Department Environmental Toxicology

Interactions of nanoparticles with biofilms

With the aim of developing an understanding of the impact of AgNP on food web interactions and ecological processes, we are currently using the fungal litter-decomposition and algal biofilm system in streams as models to assess effects of nanomaterials in ecosystems. Emphasis is being placed on testing for effects on interactions in simplified food webs (fungi-bacteria-detritivores and algae-bacteria-herbivores, respectively) and on key ecosystem processes (leaf litter decomposition and primary production) upon short-term and long-term exposure. The experiments are carried out in microcosms and indoor experimental stream channels. Assessed responses include growth and activity parameters of fungi, bacteria, algae and invertebrates as well as rates of litter decomposition and primary production.  Main results show that depending on the assessed endpoints, AgNP toxicity to litter associated fungi and bacteria as well as biofilms in streams can either be mediated by dissolved Ag+ ions or be directly caused by the particles. Trophic transfer experiments with snails feeding on biofilms contaminated with AgNP show no direct effects on the snail but on the development of hatched eggs as evidenced by malformations of the early snail embryos (SNEP, SNFS NFP 64: PhD thesis C. Gil Allué, Dr. A. Tlili).

Examination of the effects of sulfidized AgNP, which represent a major transformation form of AgNP in waste water effluents, show compared to AgNP no toxicity to various functions of biofilms. However, effects become manifest upon ozonation of the sulfidized particle, due to formation of dissolved Ag+ ions ((SNEP, SNFS NFP 64, Dr. B. Bonet).

A long-term study comparing the effects of ionic and NP silver to periphyton focused on the composition of extracellular substances (EPS) and 3D structure, which are relevant to the interaction with other pollutants and to near-bed hydrology. Reduced diversity, abundance of diatoms and EPS composition were related to the amount of ionic silver, whereas 3D structure was specifically affected by both, particulate and dissolved silver (Ambizione, Dr. A. Kroll, L. Sgier).

Contact

Prof. Dr. Kristin Schirmer Head of department Tel. +41 58 765 5266 Send Mail

Publications

Chronic exposure effects of silver nanoparticles on stream microbial decomposer communities and ecosystem functions

With the accelerated use of silver nanoparticles (AgNP) in commercial products, streams will increasingly serve as recipients of, and repositories for, AgNP. This raises concerns about the potential toxicity of these nanomaterials in the environment. Here we aimed to assess the impacts of chronic AgNP exposure on the metabolic activities and community structure of fungal and bacterial plant litter decomposers as central players in stream ecosystems. Minimal variation in the size and surface charge of AgNP indicated that nanoparticles were rather stable during the experiment. Five days of exposure to 0.05 and 0.5 μM AgNP in microcosms shifted bacterial community structure but had no effect on a suite of microbial metabolic activities, despite silver accumulation in the decomposing leaf litter. After 25 days, however, a broad range of microbial endpoints, as well as rates of litter decomposition, were strongly affected. Declines matched with the total silver concentration in the leaves and were accompanied by changes in fungal and bacterial community structure. These results highlight a distinct sensitivity of litter-associated microbial communities in streams to chronic AgNP exposure, with effects on both microbial functions and community structure resulting in notable ecosystem consequences through impacts on litter decomposition and further biogeochemical processes.
Tlili, A.; Jabiol, J.; Behra, R.; Gil-Allue, C.; Gessner, M. O. (2017) Chronic exposure effects of silver nanoparticles on stream microbial decomposer communities and ecosystem functions, Environmental Science and Technology, 51(4), 2447-2455, doi:10.1021/acs.est.6b05508, Institutional Repository

