3 research outputs found
Acute and Chronic Responses of Activated Sludge Viability and Performance to Silica Nanoparticles
Recently, the potential health and environmental risks
of silica
nanoparticles (SiO<sub>2</sub> NPs) are attracting great interest.
However, little is known about their possible impacts on wastewater
biological nitrogen and phosphorus removal. In this study, the acute
and chronic effects of SiO<sub>2</sub> NPs on activated sludge viability
and biological nutrient removal performance were investigated. It
was found that the presence of environmentally relevant concentration
(1 mg/L) of SiO<sub>2</sub> NPs caused no adverse acute and chronic
effects on sludge viability and wastewater nitrogen and phosphorus
removal. However, chronic exposure to 50 mg/L SiO<sub>2</sub> NPs
induced the increase of effluent nitrate concentration, and thus depressed
the total nitrogen (TN) removal efficiency from 79.6% to 51.6% after
70 days of exposure, which was due to the declined activities of denitrifying
enzymes, nitrate reductase and nitrite reductase. Wastewater phosphorus
removal was insensitive to 1 and 50 mg/L SiO<sub>2</sub> NPs after
either the acute or chronic exposure, because the critical factors
closely related to biological phosphorus removal were not significantly
changed, such as the activities of exopolyphosphatase and polyphosphate
kinase and the intracellular transformations of polyhydroxyalkanoates
and glycogen. Denaturing gradient gel electrophoresis (DGGE) analysis
revealed that the bacterial community structure was changed after
long-term exposure to 50 mg/L SiO<sub>2</sub> NPs, and the quantitative
PCR assays indicated that the abundance of denitrifying bacteria was
decreased, which was consistent with the declined wastewater nitrogen
removal
Effect of CO<sub>2</sub> on Microbial Denitrification via Inhibiting Electron Transport and Consumption
Increasing
anthropogenic CO<sub>2</sub> emissions have been reported
to influence global biogeochemical processes; however, in the literature
the effects of CO<sub>2</sub> on denitrification have mainly been
attributed to the changes it causes in environmental factors, while
the direct effects of CO<sub>2</sub> on denitrification remain unknown.
In this study, increasing CO<sub>2</sub> from 0 to 30 000 ppm
under constant environmental conditions decreased total nitrogen removal
efficiency from 97% to 54%, but increased N<sub>2</sub>O generation
by 240 fold. A subsequent mechanistic study revealed that CO<sub>2</sub> damaged the bacterial membrane and directly inhibited the transport
and consumption of intracellular electrons by causing intracellular
reactive nitrogen species (RNS) accumulation, suppressing the expression
of key electron transfer proteins (flavoprotein, succinate dehydrogenase,
and cytochrome c) and the synthesis and activity of key denitrifying
enzymes. Further study indicated that the inhibitory effects of CO<sub>2</sub> on the transport and consumption of electrons were caused
by the decrease of intracellular iron due to key iron transporters
(AfuA, FhuC, and FhuD) being down-regulated. Overall, this study suggests
that the direct effect of CO<sub>2</sub> on denitrifying microbes
via inhibition of intracellular electron transport and consumption
is an important reason for its negative influence on denitrification
Zinc Oxide Nanoparticles Cause Inhibition of Microbial Denitrification by Affecting Transcriptional Regulation and Enzyme Activity
Over the past few decades, human
activities have accelerated the
rates and extents of water eutrophication and global warming through
increasing delivery of biologically available nitrogen such as nitrate
and large emissions of anthropogenic greenhouse gases. In particular,
nitrous oxide (N<sub>2</sub>O) is one of the most important greenhouse
gases, because it has a 300-fold higher global warming potential than
carbon dioxide. Microbial denitrification is a major pathway responsible
for nitrate removal, and also a dominant source of N<sub>2</sub>O
emissions from terrestrial or aquatic environments. However, whether
the release of zinc oxide nanoparticles (ZnO NPs) into the environment
affects microbial denitrification is largely unknown. Here we show
that the presence of ZnO NPs lead to great increases in nitrate delivery
(9.8-fold higher) and N<sub>2</sub>O emissions (350- and 174-fold
higher in the gas and liquid phases, respectively). Our data further
reveal that ZnO NPs significantly change the transcriptional regulations
of glycolysis and polyhydroxybutyrate synthesis, which causes the
decrease in reducing powers available for the reduction of nitrate
and N<sub>2</sub>O. Moreover, ZnO NPs substantially inhibit the gene
expressions and catalytic activities of key denitrifying enzymes.
These negative effects of ZnO NPs on microbial denitrification finally
cause lower nitrate removal and higher N<sub>2</sub>O emissions, which
is likely to exacerbate water eutrophication and global warming