|Occurrence, effects and risks of marine microplastics|
Van Cauwenberghe, L. (2015). Occurrence, effects and risks of marine microplastics. PhD Thesis. Universiteit Gent - Faculteit Bio-ingenieurswetenschappen: Gent. ISBN 978-90-5989-869-1. 215 pp.
Pollution > Water pollution > Marine pollution
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Plastics are present in every aspects of our everyday life. The combination of properties such as its versatility, light weight, strength and durability, have made plastic a very popular material for use in a myriad of applications. Their widespread use has driven the annual global production from 1.7 million tonnes (MT) in the 1950s to 299 MT in 2013. Although the societal benefits of plastic are undeniable, there are serious environmental concerns associated with it. One aspect of this is microplastic pollution. Plastics are present in the environment in a wide variety of sizes, but the smallest form is called microplastic, and comprises a heterogeneous mixture of plastic particles ranging in size from several millimetres to a few micrometres. These microplastics are present in the environment as ‘microplastics by design’, so-called primary microplastics, or arise from the degradation of larger plastic litter. While the former are typically resin pellets and microbeads associated with industrial spillages and the use of cosmetics, the latter (or secondary microplastics) are formed through the action of degrading forces such as UV radiation and physical abrasion. Another important source comprises fibres originating from synthetic clothing. The presence of these microplastics has been demonstrated in different marine compartments worldwide such as inter- and subtidal sediments and in (sub)surface waters. Because of their small dimensions, microplastics have a similar size range as planktonic organisms and other suspended particles, making them available to an array of marine invertebrates commonly not affected by larger marine debris. The potential for ingestion and potential associated (adverse) effects have resulted in the recognition of microplastics as contaminants of concern. This type of pollution is, however, scarcly regulated in terms of production, use and emissions neither in Europe nor in the rest of the world. Although there is an increasing number of studies available on the presence and potential effects of microplastic pollution and it is likely that the amount of microplastics in the oceans will continue to increase in the future, so far no real risk assessment of present and future risks of microplastic to marine systems and human health has been performed. Therefore, the main aim of this thesis was to perform a assessment of the environmental and human health risks associated with microplastic pollution using both data generated during this thesis as well as those available in literature. While the main theme of this dissertation was the marine environment, we started off in the freshwater environment, more specifically a river, as they are often considered major contributors of microplastics to the marine environment. In Chapter 2, we therefore investigated the occurrence and distribution of microplastics in sediment of the Belgian Scheldt river. These sediments showed high spatial variability in microplastic abundance, with the highest concentrations detected in the vicinity of suspected point sources, i.e. a plastic production plant in the harbour of Antwerp and a sewage treatment plant (STP) near Ghent. Near these facilities, sediment microplastic abundances were up to 10 times higher those observed at other sampling stations, indicating an important contribution of these land-based point sources to environmental microplastic abundances. In fact, at the majority of sampling locations, microplastic abundances were higher than those reported for marine compartments. The sewage treatment plant (STP) discharging directly into the river was investigated in more detail, since a lot of household and industrial applications will generate microplastics that are discharged together with the domestic and industrial sewage. Our findings confirm the results of previous pilot projects: sewage contains large amounts of microplastics, which are insufficiently removed during the sewage treatment process. As a result, large amounts of microplastics are discharged into the environment on a daily basis. With this initial assessment of river sediments, we were able to identify important point sources of microplastics and demonstrate the magnitude of microplastic pollution in rivers. In Chapter 3, a comprehensive assessment of marine litter in three environmental compartments of Belgian coastal waters was performed to establish a baseline for future marine litter monitoring and research. Although microplastic pollution is not as obvious as macrolitter, it represents an important part of the overall plastic pollution problem. While at Belgian beaches macroplastics are dominant with respect to total weight, this relationship shifts towards a dominance of microplastic at the sea surface and especially on the seafloor. On the beach, the weight of macroplastic litter is over an order of magnitude higher than that of microplastics. On the Belgian Continental Shelf (BCS), micro- and macroplastics represent the same weight fraction at the sea surface, while on the seafloor there is a kilogram of macroplastics for every 400 kg of microplastics. In Chapter 4, the techniques used to assess microplastic abundance in Chapter 3 were applied on deep-sea sediments, in the first ever assessment of microplastic pollution of the deep sea. In this way, we established that microplastic presence in sediments is not only limited to accumulation hot spots such as the continental shelf, but that they are also ubiquitously present in some of the most remote of marine environments, the deep sea (up to 4800 m depth). Microplastic concentrations observed here were substantially lower than those observed on the BCS. Nonetheless, our findings demonstrate that microplastic pollution has spread throughout the world’s seas and oceans, and has reached remote and largely unknown environments such as the deep