A trophic transfer study: accumulation of multi-walled carbon nanotubes associated to green algae in water flea Daphnia magna

Abstract

Carbon nanotubes (CNT) are promising nanomaterials in modern nanotechnology and their use in many different applications leads to an inevitable release into the aquatic environment. In this study, we quantified trophic transfer of weathered multi-walled carbon nanotubes (wMWCNT) from green algae to primary consumer Daphnia magna in a concentration of 100 μg L−1 using radioactive labeling of the carbon backbone (14C-wMWCNT). Trophic transfer of wMWCNT was compared to the uptake by daphnids exposed to nanomaterials in the water phase without algae. Due to the rather long observed CNT sedimentation times (DT) from the water phase (DT50: 3.9 days (d), DT90: 12.8 d) wMWCNT interact with aquatic organisms and associated to the green algae Chlamydomonas reinhardtii and Raphidocelis subcapitata. After the exposition of algae, the nanotubes accumulated to a maximum of 1.6 ± 0.4 μg 14C-wMWCNT mg−1 dry weight−1 (dw−1) and 0.7 ± 0.3 μg 14C-wMWCNT mg−1 dw−1 after 24 h and 48 h, respectively. To study trophic transfer, R. subcapitata was loaded with 14C-wMWCNT and subsequently fed to D. magna. A maximum body burden of 0.07 ± 0.01 μg 14C-wMWCNT mg−1 dw−1 and 7.1 ± 1.5 μg 14C-wMWCNT mg−1 dw−1 for D. magna after trophic transfer and waterborne exposure was measured, respectively, indicating no CNT accumulation after short-term exposure via trophic transfer. Additionally, the animals eliminated nanomaterials from their guts, while feeding algae facilitated their excretion. Further, accumulation of 14C-wMWCNT in a growing population of D. magna revealed a maximum uptake of 0.7 ± 0.2 μg mg−1 dw−1. Therefore, the calculated bioaccumulation factor (BAF) after 28 d of 6700 ± 2900 L kg−1 is above the limit that indicates a chemical is bioaccumulative in the European Union Regulation REACH. Although wMWCNT did not bioaccumulate in neonate D. magna after trophic transfer, wMWCNT enriched in a 28 d growing D. magna population regardless of daily feeding, which increases the risk of CNT accumulation along the aquatic food chain.