Microplastics may have very different types of effects on the environment: they may physically (mechanically) affect organisms, act as vectors for hydrophobic pollutants and as substrates for organisms, and affect sediment properties.

Physical effects of microplastics

Macroplastics physically affect marine organisms. Especially for air-breathing animals, entanglement may result in death. The ingestion of macroplastic items may reduce the amount of consumed food and, consequently, the organisms’ fitness. Macroplastics can also block the intestinal tract and cause internal injuries [58, 75, 169–171]. It has been assumed that microplastics cause similar effects in smaller organisms, mainly with regard to the physical obstruction of feeding and digestion [6, 7, 71, 75]. Sharp-edged microplastics may injure gill tissues and the intestinal tract [7, 144]. In the following, the available data on physical effects of microplastics on aquatic organisms are summarised (see also Additional file 1: Table S3). While studies dealing exclusively with effects of nanoplastics were not considered, results of comparative studies of nano- and microplastics have been included.

Physical effects of microplastics on marine organisms

To date, most studies were performed with marine invertebrates. In larvae of the sea urchin T. gratilla exposed for 5 days to fluorescent PE microspheres (10–45 µm, 103–3 × 105 items/L), effects were only observed at the highest tested concentration. Body width was significantly lower than in the control; survival was reduced to approx. 50 % of the control level, but this effect was not significant [151].

In the marine copepod Centropages typicus, a 24-h exposure to concentrations ≥7 × 106 items/L of fluorescent PS microspheres (7.3 µm) led to significantly reduced ingestion of algae [146]. Similarly, Calanus helgolandicus exposed for 24 h to PS spheres (20 µm, 7.5 × 104 items/L) ingested 11 % less algae than the controls. In addition, exposed copepods preferentially ingested smaller algae. During a 6-day exposure of C. helgolandicus to the same type and concentration of PS spheres, egg production was not significantly affected, but egg size was reduced during the second half of exposure. This effect was attributed to energy depletion [172].

In 96-h acute tests, survival of adults and nauplii of the copepod T. japonicus was not affected by nano- (50 nm) and microsized (0.5 and 6 µm) PS spheres at concentrations up to 1.1 × 1015 (50 nm), 1.1 × 1012 (0.5 µm) and 6.6 × 108 items/L (6 µm). Chronic effects of these three sizes of PS spheres were studied in a two-generation test with T. japonicus. The nanosized (50 nm) spheres led to a significant reduction in survival of the first (F 0 ) and the second generation (F 1 ) at concentrations ≥4.6 × 1012 items/L. In F 0 and F 1 , the development from nauplius to copepodid was delayed at 4.6 × 1012 items/L (see also Additional file 1: Table S3). For both sizes of microspheres, the pattern of toxicity was different. The 0.5-µm microspheres caused an increased development time (both from nauplius to copepodite and from nauplius to adult) and a reduced survival of the F 1 at the highest concentration (9.1 × 1010 items/L). The 6-µm microspheres, for which the highest concentration contained 5.2 × 107 items/L, had no effect on survival and development of both copepod generations. However, microspheres of both sizes significantly reduced fecundity of the F 0 and the F 1 at all tested concentrations, i.e. the lowest observed effect concentration (LOEC) was ≤4.6 × 108 items/L for the 0.5 µm spheres and ≤2.6 × 105 items/L for the 6 µm spheres. The effects on fecundity may have been a consequence of a reduced quantity of ingested food associated to the presence of larger amounts of ingested microspheres [149]. It is well known that reduced growth often leads to a reduced fecundity [173–175].

In juveniles of the marine isopod I. emarginata fed for 6–7 weeks (2 moult cycles) with agar-based food containing seaweed powder and fluorescent microspheres (10 µm, approx. 12 items/mg food), PS fragments (1–100 µm, 20 fragments/mg food) or acrylic fibres (0.02–2.5 mm, 0.3 mg/g food), no significant effects on survival, growth and duration of the intermoult period were recorded [156].

