Carbon Nanomaterials in Freshwater Ecosystems: An Chronic, Multi-generational, and Genomic Assessment of Toxicity to Daphnia Magna

Carbon nanomaterials are synthesized with a variety of core structures and surface chemistries to make them more biocompatible for application in different industries, but variation in core structure and functionalization can change the toxicity of carbon nanomaterials to organisms. In addition, current literature is dominated by data from acute toxicity assays, but meta-data is necessary to improve our understanding of nanomaterial toxicity. This project identifies specific core structures and surface chemistries that make carbon nanomaterials more and less toxic using chronic toxicity assays and multi-generational assays to generate a dataset on the sub-lethal impacts of nanomaterials to Daphnia magna. In addition, gene expression was evaluated on organisms from these experiments to assess biochemical pathways that are important in an organism\u27s response to nanomaterial exposure. Results indicate that core structure and surface chemistry influence nanomaterial toxicity to Daphnia. Fullerene gamma-cyclodextrin complexes (C60-GCD) induced 100% mortality to daphnids after 17 days of exposure at 5 ppm, while fullerenes that were not bound to gamma-cyclodextrin were the least toxic particle types to daphnids. Carbon nanotubes induced the most consistent negative impacts to Daphnia reproduction and growth, as all types of carbon nanotubes reduced reproduction or adult size at concentrations of 10 ppm or 50 ppm. Multi-generational impacts to mortality and reproduction were observed in Daphnia for several particles types, and single-walled carbon nanotubes functionalized with carboxy-amides (SWCNT-CONH2) significantly reduced reproduction in the F0, F1, and F2 generations. Many of the carbon nanomaterial exposures did not change the expression of glutathione-s-transferase (GST), vitellogenin fused with superoxide dismutase (VTG-SOD), or NADH dehydrogenase, indicating that other biochemical pathways are important in the toxicity of these materials. However, GST expression was reduced in F0 and F2 daphnids from SWCNT-CONH2 exposures, indicating a possible impairment of this particle type to the oxidative response system. Fullerenes functionalized with malonate and non-covalently bound to GCD (C60-malonate-GCD) induced significant changes in the expression of all three investigated genes, and these changes are indicative of oxidative stress in daphnids (increases in GST transcripts), impacts to reproduction (increased VTG-SOD transcripts), and impacts to mitochondrial metabolism (decreased NADH dehydrogenase transcripts). In addition, these data indicate that GST, VTG-SOD, and NADH dehydrogenase have the potential to be used as biomarkers of early detection of nanomaterial exposure for SWCNT-CONH2 and C60-malonate-GCD particle types. Daphnia have a central role in the trophic structure of ecosystems, as they feed on algae and transfer energy from lower trophic structure to higher trophic structures. A decline in daphnid populations would change the composition of phytoplankton communities, with the potential for eutrophication of small lakes and ponds if daphnid populations are completed eliminated from the ecosystem. Daphnia are also essential food sources for juvenile and adult fish, and a loss of daphnid populations would reduce available nutrition for higher trophic organisms. This research provides a thorough and detailed expression of sub-lethal nanomaterial toxicity to Daphnia in long-term and multi-generational scenarios, and it can be used to inform the synthesis of nanomaterials such that they cause minimal harm to organisms and the environment