Embrace complexity to understand microplastic pollution

Abstract

Environmental cycling of microplastics and nanoplastics is complex; fully understanding these pollutants is hindered by inconsistent methodologies and experimentation within a narrow scope. Consistent methods are needed to advance plastic research and policy within the context of global environmental change.

Plastics and products of their degradation — microplastics (MPs), nanoplastics (NPs) and their associated chemicals — are transported through ecosystems. MP cycling is complex and incompletely understood owing to analytical barriers and the narrow scope and scale of many experiments. Moreover, plastics pollution is frequently framed as an ecotoxicological or ecosystem health issue, limiting the research questions asked and disciplines engaged in its research. Complexity is a central theme in the plastics pollution cycle. Such system level thinking is critical for policy relevant, source apportionment modelling and regional and global mass balance estimates of plastics pollution across relevant spatial and temporal scales.

Here, we argue that to understand the cycling and impacts of plastics (including MPs and NPs), researchers must embrace complexity, reframe the issue from a solely ecotoxicological one, and develop more analytical techniques and laboratory experiments that better measure and mimic real-world scenarios.

Microplastics as a GEC factor

As a novel environmental stressor, plastics pollution has already been added to the canon of global environmental change (GEC) factors (Fig. 1), such as climate and land use change. However, plastics are a diverse suite of contaminants and substances¹, with some compounds characterized as synthetic pollutants. Ultimately, this diversity of physical and chemical characteristics will result in different fates and hazards². Therefore, MPs and NPs should collectively, and separately, be considered unique GEC factors, distinct from macroplastic pollution. This perspective shift is subtle but critical, as investigating synthetic chemicals and their toxicological consequences has been the domain of ecotoxicology and geoscience, with less input from research spheres such as complex systems analyses or global environmental change studies. Additionally, by reframing MPs and NPs as GEC factors, more disciplines can engage with plastics research, from environmental sciences to geography to policy.

Fig. 1.

Plastic pollution and GEC. Examples of global environmental change (GEC) factors and their physical, chemical, and biological nature, including microplastic particles.

Shifting from an ecotoxicology to a GEC and complexity perspective matters for three main reasons. First, the choice in experimental treatment conditions will change. Ecotoxicology is concerned with estimating exposure and overall risk, often focusing on current contamination exposures, and sometimes using probability-based modelling to hindcast and forecast ecological conditions and impacts across gradients. In contrast, global change biology is concerned with collectively studying the effects of current and future environmental conditions. By applying these approaches to MPs and NPs research, scientists can probe the impacts of a wider range of plastics chemistries, sizes, and concentrations — not just those that are harmful doses to specific organisms — over many spatiotemporal scales. Secondly, whereas ecotoxicology primarily focuses on negative impacts on organisms and ecosystems, global change science considers any effects — positive or negative — that are a deviation from the natural state. Considering all effects is crucial because, for example, in some cases MPs in terrestrial ecosystems have shown to have nominally positive effects, such as on plant growth. Such effects likely would not be in the remit of ecotoxicology studies but would in GEC investigations. Thirdly, the GEC approach leads to more comprehensive testing of interactions between MPs, NPs, plastic-added chemicals, associated contaminants, their potential fate, and biological pathways in a changing world (Fig. 1). Indeed, plastics pollution is usually investigated as a single stressor or in conjunction with few others. However, nature is far more complex, with multiple stressors interacting and potentially having synergistic (or antagonist) impacts. For example, MPs incorporation into marine snow could change particle settling rates and sedimentation dynamics, influencing decomposition rates and biogeochemical cycling³.

The importance of MPs and NPs pollution might only be revealed when looking at complex, whole ecosystem-scale interactions. MPs and NPs research must consider ‘ecological surprises’ or ‘episodic events’ that become evident only when testing higher-order interactions, including impacts from a rapidly changing climate. Thus, the GEC perspective will lead to the use of broader approaches that consider ecological complexity, experimental design, and key parameters of MPs and NPs properties to better refine and effectively guide research priorities.

The adoption of plastic pollution into the GEC realm is even more critical to communicating and translating science. Plastic pollution has garnered substantial public attention, and in many cases, proposed regulatory action has been swift because of its overt physical visibility and its close connection to human behaviour. In turn, this attention has led to concerns that plastics pollution is ‘stealing’ attention, visibility, and funding from other pressing societal issues, such as climate change. If plastics pollution is conceptualized as a GEC factor, then it could be easier to communicate that plastics pollution, including MPs and NPs, and globally rising temperatures are two sides of the same coin; both distinct signatures of how humans are altering and impacting the Earth’s ecosystems and planetary boundaries.

Harmonize analytical approaches

Although MPs and NPs should be investigated within a GEC framework, mechanistic research is limited by analytical technology, cost, and time. Moreover, there is still a lack of accepted procedures for robust, quantitative, and non-targeted particulate plastic analysis in any environmental compartment. For example, despite efforts to harmonize environmental MPs protocols, many laboratories still rely on separation and analysis protocols that are derived in house. Such variability undermines the comparability of analyses across laboratories4 and different environments; the field is constrained to geographically and temporally heterogeneous monitoring, often in remote locations. Therefore, extrapolating concentration, composition (such as polymer chemistry and plastic additives), morphology, size, and impacts of MPs are inadequate for understanding cycling of MPs. There are a few examples of NPs measured in field studies, but the added analytical challenges associated with measuring particles of even smaller sizes hinder widespread investigation outside the laboratory.

We suggest two tiers of MPs and NPs analysis to gain a better understanding of their impacts on GEC: identifying metrics associated with MPs and NPs themselves (particle composition, size, morphology, concentration, and plastic added chemicals); and contextualizing this information by understanding how plastics influence biogeochemical cycles, ecological complexity dynamics and impacts on biota. Characterizing MPs and NPs is still analytically challenging, so further developments in metrology are needed for more accurate, precise, and expedited measurements. However, measuring plastics in complex matrices should not be limited to only measuring the polymer. Many added chemicals found in plastics have ecotoxicological effects and their impacts cannot be completely decoupled from the plastics pollution issue. A holistic view contextualizing the influence of plastic particles and added chemicals in environmental compartments and food webs should be addressed in tandem.

Measuring NPs is even more fraught. The analytical and conceptual toolboxes for MPs are not able to be directly applied to NPs due to both size and/or concentration limitation(s). Micro-Fourier Transform infrared spectroscopy is one of the most used techniques for identification of MPs but is best suited2 for individual particles larger than ~20 μm, so it is incapable of quantifying NPs. In addition, many techniques5 have high detection limits and, thus dilute NPs concentrations are challenging to measure. Labelled or doped MPs and NPs are an approach to trace and characterize these materials in complex media. This approach cannot be used in field samples, but in laboratory settings it can be a potential methodology to assess the biological fate of MPs and NPs in organisms. Doping MPs and NPs with rare elements, isotopes or molecules that are not typically present in the environment and using these materials as tracers can circumvent some of the current analytical challenges.

Outlook

As plastic pollution is installed in the canon of GEC, a major research effort is underway to understand potential interactions with other factors of global change; such work must strive to capture the complexity of this contaminant suite and the broad diversity of impacts across ecological scales. By embracing complexity, and by reframing MPs and NPs as a GEC factor, novel lines of thinking can be integrated into plastics research, leading to more effective public policy.