Rapid ingestion and egestion of spherical microplastics by bacteria-feeding nematodes.

Microplastics, anthropogenically released into freshwaters, settle in sediments, where they are directly ingested by benthic organisms. However, to the best of our knowledge, fine-scale studies of microplastic ingestion and egestion by nematodes, one of the most abundant meiofaunal taxa, are lacking. We therefore conducted a time series of the ingestion and egestion by adult Caenorhabditis elegans and Pristionchus pacificus of 0.5- and 1.0-µm fluorescent polystyrene (PS) beads along with bacteria. The nematodes were exposed to 107 beads ml-1 in aqueous medium for 5 min-24 h and pumping rates of C. elegans were determined. In the egestion study, PS bead egestion was monitored in nematodes with high microplastic body burdens for 5 min-24 h in microplastic-free medium. Ingested beads were detected already within 5 min and up to 203 ± 15 PS beads (1.0 µm; C. elegans) were found after 30 min. Overall, significantly more 1.0-µm than 0.5-µm PS beads were taken up. The distinct feeding behaviors of the two species influenced their PS bead body burdens. Ingested PS beads were almost completely egested within the first 20-40 min in the presence of sufficient food. In C. elegans, 1.0-µm beads were egested less rapidly than 0.5-µm PS beads. Given the rapid ingestion and egestion of the beads, our study demonstrates that the actual amount of ingested and egested microplastics by nematodes in the environment may be several times higher than the microplastic body burdens may imply. However, spherical PS beads did not bioconcentrate in nematodes.

Ingestion of microplastics by meiobenthic communities in small-scale microcosm experiments.

Microplastics have been detected in many different environments. Nematodes are a rife meiofaunal taxon and occupy an important trophic position in benthic food webs. Laboratory-based ingestion experiments have demonstrated the susceptibility of single nematode species to microplastic uptake. However, the determinants of ingestion by meiofaunal assemblages, especially those of nematodes, have yet to be fully examined. We therefore conducted a microcosm study in which field-collected freshwater sediment was spiked with fluorescent polystyrene (PS) beads (1.0, 3.0 and 6.0 µm) in concentrations of 103 and 107 PS beads ml-1 and the ingestion by the most dominant indigenous meiofaunal taxa (nematodes, rotifers, chironomids, copepods) was investigated after 2, 4 and 8 days using fluorescence microscopy. In additional small-scale microcosms, PS bead ingestion by nematode assemblages was quantified as a function of feeding type, exposure time (1-10 days), concentration (103, 105, 107 PS beads ml-1) and bead size (0.5, 1.0, 3.0, 6.0 µm). PS beads at 107 beads ml-1 were largely ingested by chironomids and copepods. Exposure time and concentration correlated positively with PS bead ingestion for all taxa. The most relevant size class for ingestion for the majority of meiofaunal taxa was PS beads of 1.0 µm. Nematode communities, especially deposit-feeding species, effectively ingested micropastics from sediment, as >30% of the exposed individuals and 56% of the species ingested 1.0-µm PS beads in <24 h. Ingestion rates were mainly influenced by PS bead size and nematode feeding type/habit, with the exception of a bead concentration of 103 beads ml-1, at which exposure time was also an important factor. Sediment particles reduced microplastic ingestion considerably for all investigated meiobenthic organisms. Our study demonstrates the ability of free-living nematodes communities to readily ingest PS beads of various sizes. If the feeding-type distribution is known, the potential exposure of nematode communities may be predicted.

Surface-Related Toxicity of Polystyrene Beads to Nematodes and the Role of Food Availability.

Microplastics released into freshwaters from anthropogenic sources settle in the sediments, where they may pose an environmental threat to benthic organisms. However, few studies have considered the ecotoxicological hazard of microplastic particles for nematodes, one of the most abundant taxa of the benthic meiofauna. This study investigated the toxic effects of polystyrene (PS) beads (0.1-10.0 µm) and the underlying mechanisms thereof on the reproduction of the nematode Caenorhabditis elegans. The observed effect of the PS beads on the nematodes correlated well with the total surface area of the beads per volume, with a 50% inhibition of reproduction at 55.4 ± 12.9 cm2/mL, independent of the bead size. The adverse effects were not explained by styrene monomers leaching from the beads because chemical activities of styrene in PS suspensions were well below the toxic levels. However, the observed effects could be related to the bead material because the same-sized silica (SiO2) beads had considerably less impact, probably due to their higher specific density. PS and SiO2 beads affected the food availability of C. elegans, with greater effects by the PS beads. Our results demonstrate the importance of including indirect food web effects in studies of the ecological risks posed by microplastics.

Ingestion of microplastics by nematodes depends on feeding strategy and buccal cavity size.

Microplastics are hardly biodegradable and thus accumulate rather than decompose in the environment. Due to sedimentation processes, meiobenthic fauna is exposed to microplastics. Within the meiofauna, nematodes are a very abundant taxon and occupy an important position in benthic food webs by connecting lower and higher trophic levels. However, the key determinants of the uptake of microplastics by freshwater nematodes are still unknown. To investigate the bioaccessibility of microplastics for nematodes, we performed single- and multi-species ingestion experiments in which the ability of seven nematode species (six bacterial and one fungal feeder), diverse in their buccal cavity morphology (1.3-10.5 µm), to ingest fluorescence-labelled polystyrene (PS) beads along with their natural diet was examined. Applied beads sizes (0.5, 1.0, 3.0 and 6.0 µm), exposure time (4, 24 and 72 h) and concentration (3 × 106 PS beads ml-1 and 107 PS beads ml-1) were varied. Ingested beads were localized and quantified via fluorescence microscopy in the nematodes. In contrast to fungal-feeding nematode species with a stylet, bacterial-feeding species ingested 0.5- and 1.0-µm PS beads with up to 249 and 255 beads after 24 h, respectively. Microplastics ≥0.5 µm could only be ingested and transported into the gastrointestinal tract, if the buccal cavities were considerably (>1.3 times) larger than the beads. At concentrations of 107 PS beads ml-1 ingestion rates were influenced by exposure time and PS bead concentration. In case of a known microplastic size distribution in the environment, predictions on the potential ingestion for nematode communities can be made based on the feeding type composition and the size of their buccal cavities.