Long-term exposure of a free-living freshwater micro- and meiobenthos community to microplastic mixtures in microcosms

Highlights

• First microcosm study on the effects of MPs on freshwater micro -and meiobenthos

• No effects on the abundance flagellates and ciliates (microbenthos)

• Significant effects of MPs on meiobenthic organisms

• No decrease of the pollution-sensitive NemaSPEAR[%]-index in the presence of MPs

• Microcosm experiments are able to reveal subtle, indirect effects of MPs.

Abstract

Microplastics in a wide range of shapes and polymer types (MPs; <5 mm) accumulate in freshwater sediments, where they may pose an environmental threat to sediment-dwelling micro- and meiobenthos. To date, the effects of MPs on those organisms have mostly been studied in single-species experiments exposed to high particle concentrations. By contrast, there have been few investigations of the effects resulting from the long-term exposure of natural communities to environmental relevant MPs. This research gap was addressed in the present study. A microcosm experiment was conducted to examine the impact of a mixture of MPs of varying polymer composition, shape, and size (50% polystyrene (PS) beads: 1-μm diameter; 37% polyethylene terephthalate (PET) fragments: 32 × 21 μm in size, and 13% polyamide (PA) fibers 104 × 15 μm in size; % based on the total particle number) provided at two concentrations (low: 4.11 × 10⁵ MPs/kg sediment dw and high: 4.11 × 10⁷ MPs/kg sediment dw) and two exposure durations (4 and 12 weeks) on a micro- and meiobenthic community collected from a freshwater sediment. MPs exposure did not alter the abundance of protozoa (ciliates and flagellates) as well as the abundance and biomass of meiobenthic organisms (nematodes, rotifers, oligochaetes, gastrotrichs, nauplii), whereas the abundance and biomass of harpacticoid copepods was affected. Neither nematode species diversity (species richness, Shannon-Wiener index, and evenness) nor the NemaSPEAR[%]-index (pollution-sensitive index based on freshwater nematodes) changed in response to the MPs. However, changes in the structure of the meiobenthic and nematode community in the presence of environmentally relevant MPs mixtures cannot be excluded, such that microcosms experiments may be of value in detecting subtle, indirect effects of MPs.

Introduction

Microplastics (MPs; <5 mm) pollution poses a threat to ecosystems worldwide (Silva et al., 2018). After entering aquatic environments, MPs undergo biofouling/sedimentation, which alters their specific densities (e.g., Galloway et al., 2017; Leiser et al., 2020) such that even MPs made up low-density polymer types (Hidalgo-Ruz et al., 2012), including polyethylene (PE) and polypropylene (PP), can sink down to the sediment. Indeed, MPs concentrations in riverine sediments can be up to 600,000-fold higher than in the water phase (Scherer et al., 2020). In a recent study of the Elbe River, the average and maximal concentrations of MPs (125–5000 μm) was 2080 and 14,100 particles/kg sediment (Scherer et al., 2020). Reliable data on particles <125 μm are currently lacking, partly because no uniformly standardized sample protocols are used limiting the comparability of different studies (Lindeque et al., 2020; Nguyen et al., 2019). However, it can be assumed that numbers for smaller particles (1–125 μm) are two to three orders of magnitude higher (Lenz et al., 2016). MPs in the sediment in general are also characterized by a larger diversity of polymer types than in the water (e.g., Horton et al., 2017; Klein et al., 2015; Scherer et al., 2020) and are mainly present as spheres, fibers, and fragments (Blair et al., 2019; Hurley et al., 2018; Tibbetts et al., 2018). Sediments thus represent a sink for MPs (Cera et al., 2020) and in turn a greater potential risk for sediment-dwelling benthic invertebrates (Frei et al., 2019; Haegerbaeumer et al., 2019a; Walkinshaw et al., 2020).

Due to their high structural and functional diversity and abundance (Giere, 2009), meiobenthos (or meiofauna; defined by their body size, that pass through a 500-μm mesh but are retained on a 44-μm mesh; Giere, 2009) occupy an important role in benthic food webs, as they connect lower and higher trophic levels (e.g., Majdi and Traunspurger, 2015). Changes in meiobenthic communities, such as in response to pollution, can thus be expected to impact the freshwater environment overall and nematodes in particular, as they comprise up to 90% of the meiobenthic community (e.g., Traunspurger et al., 2020; Traunspurger, 2021).