Silver nanoparticle effects on stream periphyton during short-term exposures

Silver nanoparticles (AgNP) are increasingly used as antimicrobials in consumer products. Subsequently released into aquatic environments, they are likely to come in contact with microbial communities like periphyton, which plays a key role as a primary producer in stream ecosystems. At present, however, very little is known about the effects of nanoparticles on processes mediated by periphyton communities. We assessed the effects of citrate-coated silver nanoparticles and silver ions (dosed as AgNO3) on five functional end points reflecting community and ecosystem-level processes in periphyton: photosynthetic yield, respiration potential, and the activity of three extracellular enzymes. After 2 h of exposure in experimental microcosms, AgNP and AgNO3 inhibited respiration and photosynthesis of periphyton and the activities of two of the three extracellular enzymes. Addition of a chelating ligand that complexes free silver ions indicated that, in most cases, toxicity of AgNP suspensions was caused by Ag(I) dissolved from the particles. However, these suspensions inhibited one of the extracellular enzymes (leucine aminopeptidase), pointing to a specific nanoparticle effect independent of the dissolved Ag(I). Thus, our results show that both silver nanoparticles and silver ions have potential to disrupt basic metabolic functions and enzymatic resource acquisition of stream periphyton.
Gil-Allué, C.; Schirmer, K.; Tlili, A.; Gessner, M. O.; Behra, R. (2015) Silver nanoparticle effects on stream periphyton during short-term exposures, Environmental Science and Technology, 49(2), 1165-1172, doi:10.1021/es5050166, Institutional Repository

Extracellular polymeric substances (EPS) of freshwater biofilms stabilize and modify CeO2 and Ag nanoparticles

Streams are potential receiving compartments for engineered nanoparticles (NP). In streams, NP may remain dispersed or settle to the benthic compartment. Both dispersed and settling NP can accumulate in benthic biofilms called periphyton that are essential to stream ecosystems. Periphytic organisms excrete extracellular polymeric substances (EPS) that interact with any material reaching the biofilms. To understand the interaction of NP with periphyton it is therefore crucial to study the interaction of NP with EPS. We investigated the influence of EPS on the physicochemical properties of selected NP (CeO2, Ag) under controlled conditions at pH 6, 7.6, 8.6 and light or dark exposure. We extracted EPS from five different periphyton communities, characterized the extracts, and exposed CeO2 and carbonate-stabilized Ag NP (0.5 and 5 mg/L, both 25 nm primary particle size) and AgNO3 to EPS (10 mg/L) over two weeks. We measured NP size distribution, shape, primary particle size, surface plasmon resonance, and dissolution. All EPS extracts were composed of biopolymers, building blocks of humic substances, low molecular weight (Mr) acids, and small amphiphilic or neutral compounds in varying concentrations. CeO2 NP were stabilized by EPS independent of pH and light/dark while dissolution increased over time in the dark at pH 6. EPS induced a size increase in Ag NP in the light with decreasing pH and the formation of metallic Ag NP from AgNO3 at the same conditions via EPS-enhanced photoreduction. NP transformation and formation were slower in the extract with the lowest biopolymer and low Mr acid concentrations. Periphytic EPS in combination with naturally varying pH and light/dark conditions influence the properties of the Ag and CeO2 NP tested and thus the exposure conditions within biofilms. Our results indicate that periphytic organisms may be exposed to a constantly changing mixture of engineered and naturally formed Ag NP and Ag+.
Kroll, A.; Behra, R.; Kaegi, R.; Sigg, L. (2014) Extracellular polymeric substances (EPS) of freshwater biofilms stabilize and modify CeO2 and Ag nanoparticles, PLoS One, 9(10), e110709 (16 pp.), doi:10.1371/journal.pone.0110709, Institutional Repository

Effect of TiO2 nanoparticles and UV radiation on extracellular enzyme activity of intact heterotrophic biofilms

When introduced into the aquatic environment, TiO2 NP are likely to settle from the water column, which results in increased exposure of benthic communities. Here, we show that the activity of two extracellular enzymes of intact heterotrophic biofilms, β-glucosidase (carbon-cycling) and l-leucin aminopeptidase (nitrogen-cycling), was reduced following exposure to surface functionalized TiO2 NP and UV radiation, depending on the particles’ coating. This reduction was partially linked to ROS production. Alkaline phosphatase (phosphorus-cycling) activity was not affected, however in contrast, an alkaline phosphatase isolated from E. coli was strongly inhibited at lower concentrations of TiO2 NP than the intact biofilms. These results indicate that enzymes present in the biofilm matrix are partly protected against exposure to TiO2 NP and UV radiation. Impairment of extracellular enzymes which mediate the uptake of nutrients from water may affect ecosystem function.
Schug, H.; Isaacson, C. W.; Sigg, L.; Ammann, A. A.; Schirmer, K. (2014) Effect of TiO2 nanoparticles and UV radiation on extracellular enzyme activity of intact heterotrophic biofilms, Environmental Science and Technology, 48(19), 11620-11628, doi:10.1021/es502620e, Institutional Repository