Mussels (M. edulis) were exposed for 3 h to fluorescent PS microspheres (3.0 and 9.6 µm, 4.3 × 104 items/L) and then transferred to control water. Effects on their feeding rate, and on haemocyte viability, phagocytic activity of the haemocytes and ability of the haemocytes to cope with oxidative stress were evaluated 3–48 days after transfer to control water. Neither the 3.0 µm nor the 9.6 µm microspheres had any significant effect on the evaluated endpoints [145]. Von Moos et al. [152] exposed M. edulis for 3–96 h to 2.5 g/L (approx. 2.7 × 107 to 3.6 × 107 items/L, see above) of HDPE fluff (0–80 µm). In exposed mussels, granulocytoma formation (indicating an inflammatory response) was significantly increased, and lysosomal membrane stability was significantly reduced. No effects were recorded on the condition index, the neutral lipid content and the accumulation of lipofuscin (a biomarker of oxidative stress) in the digestive tract. A 14-day exposure of M. edulis to PS microspheres (1.1 × 105 items/L) with diameters of 10, 30 and 90 µm led to a significant increase in energy consumption, but did not significantly affect the overall energy budget of the mussels [39].

Acute effects of PE microspheres (1–5 µm) were studied in juveniles of common goby (Pomatoschistus microps). After 96 h exposure to two concentrations of microspheres (18.4 and 184 µg/L), acetylcholinesterase activity was significantly reduced (to approx. 80 % of the control level), while survival and other biomarkers (see Additional file 1: Table S3) were not affected [176].

In a water/sediment study, lugworms (A. marina) were exposed for 28 days to sediment containing unplasticised PVC (uPVC) granules (mean size: 130 µm; 5, 10 and 50 g uPVC/kg sediment ww). During the first 2 weeks of exposure, feeding rate of lugworms was significantly reduced at 50 g/kg sediment ww. However, during the third and fourth week of exposure, feeding rate in the controls decreased to levels close to those observed at 50 g/kg ww. No clear effect on feeding was seen at 5 and 10 g/kg ww. Phagocytic activity of coelomic fluid was significantly increased at 5 and 50, but not at 10 g/kg ww. At 10 and 50 g/kg ww, total available energy reserves were significantly lower than in the controls. Yet, weight of the uPVC exposed worms at the end of the experiment did not significantly differ from the control value. In a second experiment, A. marina was exposed for 51 h to 50 g uPVC/kg ww. The frequency of egestion events (evaluated during the last 3 h of exposure) was significantly reduced at 50 g/kg ww. In a further experiment, lugworms were exposed for 7 days to sediment with 10, 50 and 100 g silica/kg ww. Since exposure to silica did not significantly affect the number of faecal casts, the reduced organic content did not appear to be the cause for the reduced feeding activity. It was assumed that the reduced egestion frequency of the worms at 50 g uPVC/kg ww might be due to a lower feeding activity or a reduced uptake efficiency, possibly caused by reduced adhesion of uPVC particles (as compared to sediment) to the feeding apparatus of A. marina [177].

In a recent 14-days water/sediment test with A. marina exposed to PS microspheres (1.1 × 105 items/kg sediment) with diameters of 10, 30 and 90 µm, protein content of the exposed lugworms was significantly increased, but the overall energy budget was not affected [39]. The shorter exposure duration and the more regular form of the microplastics in this study [39] have probably contributed to the difference between the results of the two water/sediment studies. In addition, the LOEC of 10 g/kg sediment ww obtained in the 28-days test [177] corresponds to a numerical concentration of roughly 8 × 105 items/kg sediment ww (see Additional file 1: Table S3), i.e. a higher concentration than used in the 14-days test [39].

