English abstract
Abstract Nanoparticles are used in many commercial products and can eventually migrate into freshwater ecosystems where they can cause toxic effects either directly or indirectly to ecosystem processes. The aims of this study were to measure the effects of ZnO and TiO2 nanoparticles (NPs) alone and in combination on leaf litter decomposition of Ficus sycomorus in freshwater streams in Oman and to measure their effect on microbial counts, microbial community metabolism, on bacterial community composition and extracellular enzymes. In experiment 1, ZnO and TiO2 NPs alone and in combination (0, 1, 10, 100 mg/L) were used to study their effect on leaf decomposition during three incubation periods (2, 4, 6 weeks). Percentage of mass loss, respiration rate, microbial (bacteria and fungi) counts and B-glucosidase activity were measured. TiO2-NPs alone had the strongest effect, causing reduced of mass loss. Respiration rate increased in ZnO-NPs and in combination solution. There was no significant effect of the NPs on chlorophyll a. There were a significant declined in bacterial number in exposure to Zn-1mg/L and Ti-1 mg/L after 6 weeks of incubation. On the other hand, the combined solution showed no significant effect on bacterial counts. There was higher bacterial diversity at 10 mg/L of all NPs and in the combined solution, which had inhibitory effect in which it was equal to the sum of each NPs. Nitrospira was the dominant genera in controls and decreased with different NPs concentrations. There were 18 genera, 10 classes in all treatments. Alphaproteobacteria, Gammaproteobacteria, Betaproteobacteria and Bacilli were the most dominant classes. Experiment 2 measured the effects of light and bacteria on decomposition. ZnO and Tio NPs alone and in combination at a concentration of 10 mg/L were added to two set of containers, one exposed to the light and the other set covered with black cardboard to provide dark condition. An antibiotic (Chloramphenicol 10 ug) was added to half the treatments. The aim of this part was to differentiate the direct effects of NPs on microbes from effects of light-generated ROS directly on leaf litter decomposition. There was a significant effect of NPs, light and antibiotics on the leaf mass loss. The mass loss in darkantibiotic was higher than light-antibiotic. While, bacterial number in light-non-antibiotic was higher than light-antibiotic except in TiO2 NPs. Also, bacterial number in dark-nonantibiotic were higher than in dark-antibiotic in all NPs treatments. The elements analyzer showed %>N%>H% under all treatments and in dark greater than light. In Experiment 3, leaf discs were exposed to NPs at 10 mg/L with antibiotics in light and dark. ZnO-NPs inhibited mass loss in light only. While there were significantly inhibition on mass loss in TiO-NPs and (Zn+Ti)-NPs in light and dark conditions. The result showed that nanoparticles in aquatic environments can affect leaf litter decomposition directly and indirectly. Further research needed to investigate the effect of these NPs alone and in combination on their behavior against microbial decomposer under different conditions.
