Biological Effects and Implications of Micro- and Nanoplastics in the Aquatic Environment

Within the past decade, it has been widely recognised that microplastics (commonly referred to as plastic particles <5 mm) are ubiquitous in freshwater as well as in marine environments globally. Owing to their small size, microplastics can interact with and potentially affect a wide range of aquatic organisms.

Although the number of studies on microplastic effects is quickly increasing, there is still limited understanding of the processes by which organisms interact with microplastics as well as impacts in natural ecosystems. Further uncertainties relate to the chemical nature of microplastics and their potential role as vectors for chemical pollutants to organisms.

More recently, questions have been raised about human exposure to microplastics and potential health effects – a topic where science still is at the very start of providing answers. In this context, the aims of this thesis are: 1) To critically evaluate and use controlled laboratory experiments for analysing uptake and effects of microplastics in aquatic invertebrates. 2) To examine the interaction between plastic particles and hydrophobic organic chemicals. 3) To review the current debate and state of knowledge on microplastic exposure and potential effects on humans.

Most effects of microplastics on aquatic invertebrates have been studied as a result of particle ingestion. In order to understand and interpret such effects, it is important to quantify ingestion and egestion of microplastics, as this determines the overall exposure that an organism is facing. In this thesis, it is shown that fluorescent particles can be used to quantify these processes.

This is especially useful for particles in the nano- and small micrometre size range. To achieve a reliable quantification, it is often necessary to digest animal tissue. Enzymes are recommended for digestion, based on the use and development of different enzymatic protocols within this thesis. Enzymatic digestion and quantification of particle fluorescence were successfully applied to measure ingestion and egestion of 100 nm and 2 µm particles in the water flea Daphnia magna and larvae of the blue mussel Mytilus edulis.

For both species, it was found that, on a mass basis, a higher amount of particles which are similar to the size of normal prey were ingested than smaller particles. Regarding particle egestion, more species-specific differences were observed in comparison to ingestion. It was found that egestion can strongly be influenced by particle size and the presence of food.

Also, particles have the potential to remain in organisms for a time exceeding the normal gut passage time. Both for D. magna and larvae of M. edulis the smaller particles were found to cause more adverse effects on the animals’ physiology. Controlled laboratory tests, as employed in this thesis, can be a useful tool to obtain a mechanistic understanding of organism-particle interactions and increase the reliability of and comparability between studies.

It was, however, found that a detailed particle and exposure characterisation is often missing and thus particle behaviour and fate in laboratory exposure systems are not well understood. Drawing on experience and developments within the field of engineered nanomaterials, it is therefore recommended to include analyses of particle size, composition, density, surface chemistry and charge, as well as particle aggregation/agglomeration, dispersion and sedimentation.

At the same time, it is important that exposure systems attain a higher degree of environmental realism. To achieve this, it is suggested to use lower particle concentrations, a variety of particle shapes (especially fragments and fibres), a variety of different plastic polymers, biofouled particles, and to include controls with natural particles, such as clay or silica.

Moreover, microplastics cannot always be treated as inert particles since they may contain a multitude of different chemicals, either stemming from plastic production or having sorbed to the plastics in the environment. In both cases, chemicals have the potential to get transported and released, and in this way microplastics may act as vectors for hydrophobic organic chemicals (HOCs) to aquatic animals.

It is therefore strongly recommended to include controls for potential chemical toxicity in microplastic effect studies. As reviewed in this thesis, sorption of HOCs to plastics is governed by diffusive mass transfer and occurs as either adsorption, absorption or a combination of both. The process strongly depends on the properties of the plastic particle, the chemical and the surrounding environment.

In comparison to natural matrices, such as water, dissolved organic carbon and colloids, the role of plastics as a vector may be negligible on a global scale. However, in this thesis it is emphasised that spatial variation on a smaller scale as well as the exposure route of microplastic-associated chemicals to organisms are important to consider.

In recent years, there has been an increasing focus on human exposure to and potential health effects of microplastics. This was mainly sparked by findings of plastic particles in aquatic species used for human consumption as well as other food products, and has evoked many concerns. While there is reason to assume that microplastics can exhibit particle- and/or chemical-related toxicity, no studies have investigated human health effects of consuming microplastics to date.

Humans are exposed to plastic particles and associated chemicals by a variety of pathways. Even though contaminated food products have received most attention, in this thesis it is argued that the main exposure is most likely related to abrasion of particles from the use of plastic materials in everyday life.

Because of many uncertainties and knowledge gaps, it is to date not possible to conclude to what degree microplastics are a threat to the environment and to humans. However, a strong public opinion against environmental plastic pollution has formed, which drives societal and legislative action. This is moving faster than consensus within the scientific community and thus entails the risk that not the most urgent issues are addressed or the most effective measures to reduce environmental plastic pollution are taken.