Interactions of PAH-degradation and nitrate-/sulfate-reducing assemblages in anaerobic sediment microbial community

Study on the accelerated biodegradation of PAHs in subsurface soil via coupled low-temperature thermally treatment and electron acceptor stimulation based on metagenomic sequencing

In situ anoxic bioremediation is a sustainable technology to remediate PAHs contaminated soils. However, the limited degradation rate of PAHs under anoxic conditions has become the primary bottleneck hindering the application of this technology. In this study, coupled low-temperature thermally treatment (<50 °C) and EA biostimulation was used to enhance PAH removal. Anoxic biodegradation of PAHs in soil was explored in microcosms in the absence and presence of added EAs at 3 temperatures (15 °C, 30 °C, and 45 °C). The influence of temperature, EA, and their interaction on the removal of PAHs were identified. A PAH degradation model based on PLSR analysis identified the importance and the positive/negative role of parameters on PAH removal. Soil archaeal and bacterial communities showed similar succession patterns, the impact of temperature was greater than that of EA. Soil microbial community and function were more influenced by temperature than EAs. Close and frequent interactions were observed among soil bacteria, archaea, PAH-degrading genes and methanogenic genes. A total of 15 bacterial OTUs, 1 PAH-degrading gene and 2 methanogenic genes were identified as keystones in the network. Coupled low-temperature thermally treatment and EA stimulation resulted in higher PAH removal efficiencies than EA stimulation alone and low-temperature thermally treatment alone.

Nitrate input inhibited the biodegradation of erythromycin through affecting bacterial network modules and keystone species in lake sediments

Antibiotic contamination and excessive nitrate loads are generally concurrent in aquatic ecosystems. However, little is known about the effects of nitrate input on the biodegradation of antibiotics. In this study, the effects of nitrate input on microbial degradation of erythromycin, a typical macrolide antibiotic widely detected in lake sediments, were investigated. The results showed that the nitrate input significantly inhibited the erythromycin removal and such an inhibitory effect was strengthened with the increased input dosages. Nitrate input significantly increased sediment nitrite concentration, indicating enhanced denitrification under high nitrate pressure. Bacterial network module and keystone species analysis showed that nitrate input enriched the keystone species involved in denitrification (e.g., Simplicispira and Denitratisoma). In contrast, some potential erythromycin-degrading bacteria (e.g., Desulfatiglandales, Pseudomonadales, Nitrospira) were inhibited by nitrate input. The variations in dominant bacterial groups implied competition between denitrification and erythromycin degradation in response to nitrate input. Based on the partial least squares path modeling analysis, keystone species (total effect: 0.419) and bacterial module (total effect: 0.403) showed strong association with erythromycin removal percentage. This indicated that the inhibitory effect of nitrate input on erythromycin degradation was mainly explained by bacterial network modules and keystone species. These findings will help us to assess the bioremediation potential of antibiotic-contaminated sediments suffering from excessive nitrogen discharge concurrently.

Moderate anthropogenic disturbance stimulates versatile microbial taxa contributing to denitrification and aromatic compound degradation

Wastewater treatment plants (WWTPs) effluent often contains a significant amount of residual organic pollutants and nutrients, causing disturbance to the coastal effluent receiving areas (ERA). Microbial communities in coastal ERA sediments may benefit from the coexistence of organic pollutants and nutrients, promoting the emergence of versatile taxa that are capable of eliminating these substances simultaneously. However, the identification and exploration of versatile taxa in natural environments under anthropogenic disturbances remain largely uncharted territory. In this study, we specifically focused on the versatile taxa coupled by the degradation of aromatic compounds (ACs) and denitrification, using Hangzhou Bay in China as our study area. We explored how WWTPs effluent disturbance would affect the versatile taxa, and particularly examined the role of disturbance intensity in shaping their composition. Intriguingly, we found that versatile taxa were mainly derived from denitrifiers like Pseudomonas, suggesting the fulfilled potential of denitrifiers regarding ACs degradation. We also discovered that moderate disturbance stimulated the diversity of versatile taxa, resulting in strengthened functional redundancy. Through correlation network analysis, we further demonstrated that moderate disturbance enhanced the community-level cooperation. Thus, moderate disturbance serves as a catalyst for versatile taxa to maintain community function, making them more resilient to effluent disturbances. Additionally, we identified COD and NO3--N concentrations as significant environmental factors influencing the versatile taxa. Overall, our findings reveal the role of effluent disturbances in the promotion of versatile taxa, and highlight moderate disturbance can foster more robust versatile taxa that are better equipped to handle effluent disturbances.

