Effects of silver nanoparticles on freshwater microbial communities

Nanoparticles (NPs) display special chemical properties because of their size, shape, composition and electronic structure. These properties lend NPs their functionality, but may also lead to toxic effects. Due to their widespread use in consumer products an exposure of the aquatic environment to NPs is anticipated and already proven in first analytical surveys. Especially metal/metaloxide NPs are widely used, of which silver nanoparticles (AgNPs) have gained considerable attention due to their broad microbiocidal properties. This implies a specific hazard for exposed aquatic primary producers (algae) and bacteria. In a case study with AgNPs, the toxicity to natural freshwater microbial communities was determined. The microbial communities were exposed to AgNPs of different sizes (10, 20, 40 and 50 nm) as well as different coatings (non-coated, citrate coated) in concentrations from 0.1 - 5000 nmol/L expcept for one silver nanoparticle type (50 nm, uncoated) which was tested in a range from 100 - 10 000 nmol/L. The selected silver nanoparticles were all purchased from commercial producers and distributers (Amepox, British Biocell, NanoTrade, Tedpella). To distinguish between particle related effects and effects caused by free silver ions silver nitrate was tested as a reference in corresponding total silver concentrations. The effects on the algal parts of the community was studied through pigment profile analysis with HPLC. The bacterial community composition was analysed by metabolic profiling on so called ecologplates[TRADEMARK] containing different carbon sources with a redox dye responding to respiration with colour change. The metabolic activity i.e. the colour change was detected as absorbance and was measured over time in several intervals for a total time period of 96 hours. Inhibition of growth detected as decrease in total biomass was seen in both bacterial and algal communities. All experiments were backed up by analytical measurements, measuring the total silver content, dissolved Ag+ (ultrafiltration) and particle size distribution (Nanosight).