Physical effects of microplastics on freshwater organisms

So far, very few data are available on effects of microplastics on freshwater organisms. Rochman et al. [178, 179] performed a study with Japanese medaka (Oryzias latipes) to evaluate the uptake of contaminants from microplastics into fish and resultant effects (see next section). This study included a treatment with virgin microplastics (pre-production LDPE pellets ground to <500 µm). Fish were fed for 2 months at a rate of 2 % bodyweight per day with a diet containing 10 % (w/w) virgin microplastics. This diet was prepared by reducing the dextrin content of the food and, instead, adding the microplastics. Consequently, it had a lower energy density than the control diet. An appropriate negative control receiving food with the same energy density as the fish exposed to microplastics would have been desirable (see e.g. [180]) but was not included. Survival of the fish receiving the microplastics-containing diet was not affected. However, 46 % of these fish exhibited severe glycogen depletion in the liver. This effect, which was not observed in the controls, was most likely caused by the reduced energy content of the microplastics-containing food. Following exposure to microplastics, the incidence of fatty vacuolar degeneration in medaka liver was slightly increased, while gonad histology was not affected. The microplastic treatment had no significant effect on expression of cyp1a, vitellogenin I and oestrogen receptor α in male and female fish, and on expression of choriogenin H in male fish. In female fish, choriogenin H expression was significantly reduced after 2 months exposure, an effect interpreted by Rochman et al. [179] as early warning sign of endocrine disruption. However, in view of the lower energy density of the microplastics-containing food it appears likely that the reduced choriogenin expression is an effect of the glycogen depletion described above, i.e. should not be considered as endocrine disruption as defined in [181]. Unfortunately, Rochman et al. [178, 179] do not provide any information on effects of the microplastics treatment on fish growth.

Summary: physical effects of microplastics

Physical effects on marine organisms were shown to occur at high concentrations of microplastics. The observed effects appear to be mainly due to the ingestion of microplastics leading to a reduced uptake of food, which in turn results in lower energy reserves and associated effects on other physiological functions. Studies on possible toxic effects of microplastics on freshwater organisms are scarce, effects on terrestrial biota have so far not been investigated [72, 77].

Microplastics as vectors for pollutants

Sorption of pollutants to microplastics

Hydrophobic organic pollutants sorb to microplastics, which are hydrophobic and have a large surface to volume ratio. There is clear evidence that contaminants such as hexachlorinated hexanes, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) are enriched on microplastics [11, 14, 56, 62, 71]. Sorption and desorption processes depend on the polymer type. For instance, phenanthrene reached much higher equilibrium concentrations on PE than on PP and PVC granules (200–250 µm): distribution coefficients (K d ) are 38,100 L/kg for PE, 2190 L/kg for PP and 1650 L/kg for PVC. These K d values are much higher than those for sorption of phenanthrene to two sediments (19 and 135 L/kg). However, when normalising distribution coefficients to the organic carbon content of plastics and sediments (i.e. when comparing K OC rather than K d values), differences between plastics and sediments are strongly reduced. The K OC for sorption of phenanthrene to PE granules (44,500 L/kg) is by a factor of 2–4 higher than those for the sediments (10,400 and 20,100 L/kg), while the K OC values for PP and PVC granules (2560 and 4340 L/kg) are lower than those derived for the sediments [182]. For PCBs, Velzeboer et al. [183] also found a similar magnitude of sorption to microplastics (PE microspheres of 10–180 µm size) and sediment organic matter.

Transport of pollutants sorbed to microplastics

Since microplastics can be transported over long distances, it has been proposed that they may function as vectors for sorbed hydrophobic pollutants. Such pollutants might e.g. be transported to remote sites as the Arctic [121, 182]. The relevance of marine plastics (including both micro- and macroplastics) as transport vectors for PCBs, PBDEs and perfluorooctanoic acid (PFOA) to the Arctic was evaluated by Zarfl and Matthies [121]. Based on estimated amounts of plastics and pollutants in the oceans, sorption of the pollutants to plastics, and ocean current velocities they derived a rough estimate of plastic-mediated mass fluxes of PCBs, PBDEs and PFOA. These mass fluxes were by factors of 103–106 lower than mass fluxes via atmospheric transport and transport with water. Therefore, it was concluded that for most substances, plastics are no relevant vectors for transport to the Arctic. Yet, plastic-mediated transport might increase the mobility of highly hydrophobic substances, which are due to their sorption to sediment quickly removed from the water column.