Species' functional traits and interactions drive nitrate-mediated sulfur-oxidizing community structure and functioning

IMPORTANCE

Understanding the processes and mechanisms governing microbial community assembly and their linkages to ecosystem functioning has long been a core issue in microbial ecology. An in-depth insight still requires combining with analyses of species' functional traits and microbial interactions. Our study showed how species' functional traits and interactions determined microbial community structure and functions by a well-controlled laboratory experiment with nitrate-mediated sulfur oxidation systems using high-throughput sequencing and culture-dependent technologies. The results provided solid evidences that species' functional traits and interactions were the intrinsic factors determining community structure and function. More importantly, our study established quantitative links between community structure and function based on species' functional traits and interactions, which would have important implications for the design and synthesis of microbiomes with expected functions.

New insights into enhanced biodegradation of 4-bromphenol in a nitrate-reducing system: Process performance and mechanism

Due to the recalcitrant nature of halogenated phenol, conventional anaerobic bioprocess is often limited by low removal efficiency and poor process stability. At the presence of electron acceptors such as nitrate, 4-bromophenol (4-BP) removal efficiency is significantly higher than that in the anaerobic control system, but the mechanism involved is still unclear. Therefore, an up-flow nitrate-reducing bioreactor (NRBR) was designed and consecutively performed for 215 days to explore the synergistic mechanism for BPs biodegradation and nitrate reduction. Complete 4-BP biodegradation could be obtained in NRBR at HRT and 4-BP loading rate of 24 h and 0.29 mol m - 3d - 1, while the TOC removal and nitrate reduction efficiencies were as high as 91.33±2.11% and 98.31±1.33%, respectively. Population evolution analyses revealed that the microorganisms involved in 4-BP debromination and biodegradation (Candidatus Peregrinibacteria, Denitratisoma, Anaerolineaceae and Ignavibacterium) as well as nitrate reduction (Denitratisoma, Anaerolineaceae, Limnobacter and Ignavibacterium) were significantly enriched in NRBR. Major intermediates during 4-BP biodegradation, including 4-bromocatechol, 4‑bromo-6-oxo-hexanoic acid and succinic acid were identified, while a distinct 4-BP biodegradation pathway via hydration, aromatic-ring cleavage, hydrolysis debromination and oxidation was expounded. Metagenomic analysis indicated that oxidation (had, pht4, boh, butA), hydrolysis debromination ((S)-2-haloacid dehalogenase) and bio-mineralization (gabD, sdhA) of 4-BP were largely enhanced in NRBR. Moreover, carbon, nitrogen, energy and amino acid metabolisms were significantly facilitated with the injection of nitrate in order to provide energy and electron, thus enhanced microbial activities and enzymatic reactions in NRBR. The proposed mechanism provides new insights into our mechanistic understanding of halogenated phenol biodegradation and the development of sustainable bioremediation strategies.

Microbial assemblages in water hyacinth silages with different initial moistures

Making silage is a green process to use the fast-growing water hyacinth (Eichhornia crassipes) biomass. However, the high moisture (∼95%) of the water hyacinth is the biggest challenge to making silage while its effects on fermentation processes are less studied. In this study, water hyacinths silage with different initial moistures were conducted to investigate the fermentation microbial communities and their roles on the silage qualities. Results show that both silages with 70% (S70) and 90% (S90) of initial moistures achieved the target of silage fermentation, however, their microbial processes were significantly different. Their succession directions of microbial communities were different: Plant cells in S70 were destroyed by the air-dry treatment, thus there were more soluble carbohydrates, which helped the inoculated fermentative bacteria become dominant (Lactobacillus spp. > 69%) and produce abundant lactic acid; In contrast, stochastic succession became dominant over time in S90 (NST = 0.79), in which Lactobacillus spp. and Clostridium spp. produced butyric that also obviously decreased the pH and promoted the fermentation process. Different microbial succession led to different metabolic patterns: S70 had stronger starch and sucrose metabolisms while S90 had stronger amino acid and nitrogen metabolisms. Consequently, S70 had higher lactic acid, crude protein and lower ammonia nitrogen and S90 had higher in vitro digestibility of dry matter and higher relative feeding value. Moreover, the variance partitioning analysis indicated that moisture could only explain less information (5.9%) of the microbial assemblage than pH value (41.4%). Therefore, the colonization of acid-producing bacteria and establishment of acidic environment were suggested as the key on the silage fermentation no matter how much is the initial moisture. This work can provide a basis for the future preparation of high-moisture raw biomasses for silage.

