Ecotoxicology of carbon nanotubes to sediment-dwelling bivalves

Abstract

Multi-walled (MWCNT) and single-walled (SWCNT) carbon nanotubes (CNTs) are high-aspect ratio nanostructures with a combination of properties making them useful in an increasing number of applications and products. Although CNTs occur in the environment as the result of natural combustion processes, significant environmental exposure to engineered CNTs would previously not have occurred and, therefore, they are considered to be xenobiotics of emerging concern. High concentrations of CNTs, when combined with contaminants found in sediments, are harmful to aquatic organisms and, therefore may be harmful to sediment-dwelling organisms. The present study examines the interaction of CNTs (MWCNTs and SWCNTs) with sediment, in terms of their bioavailability to the marine cockle, Cerastoderma edule, through in vivo exposure. Observations with a light microscope, confirmed by Raman spectroscopy and transmission electron microscopy (TEM), showed that MWCNTs and SWCNTs accummulated in the mantle cavity. A Raman mapping technique was also performed to detect the availability of CNTs within sediments. The present study also examined the interaction of CNTs (MWCNTs and SWCNTs) with sediment, in terms of their ecotoxicological impact on C. edule, with and without other sedimentassociated contaminants, through three different in vivo exposure conditions. The MWCNTs and SWCNTs were spiked into the water column (water-spiked), spiked onto the surface of the sediment (surface-spiked) or mixed with the sediment (sediment-spiked). Their ecotoxicological impact was assessed as DNA damage using the comet assay and oxidative stress biomarkers (superoxide dismutase activity and lipid peroxidation). The bioavailability and subsequent toxicity of CNTs were governed by their interaction with sediments. It was found that CNTs mixed with sediment were less toxic than the water-spiked or surface-spiked CNTs, reflecting the feeding habits of cockles and the bioavailability of CNTs. Under the experimental exposure conditions, although SWCNTs were more toxic than MWCNTs, both CNT types were significantly toxic only at concentrations ≥ 50 μg L-1 [0.050 ppm] for waterspiked treatment, ≥ 100 μg L-1 [0.1 ppm] for surface-spiked treatment, and ≥ 0.2 µg.g - 1 [200 ppm] for sediment-spiked treatment. In a genotoxicity assay, sedimentassociated contaminants (Cd+2 and Zn+2) were found to have a toxic effect when combined with CNTs, as they became toxic even at low concentrations of 0.1 µg.g-1 . Although SWCNTs were found to be more toxic to exposed cockles than MWCNTs, the DNA damage and oxidative stress in MWCNTs exposures were almost equal to those for SWCNTs when combined with sediment-associated contaminants. These results will assist the understanding of the ecotoxicology of CNTs in the marine environment, particularly in the sediment compartment, where they are expected to accumulate.