Uptake of pollutants sorbed to microplastics and resultant effects

Given that (1) concentrations of pollutants on microplastics can be several orders of magnitude higher than in the surrounding water and (2) microplastics are ingested by a wide variety of organisms, it has been postulated that microplastics may lead to an increased uptake of pollutants by aquatic organisms (see e.g. [25]). Such an uptake requires desorption of the contaminants in the organisms. Addition of the digestive surfactant sodium taurocholate was shown to enhance desorption of phenanthrene from PE, PP and PVC microplastics [182]. Similar results were obtained by Bakir et al. [184] for various organic pollutants sorbed to PE and PVC microplastics. Several studies have demonstrated that contaminants, which had been sorbed to microplastics, are transferred to organisms ingesting these microplastics. For instance, nonylphenol and phenanthrene that had been sorbed to PVC microplastics were detected in the tissue of A. marina exposed for 10 days to these microplastics [185].

Besseling et al. [155] exposed A. marina for 28 d to sediment contaminated with low PCB concentrations (5.28 µg PCBs/kg dw)—either alone or in combination with pre-production PS particles (400–1300 µm; 0.074, 0.74 and 7.4 % of sediment dw) previously equilibrated for 6 weeks with the sediment. In the presence of 0.074 % PS particles, PCB concentrations in A. marina were by a factor of approx. 1.1–1.5 higher than in PCB-contaminated sediment without microplastics. At 0.74 and 7.4 % PS particles, PCB concentrations in the worms were lower than with 0.074 % PS, but remained in most cases above levels in the PCB-contaminated sediment without microplastics. The authors concluded that PS microparticles had a relatively limited effect on uptake of PCBs by A. marina. Feeding activity of the lugworms decreased with increasing microplastics content. Worms in all treatments lost weight, and weight loss increased with increasing microplastics concentration. It was suggested that ingestion of the relatively large microplastic particles might have led to physical stress. In addition, organic matter content of the sediment was reduced in the presence of microplastics, i.e. the worms had to ingest larger amounts of sediment.

Rochman et al. [178, 179] performed a two-month experiment with adult medaka (O. latipes) that received control food, or food containing virgin microplastics (see previous section) or contaminated (‘marine’) microplastics. The latter were prepared by exposing pre-production LDPE pellets for 3 months at a marine site. Pellets were then ground to <500 µm and incorporated into fish food. The diets containing microplastics (10 % w/w) had a lower energy density than the control diet. At the end of exposure, levels of PAHs, PCBs and PBDEs in fish that had received marine microplastics were higher than in the control and in the virgin microplastic treatment. Marine microplastics had no significant effect on survival and expression of cyp1a. However, they caused more pronounced histopathological changes in the liver than virgin microplastics: 74 % of the fish exposed to marine microplastics exhibited severe glycogen depletion (virgin microplastics: 46 %), 47 % fatty vacuolar degeneration (virgin microplastics: 29 %) and 11 % single cell necrosis (virgin microplastics: 0 %). In female medaka fed with marine microplastics, the expression of vitellogenin, choriogenin H and oestrogen receptor α was slightly lower than in fish fed with virgin microplastics and significantly lower than in the control fish. These effects were considered as indicators of endocrine disruption [179], but are most likely related to the observed energy depletion. In this context, it is of note that reduced vitellogenin levels can only be interpreted as indicator for endocrine activity, if there is no systemic toxicity [186].

Relevance of microplastics as vector for pollutants

As outlined above, there is clear evidence that hydrophobic contaminants are enriched on microplastics and transferred to organisms ingesting these microplastics. However, it has been questioned, if the transport of sorbed pollutants by microplastics is a relevant factor contributing to accumulation and adverse effects in the environment, i.e. if microplastics transport pollutants in sufficiently high concentrations to biota [99, 187]. In this context, the contribution of the uptake via microplastics to the total uptake of a pollutant (including uptake via integument, gills and food) has to be considered. Since microplastics are in most cases excreted by the organisms that have ingested them, desorption rates of the pollutants from the microplastics in the intestinal tract are important. Modelling approaches have been used to assess the relative contribution of microplastics as vectors to the overall uptake of hydrophobic organic pollutants by A. marina [187, 188] and piscivorous fish [104].

