Gold and silver nanoparticle effects on ammonia-oxidizing bacteria cultures under ammoxidation.

Owing to their wide application in industry and manufacturing, understanding the environmental safety of gold (Au) and silver (Ag) nanoparticles entering aquatic environment is a global issue of concern. For this study, ammonia-oxidizing bacteria (AOB) enrichment cultures reproduced from surface sediments taken from the Jiulong River estuary wetlands (Fujian Province, China) were spiked with nano-Ag and nano-Au to determine their impact on ammoxidation and the mechanisms involved in the process. Results showed that nano-Ag significantly inhibited bacterial ammoxidation in aquatic environment, with the average ammoxidation rate decreasing with increasing nano-Ag concentration. The average ammoxidation rate was significantly correlated to the Shannon index, the Simpson index, and AOB abundance. This suggested that ammoxidation inhibition resulted primarily from AOB biodiversity and abundance reduction, caused by the antibacterial property of nano-Ag. However, AOB biodiversity and abundance as well as bacterial ammoxidation were not inhibited by nano-Au (with a maximum experimental concentration of 2 mg L(-1)). Moreover, an insignificant correlation was found between AOB biodiversity and abundance and the average ammoxidation rate under the nano-Au treatment. Given that ammoxidation is regarded as a rate-limiting procedure in nitrogen (N) circulation, nano-Ag would affect N cycling but nano-Au would not after entering aquatic environments. Identified nano-Ag and nano-Au impacts on ammonium nitrogen transformation could be generalized in aquatic environment according to their extensive representation in the phylogenetic tree.

Impact of TiO2 and ZnO nanoparticles at predicted environmentally relevant concentrations on ammonia-oxidizing bacteria cultures under ammonia oxidation.

Increased application of titanium dioxide and zinc oxide nanoparticles (nano-TiO2 and nano-ZnO) raises concerns related to their environmental impacts. The effects that such nanoparticles have on environmental processes and the bacteria that carry them out are largely unknown. In this study, ammonia-oxidizing bacteria (AOB) enrichment cultures, grown from surface sediments taken from an estuary wetland in Fujian Province, China, were spiked with nano-TiO2 and nano-ZnO (with an average size of 32 and 43 nm, respectively) at predicted environmentally relevant concentrations (≤2 mg L(-1)) to determine their impacts on ammonia oxidation and the mechanisms involved. Results showed that higher nano-TiO2 concentrations significantly inhibited ammonia oxidation in enrichment cultures. It is noteworthy that the average ammonia oxidation rate was significantly correlated to the Shannon index, the Simpson's index, and AOB abundance. This suggested that ammonia oxidation inhibition primarily resulted from a reduction of AOB biodiversity and abundance. However, AOB biodiversity and abundance as well as the average ammonia oxidation rate were not inhibited by nano-ZnO at predicted environmentally relevant concentrations. Accordingly, an insignificant correlation was established between biodiversity and abundance of the AOB amoA gene and the average ammonia oxidation rate under nano-ZnO treatments. AOB present in samples belonged to the ß-Proteobacteria class with an affinity close to Nitrosospira and Nitrosomonas genera. This suggested that identified impacts of nano-TiO2 and nano-ZnO on ammonia oxidation processes can be extrapolated to some extent to natural aquatic environments. Complex impacts on AOB may result from different nanomaterials present in aquatic environments at various ambient conditions. Further investigation on how and to what extent different nanomaterials influence AOB diversity and abundance and their subsequent ammonia oxidation processes is therefore required.

[Contribution of plant litters to sediments organic matter in Jiulong river estuary wetland].

The purpose of this study was to characterize the decomposition process of different plant litters and its controlling factors, and to quantify the different contribution rates to sediments organic matter throughout the decomposition of different plant litters. Results showed that the decomposition rates of plant litters buried at medium tidal level were 0.655 a(-1) for mangrove and 1.723 a(-1) for Spartina, which were greater than those with 0.651 a(-1) for mangrove and 1.586 a(-1) for Spartina at high tidal level. The reduction of carbon concentration in plant litters at high tidal level was lower than that at medium tidal level, while the increment of nitrogen and sulfur concentrations in plant litters at high tidal level was greater than those at medium tidal. And the isotope abundance of carbon (delta13C) in plant litters at medium tidal level reduced much more significantly than that at high tidal level. The contribution rates of plant litters carbon to sediments organic matter differed among tidal levels, plant species and decomposition duration. Specifically, the decomposition of mangrove litters contributed 5.96% to the sediment organic matter at medium tidal level, which was greater than that (3.03%) at high tidal level. Similarly, the decomposition of Spartina litters contributed 14.81% to the sediment organic matter at medium tidal level, which was also greater than that (13.97%) at high tidal level. The contribution of the decomposition of Spartina litters organic matter (average with 14.39%) was greater than that of mangrove litters (4.50%). The decomposition of plant litters requires a long process. The contribution of plant litters to sediments organic matter throughout one year decomposition was lower than that in complete decomposition, in particular, mangrove litters. Our study showed that the quantitative differences in plant litters-derived sediment organic matter would improve the proper estimation of the contribution of litters to wetland organic matter.

[Effect of dissolved oxygen on diversity of ammonia-oxidizing microorganisms in enrichment culture from estuarine wetland surface sediments and ammonia-oxidizing rate].

Dissolved oxygen (DO) is one of the important environmental factors influencing the ammonia oxidation process. In order to examine the effects of DO on ammonia oxidation process and its potential mechanisms, surface sediments from Jiulong River Estuarine Wetland were collected and cultured to obtain enrichment cultures. Then the enrichment cultures were inoculated under different levels of DO, and the diversity of ammonia-oxidizing microorganisms was analyzed using PCR-DGGE technique to determine the effect of DO on the ammonia oxidation rate and the ammonia-oxidizing microorganism diversity. Results showed that the Shannon index was 2. 00 and 2.05 for ammonia-oxidizing bacteria (AOB) under saturated and aerobic conditions, respectively, and the values were 2.49 (saturated) and 2.03 (aerobic) for ammonia-oxidizing archaea (AOA). However, this index was 1.76 and 1.80 for AOB under hypoxia and anaerobic condition, and 1.27 and 2. 21 for AOA. Under saturated and aerobic conditions ( higher DO level), the ammonia-oxidizing rates were 14.20 mg.(L.d)-1 and 13.36 mg.(L.d)-1 and the related conversation rates of NH+4 -N were 93.8% and 88. 2% , respectively. In comparison, under hypoxia and anaerobic conditions (lower DO level), the ammonia-oxidizing rates were 7.82 mg.(L.d) -1 and 5.66 mg.(L.d)-1 and the related conversation rates of NH+4 -N were 51.7% and 37.4% , respectively. The correlation analysis showed that DO concentration was highly significantly positively correlated with the ammonia oxidation rate, and was significantly positively correlated with the AOB diversity index; DO and ammonia oxidation rate had no correlation with indices of AOA community.