Metabolic capacity to alter polycyclic aromatic hydrocarbons and its microbe-mediated remediation

The elimination of contaminants caused by anthropogenic activities and rapid industrialization can be accomplished using the widely used technology of bioremediation. Recent years have seen significant advancement in our understanding of the bioremediation of coupled polycyclic aromatic hydrocarbon contamination caused by microbial communities including bacteria, algae, fungi, yeast, etc. One of the newest techniques is microbial-based bioremediation because of its greater productivity, high efficiency, and non-toxic approach. Microbes are appealing candidates for bioremediation because they have amazing metabolic capacity to alter most types of organic material and can endure harsh environmental conditions. Microbes have been characterized as extremophiles that can survive in a variety of environmental circumstances, making them the treasure troves for environmental cleanup and the recovery of contaminated soil. In this study, the mechanisms underlying the bioremediation process as well as the current situation of microbial bioremediation of polycyclic aromatic hydrocarbon are briefly described.

Polyaromatic hydrocarbons (PAHs) in the water environment: A review on toxicity, microbial biodegradation, systematic biological advancements, and environmental fate

Polycyclic aromatic hydrocarbons (PAHs) are considered a major class of organic contaminants or pollutants, which are poisonous, mutagenic, genotoxic, and/or carcinogenic. Due to their ubiquitous occurrence and recalcitrance, PAHs-related pollution possesses significant public health and environmental concerns. Increasing the understanding of PAHs' negative impacts on ecosystems and human health has encouraged more researchers to focus on eliminating these pollutants from the environment. Nutrients available in the aqueous phase, the amount and type of microbes in the culture, and the PAHs' nature and molecular characteristics are the common factors influencing the microbial breakdown of PAHs. In recent decades, microbial community analyses, biochemical pathways, enzyme systems, gene organization, and genetic regulation related to PAH degradation have been intensively researched. Although xenobiotic-degrading microbes have a lot of potential for restoring the damaged ecosystems in a cost-effective and efficient manner, their role and strength to eliminate the refractory PAH compounds using innovative technologies are still to be explored. Recent analytical biochemistry and genetically engineered technologies have aided in improving the effectiveness of PAHs' breakdown by microorganisms, creating and developing advanced bioremediation techniques. Optimizing the key characteristics like the adsorption, bioavailability, and mass transfer of PAH boosts the microorganisms' bioremediation performance, especially in the natural aquatic water bodies. This review's primary goal is to provide an understanding of recent information about how PAHs are degraded and/or transformed in the aquatic environment by halophilic archaea, bacteria, algae, and fungi. Furthermore, the removal mechanisms of PAH in the marine/aquatic environment are discussed in terms of the recent systemic advancements in microbial degradation methodologies. The review outputs would assist in facilitating the development of new insights into PAH bioremediation.

Characteristics of groundwater microbial communities' structure under the impact of elevated nitrate concentrations in north China plain

In groundwater environments, the interaction between microbial communities and the hydrogeochemical parameters have been investigated extensively in the past years. However, little is known whether the maximum contamination level (MCL) is a threshold value that dictates the microbial composition. In this study, we analyzed 10 groundwater samples for their nitrate, nitrite, COD and sulfate concentrations, and characterized their microbial compositions using 16 S rRNA based high-throughput sequencing methods. All the 10 samples had oxygen demands higher than the corresponding MCL of China (10 mg L^-1); moreover, 4 out of 10 samples also had nitrate concentrations higher than the corresponding MCL, which indicated that the groundwater quality was negatively impacted by anthropogenic activities. Comparing the microbial composition of groundwater that had higher-than-MCL nitrate concentrations to those that had lower-than-MCL nitrate concentrations, no significant differences were detected in communities' richness and diversity. However, the non-metric multi-dimensional analysis suggested that the 4 groundwater samples whose nitrate concentration exceed MCL are distinctly different from those of the rest 6 samples, indicating that MCL does have a significant impact on microbial structures. Pearson's correlation analysis suggested that none of the four analyzed hydrochemical parameters had significant impact on microbial communities' richness and diversity; however, at the genus level, the correlation results suggested that JG30-KM-CM45, Sphingomonas and Rhodococcus are closely correlated with nitrate concentration. The findings of this study deepened our understanding with respect to the relationships between the environmental quality indices and the microbial compositions of groundwater.

