Sublethal effects of nanodiamonds and copper oxide on the freshwater shrimp, Caridina Africana

Abstract

Nanomaterials are defined as materials where one dimension is sized 1 nm – 100 nm and are emerging technologies that are used for the benefit of mankind. Two noteworthy materials include nanodiamonds (NDs) and nano copper oxide (nCuO) as they have intrinsic properties that make them appealing for their uses in the medical and agricultural fields. However, once in an aquatic environment, they can negatively affect the organisms that inhabit it. The effects that contaminants exert on aquatic organisms can be organized into adverse outcome pathways (AOP), starting from the first anchor point, where the organism is affected on a molecular level, and in turn how this molecular initiating event is responsible for any changes in their physiological processes, influencing an apical endpoint within the organism or its population. Although these pathways are available for various contaminants, they are still scarce for nanomaterials. This study aimed to construct an AOP for NDs and nCuO and copper chloride for the freshwater shrimp, Caridina africana. Firstly, C. africana were acutely exposed to sublethal concentrations (LC10 and LC20) of two nanomaterials and copper ion to assess the sublethal effects on the shrimp’s physiological processes, such as their respiration rate, heart rate as well as behaviour. Thereafter, chronic toxicity tests were carried out to determine mortality rates after long-term exposure. The shrimp were subjected to metabolomic analyses to determine the anchor point responsible for the physiological changes exerted. Of the two nanomaterials, nCuO was more toxic than NDs at sublethal concentrations. The toxicity was expressed throughout the altered physiological processes, such as elevated respiration rates, decreased heart rate and reduced locomotor activity. The nCuO toxicity can be attributed to its particle size, agglomeration potential that results in the uptake of the particle, its adherence to the external surface of the shrimp and the uptake of copper ions following dissolution from the material by the shrimp. This resulted in nCuO causing the generation of reactive oxygen species. The copper chloride’s toxicity resulted in an increase for all physiological endpoints. Its toxicity can also be caused by the dissolution of copper ions, which accumulates in the shrimp and generates reactive oxygen species, altering their physiological processes. Nanodiamonds toxicity is attributed to the same parameters as nCuO, and although considered inert, it caused the generation of reactive oxygen species, resulting in an elevated respiration rate and a decrease in locomotor activity. The different nanomaterials’ effects on the endpoints were related to their physicochemical characteristics. These hierarchical biological responses were successfully integrated into the AOP construct.