Sublethal Concentrations of Silver Nanoparticles Stimulate Biofilm Development

Although silver nanoparticles (AgNPs) are used as antimicrobial agents in a wide variety of commercial products, sublethal exposure can counterproductively promote the development of biofilms. We observed by fluorescence microscopy denser biofilm growth with mixed cultures from a wastewater treatment plant after exposure to 21.6 μg/L 10 nm AgNPs. To further understand biofilm promotion mechanisms, experiments were conducted with a pure culture of <i>Pseudomonas aeruginosa</i> PAO1. Sublethal exposure of PAO1 to AgNPs (10.8 and 21.6 μg/L) also enhanced biofilm development and upregulated quorum sensing, lipopolysaccharide biosynthesis, and antibiotic resistance (efflux pump) genes. An increase in the sugar and protein contents of the PAO1 biofilm matrix (by 55 ± 3 and 114 ± 32%, respectively, relative to unexposed controls) corroborated the transcriptional upregulation of PAO1 biofilm-related genes. Enhanced biofilm development by exposure to low AgNP concentrations might accelerate biofouling and biocorrosion and harbor pathogens that pose a risk to public health

Manganese Peroxidase Degrades Pristine but Not Surface-Oxidized (Carboxylated) Single-Walled Carbon Nanotubes

The transformation of engineered nanomaterials in the environment can significantly affect their transport, fate, bioavailability, and toxicity. Little is known about the biotransformation potential of single-walled carbon nanotubes (SWNTs). In this study, we compared the enzymatic transformation of SWNTs and oxidized (carboxylated) SWNTs (O-SWNTs) using three ligninolytic enzymes: lignin peroxidase, manganese peroxidase (MnP), and laccase. Only MnP was capable of transforming SWNTs, as determined by Raman spectroscopy, near-infrared spectroscopy, and transmission electron microscopy. Interestingly, MnP degraded SWNTs but not O-SWNTs. The recalcitrance of O-SWNTs to enzymatic transformation is likely attributable to the binding of Mn²⁺ by their surface carboxyl groups at the enzyme binding site, which inhibits critical steps in the MnP catalytic cycle (i.e., Mn²⁺ oxidation and Mn³⁺ dissociation from the enzyme). Our results suggest that oxygen-containing surface functionalities do not necessarily facilitate the biodegradation of carbonaceous nanomaterials, as is commonly assumed

Photochemical Transformation of Carboxylated Multiwalled Carbon Nanotubes: Role of Reactive Oxygen Species

The study investigated the photochemical transformation of carboxylated multiwalled carbon nanotubes (COOH-MWCNTs), an important environmental process affecting their physicochemical characteristics and hence fate and transport. UVA irradiation removed carboxyl groups from COOH-MWCNT surface while creating other oxygen-containing functional groups with an overall decrease in total surface oxygen content. This was attributed to reactions with photogenerated reactive oxygen species (ROS). COOH-MWCNTs generated singlet oxygen (¹O₂) and hydroxyl radical (^•OH) under UVA light, which exhibited different reactivity toward the COOH-MWCNT surface. Inhibition experiments that isolate the effects of ^•OH and ¹O₂ as well as experiments using externally generated ^•OH and ¹O₂ separately revealed that ^•OH played an important role in the photochemical transformation of COOH-MWCNTs under UVA irradiation. The Raman spectroscopy and surface functional group analysis results suggested that ^•OH initially reacted with the surface carboxylated carbonaceous fragments, resulting in their degradation or exfoliation. Further reaction between ^•OH and the graphitic sidewall led to formation of defects including functional groups and vacancies. These reactions reduced the surface potential and colloidal stability of COOH-MWCNTs, and are expected to reduce their mobility in aquatic systems

Pyrolytic Remediation of Oil-Contaminated Soils: Reaction Mechanisms, Soil Changes, and Implications for Treated Soil Fertility