Electron acceptors determine the BTEX degradation capacity of anaerobic microbiota via regulating the microbial community

Anaerobic degradation is the major pathway for microbial degradation of benzene, toluene, ethylbenzene, and xylenes (BTEX) under electron acceptor lacking conditions. However, how exogenous electron acceptors modulate BTEX degradation through shaping the microbial community structure remains poorly understood. Here, we investigated the effect of various exogenous electron acceptors on BTEX degradation as well as methane production in anaerobic microbiota, which were enriched from the same contaminated soil. It was found that the BTEX degradation capacities of the anaerobic microbiota gradually increased along with the increasing redox potentials of the exogenous electron acceptors supplemented (WE: Without exogenous electron acceptors < SS: Sulfate supplement < FS: Ferric iron supplement < NS: Nitrate supplement), while the complexity of the co-occurring networks (e.g., avgK and links) of the microbiota gradually decreased, showing that microbiota supplemented with higher redox potential electron acceptors were less dependent on the formation of complex microbial interactions to perform BTEX degradation. Microbiota NS showed the highest degrading capacity and the broadest substrate-spectrum for BTEX, and it could metabolize BTEX through multiple modules which not only contained fewer species but also different key microbial taxa (eg. Petrimonas, Achromobacter and Comamonas). Microbiota WE and FS, with the highest methanogenic capacities, shared common core species such as Sedimentibacter, Acetobacterium, Methanobacterium and Smithella/Syntrophus, which cooperated with Geobacter (microbiota WE) or Desulfoprunum (microbiota FS) to perform BTEX degradation and methane production. This study demonstrates that electron acceptors may alter microbial function by reshaping microbial community structure and regulating microbial interactions and provides guidelines for electron acceptor selection for bioremediation of aromatic pollutant-contaminated anaerobic sites.

Polycyclic aromatic hydrocarbon removal from subsurface soil mediated by bacteria and archaea under methanogenic conditions: Performance and mechanisms

In situ anoxic bioremediation is an easy-to-use technology to remediate polycyclic aromatic hydrocarbon (PAH)-contaminated soil. Degradation of PAHs mediated by soil bacteria and archaea using CO₂ as the electron acceptor is an important process for eliminating PAHs under methanogenic conditions; however, knowledge of the performance and mechanisms involved is poorly unveiled. In this study, the effectiveness and efficiency of NaHCO₃ (CO₂) as an electron acceptor to stimulate the degradation of PAHs by bacteria and archaea in highly contaminated soil were investigated. The results showed that CO₂ addition (EC2000) promoted PAH degradation compared to soil without added CO₂ (EC0), with 4.18%, 9.01%-8.05%, and 6.19%-12.45% increases for 2-, 3- and 4-ring PAHs after 250 days of incubation, respectively. Soil bacterial abundances increased with increasing incubation time, especially for EC2000 (2.90 × 10⁸ g^-1 soil higher than EC0, p < 0.05). Different succession patterns of the soil bacterial and archaeal communities during PAH degradation were observed. According to the PCoA and ANOSIM results, the soil bacterial communities were greatly (ANOSIM: R = 0.7232, P = 0.001) impacted by electron acceptors, whereas significant differences in the archaeal communities were not observed (ANOSIM: R = 0.553, P = 0.001). Soil bacterial and archaeal co-occurrence network analyses showed that positive correlations outnumbered the negative correlations throughout the incubation period for both treatments (e.g., EC0 and EC2000), suggesting the prevalence of coexistence/cooperation within and between these two domains rather than competition. The higher complexity, connectance, edge, and node numbers in EC2000 revealed stronger linkage and a more stable co-occurrence network compared to EC0. The results of this study could improve the knowledge on the removal of PAHs and the responses of soil bacteria and archaea to CO₂ application, as well as a scientific basis for the in situ anoxic bioremediation of PAH-contaminated industrial sites.

Risk control and assessment of sulfide-rich sediment remediation by controlled-release calcium nitrate

Nitrate stimulation is widely used in sediment remediation to eliminate sulfides, degrade organic pollutants and immobilize phosphorus. However, the environmental risks of nitrate escape and the subsequent release of pollutants (e.g. nitrite, ammonium and trace metals) to water bodies during its application has received less attention. In this study, controlled-release nitrate pellets (SedCaN pellets) were manufactured and applied at different sediment depths to examine their effectiveness in controlling the risk of nitrate escape and subsequent pollutant release. The germination of submerged plant was also analyzed to assess the ecological risks associated with the remediated sediment. The results showed that the SedCaN pellets slowly released calcium nitrate, which led to denitrifying sulfide oxidation, organic matter degradation and the immobilization of phosphorus as a calcium-bound species. Gas production by denitrification increased the sediment porosity (0.3-2.2%) and led to the concomitant release of nitrite, ammonium, and heavy metals, creating secondary risks. Application of the SedCaN pellets at depth decreased the nitrate escape and the secondary risks, presumably by means of a capping effect of the upper sediment. The release of nitrate, ammonium, Ni and Cu were partially limited by 91.6%, 19.0%, 61.6% and 57.4% when SedCaN pellets were incorporated into deeper sediments (7-9 cm). Moreover, the range of sulfide oxidation extended to the upper and lower sediments in the profile (column), while the sulfide oxidation efficiency reached 85.9-95.0%. Finally, increased germination of Bacopa monnieri (20.0-26%) demonstrated that in comparison to reference materials the ecological risks of the treated sediments was reduced and the habitat function of sediment was restored after nitrate-stimulating remediation. The results of this study provide valuable guidelines for nitrate-stimulating remediation of sulfide-rich (black-odor) sediments.

