Environmentally Accurate Microplastic Levels and Their Absence from Exposure Studies

Integrative and Comparative Biology Integrative and Comparative Biology, volume 59, number 6, pp. 1485–1496 doi:10.1093/icb/icz068 Society for Integrative and Comparative Biology INVITED PAPER Environmentally Accurate Microplastic Levels and Their Absence from Exposure Studies Eoghan M. Cunningham1,*,† and Julia D. Sigwart*,† 1 E-mail: Synopsis Microplastics (synthetic polymers; <5 mm) are ubiquitous, in the environment and in the news. The associated effects of microplastics on flora and fauna are currently only established through laboratory-based exposure trials; however, such studies have come under scrutiny for employing excessive concentrations with little environmental relevance. This critical review is intended to summarize key issues and approaches for those who are considering the need for local microplastics research, both in terms of environmental pollution and the impacts on aquatic species. A metaanalysis of results from published experimental (n ¼ 128) and environmental (n ¼ 180) studies allowed us to compare the reported impacts from experiments that expose organisms to microplastics, and the concentrations of environmental microplastics found in the wild. The results of this meta-analysis highlight three issues that should be modified in future work (1) use of extreme dosages, (2) incompatible and incomparable units, and (3) the problem of establishing truly informative experimental controls. We found that 5% of exposure trials examined did not use any control treatment, and 82% use dramatically elevated dosages without reference to environmental concentrations. Early studies in this field may have been motivated to produce unequivocal impacts on organisms, rather than creating a robust, environmentally relevant framework. Some of the reported impacts suggest worrying possibilities, which can now inspire more granular experiments. The existing literature on the extent of plastic pollution also has limited utility for accurately synthesizing broader trends, as has been raised in previous reviews; environmental extraction studies use many different units, among which only 76% (139/180) could be plausibly converted for comparison. Future research should adopt the units of microparticles/kg (of sediment) or mp/L (of fluid) to improve comparability. Now that the global presence of microplastic pollution is well established, with more than a decade of research, new studies should focus on comparative aspects rather than the presence of microplastics. Robustly designed, controlled, hypothesis-driven experiments based on environmentally relevant concentrations are needed now to understand our future in the new plastic world. Introduction The unprecedented production of synthetic polymers (plastic) since the 1940s has improved the lives of billions of people, while simultaneously creating one of the most pressing environmental concerns the world faces today—the plastic pollution crisis. Due to global population and consumer pressure, plastic production has increased exponentially since the mid-20th century to become an industry worth billions to the worldwide economy (Cole et al. 2011). Consequently, the manufacturing and subsequent waste of plastic items is one of the leading factors that scientists have used to propose a new transition in Earth’s history, the Anthropocene (Zalasiewicz et al. 2016). It is believed that a distinct layer of plastic, among other factors, integrated in Earth’s sedimentary record will separate this contemporary geological epoch from the Holocene (Waters et al. 2016; Geyer et al. 2017). Roughly 335 million tons of plastic are produced globally every year, and of the 60 million tons deriving from within the European Union alone, 70% are wasted without recycling (PlasticsEurope 2018). Studies of potential impacts in aquatic systems have focused mainly on the marine environment, with estimates as a high as 10% of all global plastic production entering marine systems annually (Mattsson et al. 2017), and between 60% and 80% of all litter in the marine environment Advance Access publication May 24, 2019 ß The Author(s) 2019. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved. For permissions please email: . *Queen’s Marine Laboratory, Queen’s University Belfast, 12-13 The Strand, Portaferry, BT22 1PF, Northern Ireland, UK; † School of Biological Sciences, Queen’s University Belfast, 19 Chlorine Gardens, Belfast, BT9 5DL, Northern Ireland, UK 1486 microplastic fibers, a secondary microplastic, are responsible for >90% of all microplastic pollution in aquatic systems (Lehtiniemi et al. 2018). Secondary microplastics continue to proliferate in freshwater (Shruti et al. 2019), marine (Zhang et al. 2019), and terrestrial ecosystems (Hüffer et al. 2019) and their ubiquity is now well established; however, publications are still aiming toward quantifying and identifying microplastics in the environment with little scientific novelty or comparative context. Most studies employ methods such as hypersaline density separations to extract plastic from sediment (Thompson et al. 2004; Hidalgo-Ruz et al. 2012; Wang and Wang 2018) and subsequent identification of plastic polymers through Raman spectrometry (Wen et al. 2018) or Fourier Transform Infrared (FT-IR) spectrometry (Kunz et al. 2016; Naji et al. 2017; Li et al. 2018). Despite the same methods being used for most extraction studies, not all researchers quantify microplastic abundance with the same units. As a result, the abundance of microplastics in sediment or water among different environments is often not comparable due to the variety of units used (Burns and Boxall 2018). At present, we know that plastic is everywhere, but there is little data to really determine how much plastic, or the rate of accumulation. This lack of quantification protocol for environmental sampling has also caused difficulties when applying known concentrations of microplastics to exposure studies (Huvet et al. 2016), which are currently the only available medium to determine the impacts of microplastics on aquatic species. Despite studies showing that microplastics are prominent in most ecosystems, are readily consumed by species (Garnier et al. 2019), and can transport persistent organic pollutants (POPs; Rodrigues et al. 2018), the general research area has come under scrutiny in recent years and has been labeled as a “bandwagon” topic. This has both advantages and disadvantages in terms of research and environmental advocacy. Recent media coverage has promoted plastic pollution as one of the greatest threats to the planet (Stafford and Jones 2019). Increased media attention, including documentaries such as BBC’s Blue Planet, and a surge in microplastic research publications has helped to influence policy to reduce plastic waste around the world, including cosmetic microbead bans in Canada, UK, and USA. Microplastics working g (...truncated)