Koelmans et al. [187, 188] developed a biodynamic model for PCB accumulation by A. marina in an environment containing PS and PE microparticles. Different uptake processes and desorption in the intestinal tract were considered. Bioaccumulation of various PCBs was modelled in the presence of three concentrations (0.1, 1 and 10 % of sediment dw) of microplastics (approx. 1 mm) and in the absence of microplastics. PS microparticles had no significant effect on bioaccumulation of PCBs in lugworms. For PE, the model predicted a decrease in steady-state bioaccumulation for PCBs with log K OW values >5–6. When tissue concentrations increase to levels typical for substances with such high K OW , the gradient between concentrations in tissue and on the ingested PE microparticles may become negative. Consequently, PCBs may be resorbed to the microplastics, i.e. bioaccumulation is attenuated. Based on these results, Koelmans et al. [187] concluded that the contribution of microplastics to bioaccumulation can be assumed to be not very relevant.

Similar results were obtained by Gouin et al. [104] with two modelling approaches. Using an equilibrium partitioning model significant (>1 %) partitioning to plastics was only predicted for environments with very high plastic concentration and limited natural organic matter. Based on the steady-state food-web component of the bioaccumulation model of Arnot and Gobas [189], into which 10 % PE microplastics were included as additional component of the diet, a reduced bioaccumulation was obtained for substances with log K OW values between 5.5 and 6.5. As outlined above, this reduction is due to the high affinity of the microplastics to the pollutants, which prevents transfer of the pollutants to the fish. Gouin et al. [104] concluded that microplastics have a limited relevance as vector for the transfer of hydrophobic pollutants to fish. A number of uncertainties were identified, which include the effects of fouling on sorption of pollutants to microplastics, gut retention times of microplastics and uptake rates of microplastics into the tissue of an organism.

Microplastics as substrates for organisms

Many marine organisms live attached to debris [190, 191]. Due to plastics, the amount of floating debris in the oceans has greatly increased [190]. This is most relevant in the open ocean, where only very limited floating substrate is available [128, 192]. Plastic debris is often colonised by microorganisms and—depending on its size—also by larger organisms. PE microplastics and small macroplastics collected in the surface layer of the North Atlantic were colonised by a variety of organisms including bacteria, cyanobacteria, diatoms, ciliates and radiolaria [191]. Bryozoans were identified on 3–5 mm-sized microplastics sampled in the surface layer at different sites around Australia [193]. Microplastic particles are also used as oviposition sites by the sea skater Halobates, an insect living at the sea/air interface in the open ocean [128, 192]. The strong increase in microplastics in the North Pacific gyre, which was observed between 1972–1987 and 1999–2010, was associated with a considerable increase in the number of eggs, juveniles and adults of Halobates sericeus. Possible effects of the increased abundance of H. sericeus on other species have so far not been investigated [128].

Given that plastics can be transported over long distances, they may contribute to the dispersal of species [75, 190]. This includes invasive species [190] and species causing harmful algal blooms [194]. Due to their low size, microplastics may especially facilitate transport of microorganisms (including pathogens) and other very small organism. So far, it is not known if the transport of species with microplastics has any significant effect on species assemblages [158, 195].

Effects of microplastics on sediment properties

Carson et al. [134] used artificially constructed beach sediment cores containing 1.5, 7.3, 15.9 and 29.4 % (w/w) of small plastic particles (<10 mm) to investigate effects on water permeability and heat transfer. Notably, mean size of plastic particles was higher than mean grain size of the sediment. At 15.9 and 29.4 % (w/w) plastics, water permeability was significantly increased. With increasing plastic content, sediments warmed more slowly. A 16 % decrease in heat transfer was recorded at the highest plastic concentration. Possible implications of such effects on physical properties of sediments remain to be investigated.