The ecotoxicology of carbon nanotubes in the marine environment

With their high aspect ratio, strength, light weight and electrical conductivity single-walled carbon nanotubes (SWCNTs) provide properties of great interest to industry, and, consequently, are finding use in an ever increasing number of products and applications, which will eventually lead to their appearance in the environment. The toxicity of SWCNT, however, is controversial, mainly because of the inconsistency of results, owing to a lack of understanding of the behaviour of SWCNTs in environmental media. The present study explored the ecotoxicology of engineered SWCNTs and their interacton with other contaminants as well as their potential for trophic transfer. A series of co-contaminant experiments was performed to understand the interaction of SWCNTs with dissolved metals at low-level and non-toxic concentrations of both. The results showed that any nano-scale effects were negated by the tendency of uncoated SWCNTs to agglomerate in seawater. However, SWCNTs, in combination with natural organic matter (NOM), remained suspended for long enough to become available to filter-feeding mussels. A potentiating toxicological effect was observed, expressed as DNA strand breaks obtained using the comet assay and oxidative stress, on divalent metals afforded by negatively charged SWCNT agglomerates in seawater at concentrations as low as 5μg L-1. This is supported by the observation that SWCNTs alone were only toxic at concentrations ≥100μg L-1 and that the SWCNT-induced DNA damage was correlated with oxidative stress only in the absence of metals The potential for trophic transfer was assessd using the green algae (Tetraselmis suecica). Light microscopical observations, confirmed by SEM and Raman spectroscopy, showed that SWCNTs adhered to the external algal cell walls and TEM results suggested internaization. A direct effect of SWCNT exposure on the algae was a significant decrease in chlorophyll a concentrations and cell viability. Algea fed to mussels in the presence of SWCNTs led to a signifiacntly increased pseudofaeces production, suggesting selective feeding. However, histological sections of the mussel digestive gland following exposure showed evidence of SWCNTs containing algal and toxicological tests signs of DNA dmage and oxidative stress. In conclusion, the observed SWCNT-algal interaction may facilitate trophic transfer of SWCNTs up the food chain with potential consequences for human health. If these laboratory experiments are confirmed in the natural environment, the present results will have implications for the understanding of the role of carbon nanotubes in environmental metal dynamics, toxicology, and consequently, regulatory requirements