Ingestion of microplastics by free-living marine nematodes, especially Enoplolaimus spp., in Mallipo Beach, South Korea

Many plastics cause pollution in the marine environment, with microplastics (0.1 µm–5 mm) representing a key research focus. The number of microplastics in sediments may increase rapidly, affecting organisms inhabiting marine sediments. The aim of this study was to determine how microplastics affect nematodes in intertidal sand. We assessed: (1) intake of microplastic particles (10 µm, 5 µm, 1 µm, or 0.5 µm) by Enoplolaimus spp. over 48 h; (2) microplastic intake by nematodes depending on feeding type (selective deposit feeders, non-selective deposit feeders, epistrate feeders, or predators/omnivores) over 48 h; and (3) microplastic egestion by Enoplolaimus spp. The proportion of Enoplolaimus spp. individuals containing microplastics was significantly less in the 10-µm microplastic treatment than in the treatments where Enoplolaimus spp. were exposed to microplastic particles of smaller sizes (5 µm, 1 µm, or 0.5 µm). The ingestion rates of microplastics by predators/omnivores, non-selective deposit feeders, and selective deposit feeders increased as the size of the microplastic decreased. After transferring Enoplolaimus spp. to filtered seawater following microplastic ingestion, the proportion of Enoplolaimus spp. individuals containing the smallest size microplastic (0.5 µm) decreased by 15% of the ingested amount in 3 days. In conclusion, there was a significant difference among microplastic-size treatments, but not among feeding types or in the interaction between microplastic size and feeding type. The size of microplastics, rather than feeding type of nematodes, impacted ingestion rates. It is possible that microplastics in the sediment are ingested by nematodes living in marine benthic ecosystems.