Effect of terminal electron acceptors on the anaerobic biodegradation of PAHs in marine sediments

The existing polycyclic aromatic hydrocarbons (PAHs) in marine sediment has become a critical threat to biological security. Terminal electron acceptor (TEA) amendment has been applied as a potential strategy to accelerate bioremediation in sediment. HCO₃^-, NO₃^-, and SO₄^2- were separately added to anaerobic sediment system containing five kinds of PAH, namely, phenanthrene, anthracene, fluoranthene, pyrene and benzo(a)pyrene. PAH concentration, PAH metabolites, TEA concentration, and electron transport system (ETS) activity were investigated. The HCO₃^- amendment group achieved the max PAH degradation efficiency of 84.98 %. SO₄^2- group led to the highest benzo(a)pyrene removal rate of 69.26 %. NO₃^- had the lowest PAH degradation rate of 76.16 %. ETS activity test showed that NO₃^- significantly inhibited electron transport activity in the sediment. The identified PAH metabolites were the same in each group, including 4,5-dimethylphenanthrene, 3-acetylphenanthrene, 9,10-anthracenedione, pyrene-7-hydroxy-8-carboxylic acid, anthrone, and dibenzothiophene. After 126 d's anaerobic degradation at 25 °C, the utilization of HCO₃^- and SO₄^2- as selected TEAs promoted the PAH biodegradation performance better than the utilization of NO₃^-.

Engineering Pseudomonas putida To Produce Rhamnolipid Biosurfactants for Promoting Phenanthrene Biodegradation by a Two-Species Microbial Consortium

Polycyclic aromatic hydrocarbons (PAHs) are a group of organic contaminants that pose a significant environmental hazard. Phenanthrene is one of the model compounds for the study of biodegradation of PAHs. However, the biodegradation of phenanthrene is often limited by its low water solubility and dissolution rate. To overcome this limitation, we engineered a strain of Pseudomonas putida to produce rhamnolipid biosurfactants and thereby promote phenanthrene biodegradation by an engineered strain of Escherichia coli constructed previously in our lab. The E. coli-P. putida two-species consortium exhibited a synergistic effect of these two distinct organisms in degrading phenanthrene, resulting in an increase from 61.15 to 73.86% of the degradation ratio of 100 mg/L phenanthrene within 7 days. After additional optimization of the degradation conditions, the phenanthrene degradation ratio was improved to 85.73%. IMPORTANCE Polycyclic aromatic hydrocarbons (PAHs), which are recalcitrant, carcinogenic, and tend to bioaccumulate, are widespread and persistent environmental pollutants. Based on these characteristics, the U.S. Environmental Protection Agency has listed PAHs as priority contaminants. Although there are many methods to treat PAH pollution, these methods are mostly limited by the poor water solubility of PAHs, which is especially true for the biodegradation process. Recent evidence of PAH-contaminated sites suffering from increasingly severe impact has emerged. As a result, the need to degrade PAHs is becoming urgent. The significance of our study lies in the development of nonpathogenic strains of biosurfactant-producing Pseudomonas aeruginosa for promoting the degradation of phenanthrene by engineered Escherichia coli.

Enhanced bioremediation of sediment contaminated with polycyclic aromatic hydrocarbons by combined stimulation with sodium acetate/phthalic acid

In this study, four groups of laboratory scale experiments were performed by adding sodium acetate (SA), phthalic acid (PA), and SA-PA to river sediment to observe the microbial response and biodegradation efficiency of polycyclic aromatic hydrocarbons (PAHs). The results showed that the amount of total organic carbon consumed and the amount of sulfate reduction were both positively correlated (p < 0.01) with the biodegradation efficiency of the sum (∑) PAHs (∼40.5%). The lower the number of rings, the more PAHs were biodegraded, with an efficiency of 63.0% for ∑ (2 + 3) ring PAHs. Based on high-throughput sequencing and molecular ecological network analysis, it was found that the combined stimulation of SA and PA not only increased the relative abundance of PAHs-degrading bacterial (eg., Proteobacteria, Desulfobacterota, Campilobacterota and Firmicutes), but also had a strengthening effect on microbes in sediments. The altered microbial structure caused a variation in metabolic functions, which increased the amino acid metabolism to 12.2%, thus increasing the positive correlations among genera and improving the connectivity of the microbial network (p < 0.01). These changes may be responsible for the enhanced biodegradation of PAHs under SA-PA dosing in comparison to SA or PA dosing alone. This study revealed that the microbial community was stimulated by the combined addition of SA and PA, and indicated its role in enhancing biodegradation of PAHs in contaminated river sediments.