Pyrolysis of hydrocarbon-contaminated soils offers the potential for rapid remediation without destroying soil fertility. Here we elucidate the fundamental mechanisms of pyrolytic treatment and advance understanding of the surface properties of pyrolyzed soils. Using thermogravimetry and evolved gas analysis, we identified the two stages of pyrolytic remediation. Desorption of light hydrocarbons is the dominant process for temperatures between 150 and 350 °C. Pyrolysis reactions dominate in the 400–500 °C range releasing gaseous products (hydrogen, methane, higher alkanes, and olefins) and forming a solid char. XPS analysis and partial combustion revealed that the char forms a layer coating the particles of treated soils. Since pyrolysis can effectively reduce the TPH of contaminated soils at temperatures below 500 °C, it avoids carbonate decomposition reactions that may lead to large soil pH increases and severe loss of fertility. This is a significant potential advantage over competing thermal processes that expose contaminated soil to temperatures above 500 °C

Biogenic versus Thermogenic H₂S Source Determination in Bakken Wells: Considerations for Biocide Application

Hydrocarbon souring represents a significant safety and corrosion challenge to the oil and gas industry. H₂S may originate from geochemical or biogenic sources, although its source is rarely discerned. Biocides are sometimes utilized during well operations to prevent or inhibit H₂S generation. Here we develop a regional temperature map showing that downhole temperatures in Bakken reservoir wells equal or exceed the upper known temperature limit for microbial life. Attempts to extract microbial DNA from produced water yielded little to no detectable quantities. Stable isotope analysis yielded ^34Sδ values from 4.4 to 9.8‰, suggesting souring had a geochemical origin. Under Bakken reservoir conditions, anhydrite can react with hydrocarbons to form H₂S. Anhydrite present near the sour areas studied could be the underlying geochemical source creating this H₂S. In cases of geochemical souring, reevaluation of the need for biocide addition may provide significant reductions in both operational costs and overall environmental footprint

Extracellular Saccharide-Mediated Reduction of Au³⁺ to Gold Nanoparticles: New Insights for Heavy Metals Biomineralization on Microbial Surfaces

Biomineralization is a critical process controlling the biogeochemical cycling, fate, and potential environmental impacts of heavy metals. Despite the indispensability of extracellular polymeric substances (EPS) to microbial life and their ubiquity in soil and aquatic environments, the role played by EPS in the transformation and biomineralization of heavy metals is not well understood. Here, we used gold ion (Au³⁺) as a model heavy metal ion to quantitatively assess the role of EPS in biomineralization and discern the responsible functional groups. Integrated spectroscopic analyses showed that Au³⁺was readily reduced to zerovalent gold nanoparticles (AuNPs, 2–15 nm in size) in aqueous suspension of <i>Escherichia coli</i> or dissolved EPS extracted from microbes. The majority of AuNPs (95.2%) was formed outside <i>Escherichia coli</i> cells, and the removal of EPS attached to cells pronouncedly suppressed Au³⁺ reduction, reflecting the predominance of the extracellular matrix in Au³⁺ reduction. XPS, UV–vis, and FTIR analyses corroborated that Au³⁺ reduction was mediated by the hemiacetal groups (aldehyde equivalents) of reducing saccharides of EPS. Consistently, the kinetics of AuNP formation obeyed pseudo-second-order reaction kinetics with respect to the concentrations of Au³⁺ and the hemiacetal groups in EPS, with minimal dependency on the source of microbial EPS. Our findings indicate a previously overlooked, universally significant contribution of EPS to the reduction, mineralization, and potential detoxification of metal species with high oxidation state

Tetracycline Resistance Gene Maintenance under Varying Bacterial Growth Rate, Substrate and Oxygen Availability, and Tetracycline Concentration

Neither amplification nor attenuation of antibiotic resistance genes (ARG) in the environment are well understood processes. Here, we report on continuous culture and batch experiments to determine how tetracycline (TC), aerobic vs anaerobic conditions, bacterial growth rate, and medium richness affect the maintenance of plasmid-borne TC resistance (Tet^R) genes. The response of <i>E. coli</i> (a model resistant strain excreted by farm animals) versus <i>Pseudomonas aeruginosa</i> (a model bacterium that could serve as a reservoir for ARGs in the environment) were compared to gain insight into response variability. Complete loss of the Tet^R RP1 plasmid (56 kb) occurred for <i>P. aeruginosa</i> in the absence of TC, and faster loss was observed in continuous culture at higher growth rates. In contrast, <i>E. coli</i> retained its smaller pSC101 plasmid (9.3 kb) after 500 generations without TC (albeit at lower levels, with ratios of resistance to 16S rDNA genes decreasing by about 2-fold). A higher rate of ARG loss was observed in <i>P. aeruginosa</i> when grown in minimal growth medium (M9) than in richer Luria broth. Faster ARG loss occurred in <i>E. coli</i> under anaerobic (fermentative) conditions than under aerobic conditions. Thus, in these two model strains it was observed that conditions that ease the metabolic burden of plasmid reproduction (e.g., higher substrate and O₂ availability) enhanced resistance plasmid maintenance; such conditions (in the presence of residual antibiotics) may be conducive to the establishment and preservation of ARG reservoirs in the environment. These results underscore the need to consider antibiotic concentrations, redox conditions, and substrate availability in efforts to evaluate ARG propagation and natural attenuation

