the sediment at the river bed. Further, sediment-bound plastics may take years or even decades to make their way into the oceans, by which time their interactions with biological materials including different types of microbial biofilms will have changed their properties and behaviour, contaminants may have sorbed on their surfaces and they may have caused long-term exposures to vulnerable organisms.

Understanding how plastics are transported and transformed in water is a first step for researchers who want to quantify their potential impacts and risks, providing the basis for determining how plastics affect human health.

Tiny particles, massive impact

Professor Iseult Lynch is an environmental nanoscientist at the University of Birmingham, focusing on behaviour of nanoplastics, the very smallest of particles, which range from 1 - 100 nanometers (nm) and microplastics. Lynch believes this tiny size makes microplastics behave in interesting and consequential ways. As particles reduce in size, the proportion of surface area to volume increases. The chemical behaviour of particles this small is then driven more by the properties of their surface area than by their constituent materials. As such, the ways that nano- and microplastics interact with the environment cannot be predicted from the behaviour of macroplastics. 

Microplastics have already found their way into zooplankton, fish, invertebrates and mammalian digestive systems. There are concerns they may accumulate in organisms via a ‘Trojan Horse’ effect in which they bind to other entities rather than enter through simple inhalation or ingestion; Professor Lynch’s work is identifying ways in which nanoplastics can bind to organic materials to create new substances. Contaminants adhere to the surface of microplastic particles and biofilms that may have formed around them, and which are a food source for some organisms. These particles are then consumed by organisms that would not normally ingest plastics. Far from being inert and inactive particles, microplastics - even those free from BPA - appear to interact with biological materials in ways that may prove to be harmful, for example, by making organisms appear full without providing nutritional content thereby impacting growth and reproduction potential. 

Sentinel of change - the water flea

Professor Lynch’s research is examining the impacts of micro- and nanoplastics on daphnia, or water fleas, which are important sentinel organisms for providing information on the impact of water pollution because they are filter-feeders, and therefore micro- and nanoplastics in the water can enter their gut and cause stiffening or constipation. Lynch's research is demonstrating that daphnia release proteins that attach in a process known as adsorption, forming an ecocorona around the nanoplastic particles, that changes how organisms interact with the nanoplastics.

Essentially, the ecocorona is a complex created between microplastics and organic materials present in the environment, as physical adsorption confers a biological identity over the existing chemical one. It is thought that this adsorption fundamentally changes the behaviour of these nanoplastics. For example, such plastics appear to be taken up more rapidly and excreted more slowly than simple plastics which can only exist in laboratory studies, as absorption of biomolecules occurs instantaneously in real environments, allowing them to accumulate in the tissues of organisms to a far greater degree than if they were absorbed based on diffusion gradients alone. Further, these coated nanoparticles appear to have certain toxic effects on their host organisms, probably related to enhanced retention in the gut. However, Professor Lynch explains that this is unlikely to manifest as acute toxicity, being more likely to lead to subtle, longer-term effects such as slow starvation or impaired development, which are harder to study in vivo. 

Microplastics - from problem to solution?

The contaminants of concern are not only the plastics themselves, but the kinds of compounds that might stick to their surfaces, like heavy metals and pharmaceutical products. These can increase in concentration at the microplastic surface and once taken into the tissues of feeders positioned low in the food chain, there is a possibility of bioaccumulation - the progressive increase in concentration by organisms higher in the chain.  Toxicity effects are not yet well understood and are thus an important avenue for research. Academics must also focus on improving technologies for waste water treatment works, which are currently effective at removal of macroplastics but continue to release large proportions of microplastics into rivers and via biosolid fertilisers and irrigation potentially also into agriculture.  

Despite concerns that micro and nanoplastics are deleterious for human health, experts are also interested whether nanoparticle science could, through the same underlying mechanisms, have human and environmental benefits too. There are already known examples of surface area adsorption technologies being applied in this way. For example, ceramic cordierite in catalytic converters removes nitrogen oxides from combustion engine exhausts. Domestic water purification filters are based on granular activated carbon. And carbon black can adsorb ingested poisons.

However, nanoplastics may have additional applications. Their affinity to bind with toxic compounds could help remove toxic substances from water, although the challenge will be how to contain the nanoplastics once they have bound to contaminants, in order not to aggravate the problem by releasing vectors into the ecosystem. Knotty problems require complex solutions. Both Professor Krause and Professor Lynch see a hopeful future as workers across disciplinary boundaries work together including hydrologists, ecologists, sedimentologists, fluvial geomorphologists, economists, human geographers and clinical scientists working with chemists, environmental scientists and ecotoxicologists.

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