New insights into denitrification and phosphorus removal with degradation of polycyclic aromatic hydrocarbons in two-sludge system

Polycyclic aromatic hydrocarbons (PAHs) have led to failure of waste water treatment plant operations. In this study, a two-sludge system was used to solve this problem of simultaneously removing phosphorus, nitrogen, and PAHs. The results showed that increasing the maximum PAHs concentration to 15 mg/L did not have any negative effect on the removal rates of total nitrogen (79.68%) and chemical oxygen demand (75.94%); however, the phosphorus removal efficiency decreased to 61.16%. The system exhibited a stronger degradation ability for phenanthrene. Thauera, Hydrogenophaga, and Hyphomicrobium were enriched, which resulted in good denitrification, and contributed to PAHs removal. PAHs mixture promoted PAHs functional genes but restrained denitrification functional genes. However, single naphthalene enhanced denitrification functional genes, which confirmed the feasibility of denitrification coupled with PAHs degradation. In conclusion, for the removal of pollutants from sewage treatment, nitrogen and phosphorus removal coupled with PAHs could be maintained by selecting a two-sludge system.

Pilot-scale bioaugmentation of polycyclic aromatic hydrocarbon (PAH)-contaminated soil using an indigenous bacterial consortium in soil-slurry bioreactors

Soil-slurry bioreactor based bioremediation of polycyclic aromatic hydrocarbons (PAHs) contaminated soil was studied through laboratory and pilot-scale trials, in which the degradation mechanism was explored. Indigenous PAH-degrading consortium was firstly screened out and it degraded 80.5% of total PAHs in lab-scale bioreactors. Then a pilot-scale trial lasting 410 days was conducted in two bioreactors of 1.5 m³ to examine the operating parameters and validate the optimum running conditions. During the initial 200 days, the crucial running parameters affecting PAH removal were evaluated and selected. Subsequently, an average PAH removal rate of 93.4% was achieved during 15 consecutive batches (210 days) under the optimum running conditions. The kinetic analysis showed that the reactor under optimum conditions achieved the highest PAH degradation rate of 0.1795 day^-1 and the shortest half-life of 3.86 days. Notably, efficient mass transfer of PAHs and high biodegradation capability by bioaugmented consortia in soil-slurry bioreactors were two key mechanisms for appreciable PAH removal performance. Under the optimal operating conditions, the degradation rate of low-molecular-weight (LMW) PAHs was significantly higher than high-molecular-weight (HMW) PAHs; when the mass transfer was limited, there was no significant difference between their degradation behaviors. Both microbial co-metabolism and collaborative metabolism might occur when all PAHs demonstrated low degradation rates. The findings provide insightful guidance on the future assessment and remediation practices of PAH-contaminated sites.

Biodegradation potential of polycyclic aromatic hydrocarbons in Taihu Lake sediments

ABSTRACTTo assess the biodegradation potential of polycyclic aromatic hydrocarbons (PAHs) in sediments, sediment microcosms were constructed with sediments collected from six lake zones with different trophic statuses in Taihu Lake. The presence and concentration of PAH-degrading bacteria (PDB) were estimated by the most probable number (MPN) method. After 85 d of aerobic and anaerobic incubation, spiked PAHs (phenanthrene, pyrene, and benzo[a]pyrene) were partially degraded by indigenous sediment microorganisms. Large differences in PAH degradation were observed depending on the molecular size of the PAHs. The PAH removal efficiency in sediments under aerobic conditions was higher than that under anaerobic conditions. MPN analyses showed a higher abundance of degrading microflora in the high PAH-contaminated sites than in the low PAH-contaminated sites. Moreover, the anaerobic PDB populations in the sediments from the six different sites were much higher than those of aerobic PDB. The PAH biodegradation capability in sediments was associated with the geochemical conditions and bacterial populations. PDB showed a broad spatial distribution, thereby implying that they played an important role in the natural attenuation and cycling of PAHs in Taihu Lake. This work indicates that PAHs remain a concern in Taihu Lake sediments and can provide useful information for further bioremediation of PAH-contaminated sediments.

