Interactive Effects of Zinc and Titanium Nanoparticles of Leaf Decomposition in Freshwater Ecosystems

Abstract

Abstract In conclusion, we observed that titanium nanoparticles inhibited leaf decomposition more strongly than zinc nanoparticles, and that the combination treatments reduced the effects of the titanium. This was opposite to what we initially expected. The nano-titanium reduced bacterial growth, suggesting that this may be one mechanism that could inhibit decomposition. Light may play a role in generation of reactive oxygen species that increase toxicity of the nanoparticles, but effects are complex. Overall, this study highlights the importance of testing the effects of mixtures that are likely to occur in the environment. Metallic nanoparticles are an emerging hazard that will continue to grow as their use expands in the future. Zinc and titanium nanoparticles are used in many consumer and industrial products. Consequently, they are increasingly being detected in the sediments of aquatic ecosystems. Despite their frequent co-occurrence, there is little information on how they interact, although previous studies on cells suggest that nano-titanium may inhibit nano-zinc toxicity by reducing Zn2+ bioavailability. Leaf decomposition is a major source of allochthanous energy in freshwater ecosystems. In this study we measured the effects of zinc and titanium nanoparticles, alone or in combination, on the rate of leaf decomposition. In Experiment 1, leaf discs, produced from senescent leaves of Ficus sycomorus were exposed to either 1, 10 or 100 mg/L of either ZnO or TiO2 nanoparticles, alone or in combination, for six weeks. Mass loss and microbial metabolism were measured at fortnightly intervals and bacterial community composition measured after six weeks using next generation Illumina MiSeq sequencing. In Experiment 2, F. sycomorus leaf discs were exposed for two weeks to10 mg/L ZnO or TiO2 nanoparticles, alone or in combination, both in the light or dark, and in presence or absence of chloramphenicol, a broad spectrum antibacterial, giving a total of 16 treatments. Mass loss, bacterial colony formation, and the C:N ratio of leaf tissue were measured. In experiment 1 there was no evidence that titanium nanoparticles reduced the concentration of Zn2+. After six weeks, there was significantly less mass loss in all titanium nanoparticle treatments, compared with controls, zinc nanoparticles alone, or the combination treatments. Microbial metabolism was initially low, but increased after four weeks, but there were no clear effects of the nanoparticles on oxygen consumption by the microbes. After six weeks, the bacterial communities of leaves treated with 10 and 100 mg/L of the combination treatment and the 100 mg/L zinc nanoparticle treatments, showed a clear separation from other treatments int terms of community composition. The dominant species in the three communities was Bacillus subtillus. In experiment 2, there was greater mass loss in the dark and lower mass loss in treatments containing chloramphenicol. Both nano-titanium and chloramphenicol inhibited bacterial growth, but there were complex three-way interactions between all three factors. The C:N ratio was lowest in controls and highest in the leaves exposed to nano-titanium. There was also an interaction between light and nanoparticle treatment.