Speciation of and the effects of silver nanoparticles on the earthworm Aporrectodea caliginosa

Abstract

Silver nanoparticles (AgNPs) were first used as antimicrobial agents. Through technological development, AgNPs are now used in a wide range of applications across the commercial sector due to their beneficial antimicrobial and other desirable traits. However these traits are also what has generated great concern to environmental scientists. There are numerous suggested pathways that can cause AgNP soil contamination. This research was designed to compare the toxic effects of low environmentally relevant doses of AgNP and AgNO₃ on the earthworm species A. caliginosa. A total of 60 earthworms were DNA typed for the cytochrome oxidase I gene to identify A. caliginosa specimens. The genetically typed earthworms were then used to construct a breeding population of A. caliginosa. Citrate coated AgNPs were synthesised and characterised for soil exposure experiments. Earthworms were exposed to 0.3 mgkg⁻¹ AgNP, 30 mgkg⁻¹ AgNP, 3 mgkg⁻¹ AgNO₃ and 0 mgkg⁻¹ control in 600 g of organic soil. RNA samples and coelomocyte samples were removed from earthworms at 3 time points, 2, 7 and 14 d. Gene expression of catalase (CAT), superoxide dismutase (SOD) and metallothionein (MT), and also coelomocyte viability and differential coelomocyte cell counts were studied. CAT gene expressions were significant across treatments notably at the 14 d time point. SOD gene expressions were also significant across time, and several - fold inductions of SOD were measured across all the treatment conditions at the 14 d time point. There was no significant effect on coelomocyte cell viability. Among the coelomocytes, the amoeobocyte proportions were significantly higher on day 14 across all treatments and granulocytes were significantly higher on day 2 across all treatments. The research showed an effect on gene expression of CAT and SOD and also on coelomocyte cell type proportion fluctuations following exposure to AgNP and AgNO₃. Overall, the threat of environmentally relevant AgNP doses in soil do not pose an immediate threat to the earthworm species A. caliginosa based on the few parameters studied.