Indigenous bacterial community and function in phenanthrene-polluted coastal wetlands: Potential for phenanthrene degradation and relation with soil properties

The Yellow River Delta, adjacent to Shengli Oilfield, has a potential risk of petroleum pollution. In this study, soil samples were collected from phenanthrene (PHE)-polluted (adjacent to abandoned oil well, Zone D) and non-polluted (far away from abandoned oil well, Zone E) coastal wetlands. The influence of PHE pollution on indigenous bacterial community and function, and their relationship with soil characteristics were investigated. The levels of PHE, salinity and NH₄⁺-N were higher in Zone D than in Zone E. PHE-degrading bacteria Achromobacter and Acinetobacter were mainly distributed in Zone E, whereas Halomonas, Marinobacter, and Roseovarius were highly abundant in Zone D. Halomonas and Marinobacter had the potential for denitrification and could achieve PHE degradation through mutual cooperation. PHE pollution could increase the abundance of functional bacteria but reduce the diversity of microbial community. PHE and salinity played key roles in shaping microbial community structure and function. High PHE level inhibited microbial metabolism but stimulated self-protection potential. PHE aerobic degradation associated with the catechol and phthalic acid pathways was found in Zone D, whereas the catechol pathway dominated in Zone E. Interestingly, PHE anaerobic degradation with nitrate reduction also dominated in Zone D, whereas the process coupled with multiple electron acceptors co-existed in Zone E, which was associated with tidal seawater carrying nutrients. This study illustrated the importance of comprehensive consideration of microbial community structure and function under PHE pollution, suggesting indigenous microorganisms as potential microbial consortium for bioremediation in coastal wetlands.

Persistent contamination of polycyclic aromatic hydrocarbons (PAHs) and phthalates linked to the shift of microbial function in urban river sediments

Urban rivers were heavily polluted, which resulted in blackening and odorization (i.e., black-odor rivers). Nevertheless, very limited information is available on sediment contamination levels of black-odor rivers and their linkage to the patterns of microbial functional genes. This study investigated distribution of polycyclic aromatic hydrocarbons (PAHs) and phthalates (PAEs) and their linkages to bacterial community and related functional genes in river sediments. The results demonstrate that higher average levels of ∑16PAHs (1405 μg/kg, dry weight) and ∑6PAEs (7120 μg/kg) were observed in sediments from heavy black-odor rivers than the moderate ones (∑16PAHs: 462 μg/kg; ∑6PAEs: 2470 μg/kg). The taxon composition and diversities of bacterial community in sediments varied with significantly lower diversity indices in heavy black-odor rivers than moderate ones. Sediments from heavy black-odor rivers enriched certain PAH and PAE degrading bacteria and genes. Unfortunately, PAH and PAE contamination demonstrated negative influences on nitrogen and phosphorus metabolism related bacteria and function genes but significant positive influences on certain sulfur metabolism related bacterial taxa and sulfur reduction gene, which might cause nitrogen and phosphorus accumulation and black-odor phenomenon in heavy black-odor rivers. This study highlights PAH and PAE contamination in urban rivers may shift bacterial community and detrimentally affect their ecological functions.

Organic matter composition, BaP biodegradation and microbial communities at sites near and far from the bioanode in a soil microbial fuel cell

Bioanodes in a soil microbial fuel cell (SMFC) can serve as sustainable electron acceptors in microbial metabolism processes; thus, SMFCs are considered a promising in situ bioremediation technology. Most related studies have focused on the removal efficiency of contaminants. Relatively few efforts have been made to comprehensively investigate the organic matter composition and biodegradation metabolites of organic contaminants and microbial communities at various distances from the bioanode. In this study, the level and composition of dissolved organic matter (DOM), biodegradation metabolites of benzo[a]pyrene (BaP), and microbial communities at two sites with different distances (S_1cm and S_11cm) to the bioanode were investigated in an SMFC. The consumption efficiency of dissolved organic carbon (R_DOC) and removal efficiency of BaP (R_BaP) at S_1cm were slightly higher than those at S_11cm after 100 days (R_DOC 47.82 ± 5.77% at S_1cm and 44.98 ± 10.76% at S_11cm; R_BaP 72.52 ± 1.88% at S_1cm and 68.50 ± 4.34% at S_11cm). More fulvic acid-like components and more low-molecular-weight metabolites (indicating a higher biodegradation degree) of BaP were generated at S_1cm than at S_11cm. The microbial community structures were similar at the two sites. Electroactive bacteria (EAB) and some polycyclic aromatic hydrocarbon degraders were both enriched at the bioanode. Energy metabolism at the bioanode could be upregulated to generate more adenosine triphosphate (ATP). In conclusion, the bioanode could modulate the metabolic pathways in the adjacent soil by strengthening the contact between the EAB and BaP degraders, and providing more ATP to the BaP degraders.

