Interaction of silver nanoparticles with the aquatic fern Azolla filiculoides: root structure, particle distribution, and silver accumulation

Abstract

The increased use of silver nanoparticles (AgNPs) is expected to result in negative environmental impact, particularly on aquatic ecosystems. The effect of AgNPs to aquatic plants has received little attention. Azolla filiculoides is an interesting aquatic fern with phytoremediation capacities. Therefore, the objective of this research was to evaluate the interaction of AgNPs with A. filiculoides, observing NPs internalization, accumulation, and effects on growth and root structure with high-resolution imaging and analytical techniques. The AgNPs bio-synthesized with Camellia sinensis extracts showed regular shape (quasi-spherical) and size (26.74 ± 0.52 nm). A. filiculoides plants were grown in Yoshida nutrient solution (pH 5) and exposed to AgNPs (0.0, 0.1, 1.0, 5.0, 10.0, 100.0 mg L−1), under a completely random experimental design. The Spearman’s correlation indicated a high negative relation between the AgNPs concentration, and dry biomass (DB), associated with a higher duplication rate (DR). AgNP internalization and distribution were studied with high-resolution scanning electron microscopy and energy-dispersive X-ray spectroscopy (EDS) combined with backscattered electron (BSE) images on surfaces and thin (70 nm) sections of roots and fronds. Our results support the hypothesis that AgNPs deposit on roots and fronds subphases and dissociate into Ag+ that enter the root and accumulate in the endodermis, where they form AgNPs. Structural damages on the rhizodermis and vascular cylinder at high concentrations (100.0 mg AgNPs L−1) suggest the impairment of the main functions of roots, reducing either Ag+ or AgNP uptake and translocation, affecting plant growth and silver accumulation in the fern.