Effect of cobalt and silver nanoparticles and ions on Lumbricus rubellus health and on microbial community of earthworm faeces and soil

Abstract

The aim of this study was to investigate the impact of silver and cobalt, supplied both as ions and nanoparticles (Ag+, Co2+, AgNPs, CoNPs) through contaminated food to earthworms (Lumbricus rubellus), on their health as well as on microbial community of both soil and earthworm faeces. Earthworms and microbes were exposed to the contaminants in laboratory microcosms with artificial soil. Contaminants were supplied once a week for 5 weeks by spiking them on horse manure. The accumulation of CoNPs and Co2+ in earthworm tissues was two and three times greater than AgNPs and Ag+, respectively. Except for AgNPs, contaminants significantly affected microbial community structure of earthworm faeces by increasing G- bacteria, thus also increasing the bacteria/fungi ratio while decreasing the G+/G- bacteria ratio. Such shift was also reflected on soil microbial community, thus suggesting a close relationship between microbial community of soil and of earthworm faeces. Neither of the Co treatments affected soil microbial basal respiration whereas they increased the microbial biomass specific respiration or metabolic quotient, suggesting some stress induction on soil microorganisms. Earthworm health was strongly affected as revealed by the reduced fluidity of fatty acids extracted from the body tissues. In addition, the histological investigations, after the depuration period, showed positive results about the NPs toxicity. In particular, TUNEL-positive nuclei in epidermis and in peritoneum, suggest the presence of toxicosis.The ESEM-EDS technique revealed the presence of Ca-P spherules (calcification) between mouth and clitellum of earthworms fed with Co2+ contaminated food.