Uptake routes of microplastics in fishes: practical and theoretical approaches to test existing theories

Abstract

Microplastics are frequently detected in the gastrointestinal tracts of aquatic organisms worldwide. A number of active and passive pathways have been suggested for fish, including the confusion of microplastic particles with prey, accidental uptake while foraging and transfer through the food chain, but a holistic understanding of influencing factors is still lacking. The aim of the study was to investigate frequently suggested theories and identify relevant biotic factors, as well as certain plastic properties, affecting microplastic intake in fish. Four species of freshwater fish, each representing a different combination of foraging style (visual/chemosensory) and domestic status (wild/farmed) were exposed to different realistic plastic concentrations and polymer types with and without the provision of genuine food. As most previous investigations of microplastic uptake routes consider only particles large enough to be perceptible to fish, the potential for accidental intake via drinking water has been somewhat neglected. This route is evaluated in the current study using a model approach. The results show that visually oriented fish forage actively on microplastic particles that optically resemble their usual food, while fish with a predominantly chemosensory foraging style are more able to discriminate inedible food items. Even so, the accidental uptake of microplastics while foraging is shown to be relevant pathway, occurring frequently in both visual and chemosensory foragers alike. Several factors were shown to increase plastic uptake, including microplastic concentration in the water, foraging behaviour promoted by availability of genuine food, and fish size. Although both wild and farmed fish ingested microplastic particles, cultured fish showed less discernment in terms of colour and were more likely to forage actively on microplastics when no food was available. Drinking has been identified as a possible source of microplastic intake specifically for large marine fish species. Particles smaller than <5 µm can pass the gastrointestinal tract wall and bioaccumulation could arise when uptake exceeds release or when particles are assimilated in tissues or organs. The effects of accumulation may be significant, especially in long-living species, with implications for food web transfer and fish as food items.

Toward an Improved Understanding of the Ingestion and Trophic Transfer of Microplastic Particles: Critical Review and Implications for Future Research

“…Under environmentally relevant concentrations, where concentrations in the gastrointestinal tract are not at steady state with the external environment, and where the rate of passage through the gastrointestinal tract may be greater than the rate of biological uptake, accumulation within the organism is unlikely. Alternatively, when the gastrointestinal tract residence time is relatively long compared with the rate of biological uptake, then an increased probability for translocation may be possible (such as observed in the theoretical MICROWEB model applied to seals by Diepens and Koelmans [2018] and to fish by Roch et al [2020]). It is thus recommended that future studies should include efforts aimed at estimating gastrointestinal tract residence times for the species under investigation; such studies should also aim to characterize potential differences between species with different feeding strategies (i.e., filter‐feeders vs nonfilter‐feeders).…”

“…The interaction between organisms and particulates such as ENMs and microplastic particles is thus dynamic, not steady state, and the processes for uptake and elimination are driven by various endocytosis‐related mechanisms (Felix et al 2017), not by passive diffusion through tissues or via solute transporters as they are for soluble chemicals (Schultz 1976; DeVito 2000). Consequently, the assessment of poorly soluble particulates with respect to their potential to bioaccumulate may require the development of new test systems, models, and mechanistic understanding (Handy et al 2018; European Centre for Ecotoxicology and Toxicology of Chemicals 2019; Petersen et al 2019; Roch et al 2020).…”

Microplastics in freshwater fishes: Occurrence, impacts and future perspectives

“…Although MP encounter by fish is assumed to occur mainly during active feeding, there is increasing evidence of MPs being encountered via the gills and/or epidermis of wild freshwater fish (Abbasi et al, 2018;Hurt et al, 2020;Park et al, 2020). Experimental studies have also demonstrated MP accumulation on the gills (Mak et al, 2019;Roch et al, 2020). Passive uptake of MPs is thus an additional source of MPs following environmental exposure during swimming and respiration.…”

Microplastic evacuation in fish is particle size‐dependent

“…Microplastics include fragments, beads, films, and fibres smaller than 5 mm, which are introduced directly into the environment or are produced by weathering from larger plastic waste (Barnes et al., 2009; Lambert et al., 2014). In fish, there are several routes by which MPs might be ingested: they may be transferred directly through the food chain (Santana et al., 2017); accidentally ingested while eating or drinking (Roch et al., 2020); or in some cases, can be confused with food, leading to active ingestion (Ory et al., 2017).…”