Phosphate Changes Effect of Humic Acids on TiO₂ Photocatalysis: From Inhibition to Mitigation of Electron–Hole Recombination

A major challenge for photocatalytic water purification with TiO₂ is the strong inhibitory effect of natural organic matter (NOM), which can scavenge photogenerated holes and radicals and occlude ROS generation sites upon adsorption. This study shows that phosphate counteracts the inhibitory effect of humic acids (HA) by decreasing HA adsorption and mitigating electron–hole recombination. As a measure of the inhibitory effect of HA, the ratios of first-order reaction rate constants between photocatalytic phenol degradation in the absence versus presence of HA were calculated. This ratio was very high, up to 5.72 at 30 mg/L HA and pH 4.8 without phosphate, but was decreased to 0.76 (5 mg/L HA, pH 8.4) with 2 mM phosphate. The latter ratio indicates a surprising favorable effect of HA on TiO₂ photocatalysis. FTIR analyses suggest that this favorable effect is likely due to a change in the conformation of adsorbed HA, from a multiligand exchange arrangement to a complexation predominantly between COOH groups in HA and the TiO₂ surface in the presence of phosphate. This configuration can reduce hole consumption and facilitate electron transfer to O₂ by the adsorbed HA (indicated by linear sweep voltammetry), which mitigates electron–hole recombination and enhances contaminant degradation. A decrease in HA surface adsorption and hole scavenging (the predominant inhibitory mechanisms of HA) by phosphate (2 mM) was indicated by a 50% decrease in the photocatalytic degradation rate of HA and 80% decrease in the decay rate coefficient of interfacial-related photooxidation in photocurrent transients. These results, which were validated with other compounds (FFA and cimetidine), indicate that anchoring phosphate - or anions that exert similar effects on the TiO₂ surface - might be a feasible strategy to counteract the inhibitory effect of NOM during photocatalytic water treatment

Methane Bioattenuation and Implications for Explosion Risk Reduction along the Groundwater to Soil Surface Pathway above a Plume of Dissolved Ethanol

Fuel ethanol releases can stimulate methanogenesis in impacted aquifers, which could pose an explosion risk if methane migrates into enclosed spaces where ignitable conditions exist. To assess this potential risk, a flux chamber was emplaced on a pilot-scale aquifer exposed to continuous release (21 months) of an ethanol solution (10% v:v) that was introduced 22.5 cm below the water table. Despite methane concentrations within the ethanol plume reaching saturated levels (20–23 mg/L), the maximum methane concentration reaching the chamber (21 ppm_v) was far below the lower explosion limit in air (50,000 ppm_v). The low concentrations of methane observed in the chamber are attributed to methanotrophic activity, which was highest in the capillary fringe. This was indicated by methane degradation assays in microcosms prepared with soil samples from different depths, as well as by PCR measurements of <i>pmoA</i>, which is a widely used functional gene biomarker for methanotrophs. Simulations with the analytical vapor intrusion model “Biovapor” corroborated the low explosion risk associated with ethanol fuel releases under more generic conditions. Model simulations also indicated that depending on site-specific conditions, methane oxidation in the unsaturated zone could deplete the available oxygen and hinder aerobic benzene biodegradation, thus increasing benzene vapor intrusion potential. Overall, this study shows the importance of methanotrophic activity near the water table to attenuate methane generated from dissolved ethanol plumes and reduce its potential to migrate and accumulate at the surface