Impact of metal (Ag, Co and Ni) and metal oxide (TiO2) engineered nanoparticles on soil microbial biomass and Lumbricus rubellus

Nanotechnology is the new frontier of research and nowadays many nanoproducts, thanks to their revolutionary physico-chemical properties, are employed in common goods, medical supplies and agricultural products. On the other hand, we have a poor knowledge of the impact of these new materials on human health and environment. Several studies sustain that the environmental compartment designed to be the major recipient of engineered nanoparticles (NPs) is soil. With the aim of investigating the impact of commercially relevant NPs on soil we compared the effect of Ag, Co, Ni, TiO2 NPs on soil chemical properties, on microbial community and on Lumbricus rubellus. Ten mature (clitellate) earthworms were placed in five hundred grams of artificial soil (sandy loam texture, pH 6.9, 4.3% TOC, 0.5% TKN) and incubated at constant temperature and controlled moisture (25°C and 60% WHC) for 28 days. Earthworms were fed once a week with polluted vegetable for a total amount of 20 µg.. The physico-chemical properties of the technological soil ( pH, total organic carbon, total nitrogen) did not significantly change after the 4 weeks of exposure to NPs. The more interesting results were obtained from the study of soil organisms. Indeed, soil microbial biomass C (Cmic) significantly decreased after exposure to metal-NPs with respect to the control, except in TiO2-NPs treatment. High amount of elements arising from NPs were found in polluted soils and earthworms tissues. The presence of NPs in earthworms tissue was confirmed by ESEM-EDS. The analysis of FAMEs in worms did not show significant differences in the total amount of fatty acids, whereas their composition changed. In fact, in the contaminated samples there was an increase of saturated FAs and a decrease of both mono- and poly-unsaturated FAs with respect to the control. Changes in degree of unsaturation affect membrane fluidity: saturated FAs are less susceptible to free radicals, they can pack together better than unsaturated ones and, therefore, make membrane more viscous and less permeable. The order with which NPs affect at the greatest extent FAs composition of earthworm tissues was Ni > TiO2 = Co > Ag. These results suggested that NPs induced stress on soil organisms: a decrease of total amount of soil microbial biomass and a physiological alteration in Lumbricus rubellus.