Microplastics as vectors for bioaccumulation of hydrophobic organic chemicals in the marine environment: A state-of-the-science review.

A state-of-the-science review was conducted to examine the potential for microplastics to sorb hydrophobic organic chemicals (HOCs) from the marine environment, for aquatic organisms to take up these HOCs from the microplastics, and for this exposure to result in adverse effects to ecological and human health. Despite concentrations of HOCs associated with microplastics that can be orders of magnitude greater than surrounding seawater, the relative importance of microplastics as a route of exposure is difficult to quantify because aquatic organisms are typically exposed to HOCs from various compartments, including water, sediment, and food. Results of laboratory experiments and modeling studies indicate that HOCs can partition from microplastics to organisms or from organisms to microplastics, depending on experimental conditions. Very little information is available to evaluate ecological or human health effects from this exposure. Most of the available studies measured biomarkers that are more indicative of exposure than effects, and no studies showed effects to ecologically relevant endpoints. Therefore, evidence is weak to support the occurrence of ecologically significant adverse effects on aquatic life as a result of exposure to HOCs sorbed to microplastics or to wildlife populations and humans from secondary exposure via the food chain. More data are needed to fully understand the relative importance of exposure to HOCs from microplastics compared with other exposure pathways. Environ Toxicol Chem 2016;35:1667-1676. © 2016 SETAC.

Microscopic investigation of single-wall carbon nanotube uptake by Daphnia magna.

The objectives of this study were to determine the extent of absorption of functionalized single-wall carbon nanotubes (SWCNTs) across the gut epithelial cells in Daphnia magna. Several microscopic techniques were utilized, including micro-Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM) and selective area diffraction (SAD). In an effort to examine the variation in uptake due to surface properties, four groups of differently functionalized SWCNTs were used: hydroxylated (OH-SWCNTs), silicon dioxide (SiO2-SWCNTs), poly aminobenzenesulfonic acid (PABS-SWCNTs) and polyethylene glycol (PEG-SWCNTs). Raman spectroscopy was able to detect OH-SWCNTs within the gut, but lacked the spatial resolution that is needed to identify lower concentrations of SWCNTs that may have been absorbed by body tissues. Initially, low-magnification imaging of exposed D. magna sections in the TEM revealed several features, which suggested absorption of SWCNTs. However, subsequent analysis with additional techniques (HRTEM, X-ray energy-dispersive spectroscopy and SAD) indicated that these features were either artifacts produced via the specimen staining process or consisted of non-graphitic, organic structures. This latter observation emphasizes the inherent difficulty in resolving SWCNTs embedded within a complex, organic matrix, as well as the care with which imaging results must be interpreted and supplemented with other, more analytical techniques.

The influence of natural organic matter on the toxicity of multiwalled carbon nanotubes.

Engineered carbon nanostructures, such as multiwalled carbon nanotubes (MWNTs), are inherently hydrophobic and are not readily stable in aqueous media. However, the aqueous stability and bioavailability of these nanotubes may be influenced by the water quality parameters such as ionic strength, pH, and natural organic matter (NOM). Natural organic matter adsorbs onto the surface of MWNTs, effectively covering the hydrophobic surface and resulting in increased aqueous stability. This enhanced stability is likely to lead to an increased residence time in the water column and increased exposure times for pelagic organisms. In the current study, NOM from three different river systems in the southeast United States increased the stability of MWNT suspensions. The effects of these suspensions were evaluated using acute and chronic bioassays with Daphnia magna and Ceriodaphnia dubia. The 96-h LC50 for D. magna exposed to MWNTs suspended in Suwannee River (USA) NOM was approximately 2.0 mg/L and was not significantly influenced by NOM concentrations ranging from 1.79 to 18.5 mg/L DOC. However, there were differences in 96-h LC50 values among different sources of NOM (Suwannee, Black, and Edisto Rivers, USA). Daphnid growth was reduced in both D. magna and C. dubia, whereas reproduction was reduced in C. dubia. Characterization of the different NOM sources and MWNT suspensions was conducted. Visual inspection using transmission electron microscopy (TEM) and gut elimination observations suggested that the toxicity was attributable to ingested MWNTs clogging the gut tract of D. magna. The TEM micrographs indicated that MWNTs can disaggregate within the gut tract, but single MWNTs are unable to absorb across the gut lumen.

Influence of multiwalled carbon nanotubes dispersed in natural organic matter on speciation and bioavailability of copper.

The dispersion of multiwalled carbon nanotubes (MWNTs) by natural organic matter (NOM) may influence the bioavailability of MWNTs and other contaminants. The speciation and bioavailability of copper (Cu) in MWNTs-associated NOM was studied using Daphnia magna. Cu titration data indicated that the binding affinity of Cu for MWNTs-associated NOM was lower than that for NOM alone. The free Cu(2+) ion activity was increased even by the addition of a low nontoxic concentration of 1.0 mg/L MWNTs. The 96 h LC(50) of MWNTs was determined to be 2.48 mg/L. The Fourier transform infrared (FTIR) spectra results showed clearly different features in Cu spiked between NOM and MWNTs-associated NOM, indicating that the interruption of Cu binding was probably due to steric stabilization of the MWNTs dispersed in NOM, which inhibited the complexation by rendering the functional groups in NOM less favorable to Cu. The mortality and biochemical reactive oxygen species (ROS) production in the D. magna bioassay were enhanced in MWNTs-associated NOM compared to NOM alone because of increased free Cu(2+) ion activity as expected from the titration and FTIR results. This study suggests the bioavailability of Cu is enhanced by the presence of MWNTs interacting with NOM.