Direct and indirect effects of zinc oxide and titanium dioxide nanoparticles on the decomposition of leaf litter in streams

As the production of metallic nanoparticles has grown, it is important to assess their impacts on structural and functional components of ecosystems. We investigated the effects of zinc and titanium nanoparticles on leaf decomposition in freshwater habitats. We hypothesized that nanoparticles would inhibit the growth and activity of microbial communities leading to decreased decomposition rates. We also hypothesized that under natural light, the nanoparticles would produce reactive oxygen species that could potentially accelerate decomposition. In the lab, whole Ficus vasta leaves were placed in containers holding one liter of stream water and exposed to either 0, 1, 10 or 100 mg/L of ZnO or TiO₂ nanoparticles for six weeks (referred to as Exp. 1). We measured leaf mass loss, microbial metabolism, and bacterial density at 2, 4, and 6 weeks. In a second experiment (referred to as Exp. 2), we measured the effects of light and 10 and 100 mg/L ZnO or TiO₂ nanoparticles on leaf mass loss, bacterial density and the bacterial and fungal community diversity over a 2 week period. In Experiment 1, mass loss was significantly reduced at 10 and 100 mg/L after 6 weeks and bacterial density decreased at 100 mg/L. In Experiment 2, there was no effect of ZnO nanoparticles on leaf mass loss, but TiO₂ nanoparticles significantly reduced mass loss in the dark but not in the light. One possible explanation is that release of reactive oxygen species by the TiO₂ nanoparticles in the light may have increased the rate of leaf decomposition. Bacterial and fungal diversity was highest in the dark, but nanoparticles did not reduce overall diversity.