Changes in bacterial diversity of activated sludge exposed to titanium dioxide nanoparticles

Response to shock load of titanium dioxide nanoparticles on aerobic granular sludge and algal-bacterial granular sludge processes

Titanium dioxide nanoparticles (TiO2 NPs) are extensively used in various fields and can consequently be detected in wastewater, making it necessary to study their potential impacts on biological wastewater treatment processes. In this study, the shock-load impacts of TiO2 NPs were investigated at concentrations ranging between 1 and 200 mg L-1 on nutrient removal, extracellular polymeric substances (EPSs), microbial activity in aerobic granular sludge (AGS), and algal-bacterial granular sludge (AB-AGS) bioreactors. The results indicated that low concentration (≤10 mg L-1) TiO2 NPs had no effect on microbial activity or the removal of chemical oxygen demand (COD), nitrogen, and phosphorus, due to the increased production of extracellular polymeric substances (EPSs) in the sludge. In contrast, the performance of both AGS and AB-AGS bioreactors gradually deteriorated as the concentration of TiO2 NPs in the influent increased to 50, 100, and 200 mg L-1. Specifically, the ammonia‑nitrogen removal rate in AGS decreased from 99.9 % to 88.6 %, while in AB-AGS it dropped to 91.3 % at 200 mg L-1 TiO2 NPs. Furthermore, the nitrate‑nitrogen levels remained stable in AB-AGS, while NO3-N was detected in the effluent of AGS at 100 and 200 mg L-1. Microbial activities change similarly as smaller decrease in the specific ammonia uptake rate (SAUR) and specific nitrate uptake rate (SNUR) was found in AB-AGS compared to those in AGS. Overall, the algal-bacterial sludge exhibited higher resilience against TiO2 NPs, which was attributed to a) higher EPS volume, b) smaller decrease in LB-EPS, and c) the favorable protein to polysaccharide (PN/PS) ratio. This in turn, along with the symbiotic relationship between the algae and bacteria, mitigates the toxic effects of nanoparticles.

Impacts of nano-titanium dioxide toward Vallisneria natans and epiphytic microbes

In this study, Vallisneria natans plants were exposed to 5 and 20 nm of titanium dioxide nanoparticles (TiO₂ NPs) anatase and 600-1000 nm of bulk at 5 and 20 mg/L for 30 days. SEM images and EDX spectra revealed that epiphytic biofilms were more prone to TiO₂ NPs adhesion than bare plant leaves. TiO₂ NPs injured plant leaf cells, ruptured epiphytic diatoms membranes and increased the ratio of free-living microbes. The TN, NH₄⁺-N and NO₃--N concentrations significantly decreased, respectively, by 44.9%, 33.6%, and 23.6% compared to bulk treatments after 30 days due to macrophyte damage and a decline in diversity of epiphytic bacterial community and abundance of nitrogen cycle bacteria. TiO₂ NPs size-dependent decrease in bacterial relative abundance was detected, including phylum Cyanobacteria, Planctomycetes, and Verrucomicrobia. Although TiO₂ NPs increased eukaryotic diversity and abundance, abundances of Bacillariophyceae and Vampyrellidae classes and Gastrotricha and Phragmoplastophyta phylum decreased significantly under TiO₂ NPs exposure compared to bulk and control. TiO₂ NPs reduced intensities of interaction relationships among epiphytic microbial genera. This study shed new light on the potential effects of TiO₂ NPs toxicity toward aquatic plants and epiphytic microbial communities and its impacts on nitrogen species removal in wetlands.

Sediment Bacteria and Phosphorus Fraction Response, Notably to Titanium Dioxide Nanoparticle Exposure

Titanium dioxide nanoparticle (TiO₂ NP) toxicity to the growth of organisms has been gradually clarified; however, its effects on microorganism-mediated phosphorus turnover are poorly understood. To evaluate the influences of TiO₂ NPs on phosphorus fractionation and the bacterial community, aquatic microorganisms were exposed to different concentrations of TiO₂ NPs with different exposure times (i.e., 0, 10, and 30 days). We observed the adhesion of TiO₂ NPs to the cell surfaces of planktonic microbes by using SEM, EDS, and XRD techniques. The addition of TiO₂ NPs resulted in a decrease in the total phosphorus of water and an increase in the total phosphorus of sediments. Additionally, elevated TiO₂ NPs enhanced the sediment activities of reductases (i.e., dehydrogenase [0.19–2.25 μg/d/g] and catalase [1.06–2.92 μmol/d/g]), and significantly decreased the absolute abundances of phosphorus-cycling-related genes (i.e., gcd [1.78 × 10⁴–9.55 × 10⁵ copies/g], phoD [5.50 × 10³–5.49 × 10⁷ copies/g], pstS [4.17 × 10²–1.58 × 10⁶ copies/g]), and sediment bacterial diversity. TiO₂ NPs could noticeably affect the bacterial community, showing dramatic divergences in relative abundances (e.g., Actinobacteria, Acidobacteria, and Firmicutes), coexistence patterns, and functional redundancies (e.g., translation and transcription). Our results emphasized that the TiO₂ NP amount—rather than the exposure time—showed significant effects on phosphorus fractions, enzyme activity, phosphorus-cycling-related gene abundance, and bacterial diversity, whereas the exposure time exhibited a greater influence on the composition and function of the sediment bacterial community than the TiO₂ NP amount. Our findings clarify the responses of phosphorus fractions and the bacterial community to TiO₂ NP exposure in the water–sediment ecosystem and highlight potential environmental risks of the migration of untreated TiO₂ NPs to aquatic ecosystems.

The Effect of TiO2 Nanoparticles on the Composition and Ultrastructure of Wheat

The present work aims to follow the influence of TiO2 nanoparticles (TiO2 NPs) on bioactive compounds, the elemental content of wheat, and on wheat leaves' ultrastructure. Synthesized nanoparticles were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and transmission electron microscopy (TEM). The concentration of phenolic compounds, assimilation pigments, antioxidant capacity, elemental content, as well as the ultrastructural changes that may occur in the wheat plants grown in the presence or absence of TiO2 NPs were evaluated. In plants grown in the presence of TiO2 NPs, the amount of assimilating pigments and total polyphenols decreased compared to the control sample, while the antioxidant activity of plants grown in amended soil was higher than those grown in control soil. Following ultrastructural analysis, no significant changes were observed in the leaves of TiO2-treated plants. Application of TiO2 NPs to soil caused a significant reaction of the plant to stress conditions. This was revealed by the increase of antioxidant capacity and the decrease of chlorophyll, total polyphenols, and carotenoids. Besides, the application of TiO2 NPs led to significant positive (K, Zn, Br, and Mo) and negative (Na, Mn, Fe, As, Sr, Sb, and Ba) variation of content.