sea In the first part of this dissertation, we demonstrated that microplastics are ubiquitously present in the marine environment, and this a wide variety of marine compartments and systems. Because of their small dimensions (ranging from a few micrometres up to five millimetres) microplastics represent a collection of particles that are of particular, biological, interest: since they are within the same size range as small particulate matter, they can be taken up by marine biota, especially invertebrates. These invertebrates often represent the lower levels of marine food webs, and are hence of great importance to marine systems. In Chapter 5, we studied the uptake of microplastics under field conditions, i.e. in organisms exposed to ambient microplastic concentrations. The two species under investigation, the blue mussel Mytilus edulis and the lugworm Arenicola marina, represented two different feeding strategies: while the bivalve species is a filter feeder, the polychaete was representative of a deposit feeding strategy. Although exposed to environmental concentrations of microplastics, which are thousands of times lower than those used in laboratory trails investigating microplastic ingestion in these species, microplastics were detected in all organisms collected in the field. The observed microplastic body burdens in these animals were, however, relatively low (less than 1 particle per gram of tissue). Yet, the accumulation of microplastics in these animals could result in the transfer of these particles to higher trophic level organisms. In a subsequent proof-of-principle laboratory experiment the potential (adverse) effects of microplastic exposure and ingestion on the organisms’ energy metabolism (cellular energy allocation) was assessed. Although organisms were exposed to high concentrations of microplastics, no significant adverse effects on the organisms’ overall energy budget were observed. As seafood, including shellfish such as mussels, is consumed by humans worldwide, the presence of marine microplastics in “naturally exposed” species indicates a risk to human health and food safety. In Chapter 6, we therefore extended the assessment of microplastic accumulation in natural systems to include two species of economic interest: the commercially grown bivalves Mytilus edulis and Crassostrea gigas. In accordance with the findings of chapter 5, microplastics were recovered from the soft tissues of both bivalve species grown for human consumption. These results hence indicate that, through the consumption of shellfish, microplastics will end up in the human food chain. Based on consumption data obtained from the European Food Safety Authority (EFSA), it was calculated that European shellfish consumers will ingest between 1,800 and 11,000 microplastics per year, depending on whether they are minor or top shellfish consumers. The implications of this presence of microplastics in seafood, i.e. whether there truly is a risk to human food safety, were investigated in the Chapter 7. Assessing possible (adverse) effects of microplastic ingestion to humans consuming contaminated seafood, was achieved using the intestinal cell line Caco-2. While no cytotoxic effects were observed in the intestinal cells exposed to microplastics, we did observe translocation of particles. Already in the first hour after administering the microplastics, a small fraction could be observed on the basal side of the cells, indicating transport of the particles through the epithelial monolayer. Based on the average concentration of microplastics in and the average consumption of shellfish in humans, we calculated that 3 to 60 microplastics will translocate to the underlying circulatory system on an annual basis. However, we are still unable to assess the adverse effects of this translocation, as data are currently lacking in literature. Finally, all aspects of microplastic pollution investigated in the previous chapters were integrated into a risk assessment. Indeed, while it is often stated that microplastics pose a risk to the marine environment, this has never been thoroughly assessed. The risk assessment, performed in Chapter 8, suggests that current microplastic levels (i.e. anno 2015) are of minor concern to marine systems. Concentrations of pelagic microplastics do not appear to constitute any risk to biota living in the water column. However, in sediments, microplastics can reach very high levels, especially in highly impacted coastal areas (e.g. industrial harbours). Here, we demonstrated that these current microplastic concentrations already exceed safe, predicted no effect concentrations (PNEC), indicating there is a risk for biota inhabiting these sediments. By the end of the century, we predict an 60-fold increase in total microplastic abundances. While we see this increase in all marine compartments studied, only highly impacted sediments (this time both coastal and deep-sea sediments) will exceed the predicted safe level by 2100. With regards to human health, we can state that microplastic ingestion through the consumption of contaminated shellfish will not pose significant risks. Current and future (i.e. 2015 vs. 2100) ingestion patterns in individuals consuming shellfish will not bring about significant health risks. Neither direct effects on intestinal cells nor any indirect effects associated with chemicals transported from plastic to shellfish for human consumption were identified. However, it has to be mentioned that currently toxicity data regarding the (adverse) effects associated with translocated microplastics in humans (and other vertebrates) are lacking. Although still very preliminary, the risk assessment presented here gives the indication that microplastics may not be as harmful as previously thought. Yet, we stress that the lack of an apparent risk associated with microplastic pollution should never be considered a safe conduct to continue our present attitude towards plastic and plastic management. It would be immensely irresponsible and negligent to seize the apparent (current) lack of adverse impacts associated with microplastics to give up on the efforts made and to be made to reduce this type of pollution.