Biochemical- and neuro-toxicity of silver nanoparticles in soil to earthworms (Aporrectodea calginosa) : A dissertation submitted in partial fulfilment of the requirements for the degree of Master of Science in Food Innovation at Lincoln University

Abstract

Silver nanoparticles (AgNPs) are one of the most common and widely used metal nanoparticles around the world due to their unique chemical and physical properties. Their application in a wide range of industries has resulted in their release into aquatic and soil ecosystems, raising environmental and food safety concerns. Their continued accumulation in soils has detrimental impacts on living organisms including soil-dwelling earthworms, and their potential accumulation in plants impacts indirectly on humans. Earthworms are an environmentally important organism, contributing greatly to soil fertility and indirectly affecting agricultural production. Earthworms are also sensitive to exposure to chemicals such as AgNPs and hence can be used as indicator organisms to evaluate terrestrial pollution. In this study, the earthworm Aporrectodea caliginosa was used to assess the effect of AgNPs in soil at different concentrations (0, 0.3, 3, 30, 300 mg/kg). Measurements were taken of their body weights, four antioxidant enzymes [(i) superoxide dismutase (SOD), (ii) glutathione transferase (GST), (iii) catalase (CAT) and (iv) glutathione peroxidase (GPx)], and lipid peroxidation (LPO), at 2 (n = 4) and 4 (n = 6) weeks after exposure. Significant increases (P < 0.05) in CAT and GPx were observed in earthworms exposed to the highest AgNP concentration (300 mg/kg) and a non-significant increase was evident in LPO compared with the controls. A highly sensitive measure, nerve conduction velocity (NCV), has been used to monitor neurotoxic effects in organisms. The effect of AgNPs on the NCV of the largest nerve fibre (medial giant fibre; MGF) in earthworms was also measured at 2 (n = 4) and 4 (n = 6) weeks, as an indirect assessment of the impact of AgNPs on their escape reflex, which is important for their survival. A dose- and time-dependent non-significant (P > 0.05) NCV decrease in MGF was observed in all earthworms exposed to soil AgNPs. The decline in MGF NCV of the earthworms exposed to the highest AgNP concentration (300 mg/kg) was significant (P < 0.05). Given the likelihood of ever-increasing AgNP concentrations in soil, my findings of an increase in LPO (although non-significant), which is a reflection of oxidative stress, a significant (P < 0.05) increase in CAT and GPx antioxidant enzyme responses to oxidative stress, and a decline in MGF NCV at the highest concentration of 300 mg/kg AgNPs in a common indicator organism earthworm are indicative of the future potential impact of AgNPs on living organisms. How the impacted earthworms may affect soil fertility and thus indirectly agricultural production in the coming years because of the ongoing increase in soil AgNP concentrations requires further investigation.