Microplastics as an emerging threat to terrestrial ecosystems

Abstract

Microplastics (plastics <5 mm, including nanoplastics which are <0.1 μm) originate from the fragmentation of large plastic litter or from direct environmental emission. Their potential impacts in terrestrial ecosystems remain largely unexplored despite numerous reported effects on marine organisms. Most plastics arriving in the oceans were produced, used, and often disposed on land. Hence, it is within terrestrial systems that microplastics might first interact with biota eliciting ecologically relevant impacts. This article introduces the pervasive microplastic contamination as a potential agent of global change in terrestrial systems, highlights the physical and chemical nature of the respective observed effects, and discusses the broad toxicity of nanoplastics derived from plastic breakdown. Making relevant links to the fate of microplastics in aquatic continental systems, we here present new insights into the mechanisms of impacts on terrestrial geochemistry, the biophysical environment, and ecotoxicology. Broad changes in continental environments are possible even in particle-rich habitats such as soils. Furthermore, there is a growing body of evidence indicating that microplastics interact with terrestrial organisms that mediate essential ecosystem services and functions, such as soil dwelling invertebrates, terrestrial fungi, and plant-pollinators. Therefore, research is needed to clarify the terrestrial fate and effects of microplastics. We suggest that due to the widespread presence, environmental persistence, and various interactions with continental biota, microplastic pollution might represent an emerging global change threat to terrestrial ecosystems.

Keywords: environmental health; global change; microplastics; nanoplastics; pollution; soil geochemistry.

Figure 1

Microplastic fate in terrestrial environments and its link to freshwaters. Potential microplastic sources (non-exhaustive) are named and represented by the red-colored objects while areas of microplastic concentration are highlighted with red-filled spheres (industrial zones (Lechner & Ramler, 2015), atmosphere (Dris et al., 2016), sewage (Mahon et al., 2017, Mason et al., 2016), agricultural soils (Huerta Lwanga et al., 2017, Nizzetto et al., 2016b), freshwater beaches (Ballent et al., 2016), harbors and dams (Zhang et al., 2017), cities and roads (Horton et al., 2017a), and landfills (Rillig, 2012)). The three upper circular panels represent zoom on selected effects on soil chemistry (Fuller & Gautam, 2016), microbiome (McCormick et al., 2016) and the biophysical environment (Huerta Lwanga et al., 2017, Liebezeit & Liebezeit, 2015, Maass et al., 2017, Rillig et al., 2017b, Zhu et al., 2018).

Figure 2

Microplastics as trigger of combined physical or chemical-like effects. Soil biogeochemistry related to agricultural mulching (Steinmetz et al., 2016), ingestion by terrestrial and continental birds (Gil-Delgado et al., 2017, Holland et al., 2016, Zhao et al., 2016), reduction in growth of earthworms (Lwanga et al., 2016), lethal toxicity to fungi (Miyazaki et al., 2014, Miyazaki et al., 2015, Nomura et al., 2016), mammal lung inflammation (Hamoir et al., 2003, Oberdorster, 2000, Schmid & Stoeger, 2016) and broad cytotoxicity (Forte et al., 2016, Kato et al., 2003) of nanoplastics.

Figure 3

Potential toxicity and uptake mechanisms of nanoplastics. Carboxyl (COOH) and amino (NH₂) terminated or lecithin coated polystyrene beads yield nanoparticles with diverse cellular fate, which influences the toxicity mechanism. Reported processes (Kato et al., 2003, Miyazaki et al., 2014, Miyazaki et al., 2015, Syberg et al., 2015) do not fully explain toxicity and uptake. Further mechanisms await discovery.