Benthic organisms (ciliates, flagellates, rotifers, annelids, nematodes, crustaceans) are able to ingest MPs of different sizes, shapes, and polymer types (reviews, e.g., by Adam et al., 2019; Scherer et al., 2018; Triebskorn et al., 2019; studies, e.g., by Fueser et al., 2019; Fueser et al., 2020a, Fueser et al., 2020b). However, despite the ecological relevance of meiobenthic organisms in MPs research (Giere, 2019), the ecotoxicological effects of MPs on nematodes have scarcely been investigated (Haegerbaeumer et al., 2019a). Most studies on the effects of MPs on nematodes made use of the model organism Caenorhabditis elegans and examined toxicity endpoints such as oxidative stress, mobility, and survival (reviewed by Bhagat et al., 2021; Hu et al., 2020), but large deviations in the effect levels were determined (Bhagat et al., 2021). For example, in water and sediments, a 50% inhibition of C. elegans reproduction was observed after 96 h at PS bead concentrations of 0.57 mg 1-μm PS beads/ml (Mueller et al., 2020b) and 4.8–11.3 mg 1-μm PS beads/g sediment dry weight (dw) (Höss et al., 2022 in press). Moreover, long-term multigenerational tests (21-day exposure) assessing the population growth of C. elegans showed a 100-fold higher susceptibility (Mueller et al., 2020a). Whether species-specific population growth responses translate into a shift in nematode species composition under more realistic exposure scenarios remains to be tested using model ecosystems (Prokić et al., 2021; de Sá et al., 2018; Stanković et al., 2021).

Model ecosystems are appropriate tools for higher-tier risk assessments of chemicals in sediments (Diepens et al., 2017; EFSA Panel on Plant Protection Products and their Residues (PPR), 2013). They include the use of microcosms containing samples from natural ecosystems but with the tests run under controlled conditions (Brock et al., 2015). Compared to standardized single-species tests, the main advantage of microcosm studies in environmental risk assessments is that intra- and interspecific interactions within a community can be investigated over the long-term (Brock et al., 2015). With this approach, both direct and indirect effects of the substances of interest at the population and community level can be taken into account (van den Brink et al., 2009). Up to now, microcosms have been used in studies of the effects of different substances on freshwater meiobenthos, especially nematodes (summarized in Höss, 2021).

The impacts of MPs on benthic invertebrates (meio- and macrofauna) and benthic ecosystem functioning have been addressed in only a few studies, conducted in outdoor (Lin et al., 2020; Redondo-Hasselerharm et al., 2020; Stanković et al., 2021) and laboratory (Huang et al., 2021; López-Rojo et al., 2020; Silva et al., 2022; Wakkaf et al., 2020; You et al., 2020) microcosm set-ups, but none have included protozoa. Although the comparability of those studies is limited, given their differences with respect to the studied environment (marine, freshwater, soil) and polymer type (single polymers: PE, polyethylene terephthalate [PET], polyvinyl chloride [PVC], PS, or polymer mixtures), deleterious effects of MPs were observed for several endpoints. For example, these included changes in leaf litter composition and an increase in the mortality of the detritivore caddisfly Sericostoma pyrenaicum after exposure to 10-μm PS microspheres (at 1 μg PS microspheres/ml; López-Rojo et al., 2020). However, MPs at a concentration of 0.05–50 g/kg sediment dw were shown to exert both positive and negative effects on recolonization by macroinvertebrates (Redondo-Hasselerharm et al., 2020). The effects of a realistic MPs composition in terms of size and shape on a native benthic meiobenthic community sampled from a freshwater sediment has yet to be analyzed in long-term experiments conducted under controlled conditions.

To fill this research gap, we examined the impact of a mixture of MPs, differing in their shape and polymer type (50% PS beads, 37% PET fragments, 13% polyamide [PA] fibers) and provided at low (4.11 × 10⁵ MPs number or 9.68 mg MPs per kg sediment) and high (4.11 × 10⁷ MPs number or 968 mg MPs per kg sediment) concentrations, on the micro- and meiobenthic communities in a freshwater sediment. The microcosms were filled with field-collected sediment that included the native fauna and were subsequently spiked with the MPs mixtures. After 4 and 12 weeks, protozoans (flagellates and ciliates) and meiobenthic invertebrates (nematodes, rotifers, oligochaetes, harpacticoid copepods, gastrotrichs and nauplii) were sampled and their abundance and biomass were measured. Nematodes, as the most abundant group, were identified to the species level, which allowed their structural and functional diversity as well as a specific pollution index to be determined.

Previous ecotoxicological studies obtained conflicting results on the sensitivity of benthic invertebrates, especially nematodes, to MPs (Haegerbaeumer et al., 2019a). Our interest here was to monitor the response of micro- and meiobenthic communities to MPs under a realistic, long-term exposure scenario. We hypothesized that direct toxic effects on protozoa and meiobenthos were unlikely but that MPs interference with food consumption (Rauchschwalbe et al., 2021) would result in indirect food web effects evidenced as changes in community structure.