Aerobic and Anaerobic Biodegradability of Organophosphates in Activated Sludge Derived From Kitchen Garbage Biomass and Agricultural Residues

Organophosphates (also known as organophosphate esters, OPEs) have in recent years been found to be significant pollutants in both aerobic and anaerobic activated sludge. Food waste, such as kitchen garbage and agricultural residues, can be used as co-substrates to treat the active sludge in sewage treatment plants (STPs). We investigated the biodegradability of nine OPEs derived from kitchen garbage biomass and agricultural residues under different conditions. Under anaerobic conditions, the rate of removal of triphenyl ester OPEs was significantly higher than that of chloride and alkyl OPEs. The addition of FeCl₃ and Fe powder increased the rate of degradation of triphenyl ester OPEs, with a DT₅₀ for triphenyl ester OPEs of 1.7–3.8 d for FeCl₃ and 1.3–4.7 d for Fe powder, compared to a DT₅₀ of 4.3–6.9 d for the blank control. Addition of an electron donor and a rhamnolipid increased the rate of removal of chlorinated OPEs, with DT₅₀ values for tris(2-carboxyethyl)phosphine) (TCEP) and tris(1,3-dichloroisopropyl)phosphate (TDCPP) of 18.4 and 10.0 d, respectively, following addition of the electron donor, and 13.7 and 3.0 d, respectively, following addition of the rhamnolipid. However, addition of an electron donor, electron acceptor, surfactant, and Fe powder did not always increase the degradation of different kinds of OPEs, which was closely related to the structure of the OPEs. No treatment increased the removal of alkyl OPEs due to their low anaerobic degradability. Tween 80, a non-ionic surfactant, inhibited anaerobic degradation to some degree for all OPEs. Under aerobic conditions, alkyl OPEs were more easily degraded, chlorinated OPEs needed a long adaptation period to degrade and finally attain a 90% removal rate, while the rates of degradation of triphenyl ester OPEs were significantly affected by the concentration of sludge. Higher sludge concentrations help microorganisms to adapt and remove OPEs. This study provides new insights into methods for eliminating emerging pollutants using activated sludge cultured with kitchen garbage biomass and agricultural residues.

Anaerobic degradation of xenobiotic organic contaminants (XOCs): The role of electron flow and potential enhancing strategies

In groundwater, deep soil layer, sediment, the widespread of xenobiotic organic contaminants (XOCs) have been leading to the concern of human health and eco-environment safety, which calls for a better understanding on the fate and remediation of XOCs in anoxic matrices. In the absence of oxygen, bacteria utilize various oxidized substances, e.g. nitrate, sulphate, metallic (hydr)oxides, humic substance, as terminal electron acceptors (TEAs) to fuel anaerobic XOCs degradation. Although there have been increasing anaerobic biodegradation studies focusing on species identification, degrading pathways, community dynamics, systematic reviews on the underlying mechanism of anaerobic contaminants removal from the perspective of electron flow are limited. In this review, we provide the insight on anaerobic biodegradation from electrons aspect - electron production, transport, and consumption. The mechanism of the coupling between TEAs reduction and pollutants degradation is deconstructed in the level of community, pure culture, and cellular biochemistry. Hereby, relevant strategies to promote anaerobic biodegradation are proposed for guiding to an efficient XOCs bioremediation.

Nitrate-assisted biodegradation of polycyclic aromatic hydrocarbons (PAHs) in the water-level-fluctuation zone of the three Gorges Reservoir, China: Insights from in situ microbial interaction analyses and a microcosmic experiment

An increase in polycyclic aromatic hydrocarbon (PAH) pollution poses significant challenges to human and ecosystem health in the Three Gorges Reservoir (TGR) of the Yangtze River. Based on the combination of PAH analysis with qPCR and high-throughput sequencing of bacteria, 32 topsoil samples collected from 16 sites along the TGR were used to investigate the distribution and biodegradation pathways of PAHs in the water-level-fluctuation zone (WLFZ). The results indicated that the concentrations of PAHs were 43.8-228.2 and 30.8-206.3 ng/g soil (dry weight) under the high- and low-water-level (HWL and LWL) conditions, respectively. The PAH concentration in urban areas was higher than that in rural areas. Under both the HWL and LWL conditions, the abundance of the bamA gene, a biomarker of anaerobic PAH biodegradation, was significantly higher than that of the ring-hydroxylating-dioxygenase (RHD) gene, a biomarker of aerobic PAH biodegradation. The abundance of the bamA gene was significantly positively correlated with PAHs (R² = 0.8), and the biodegradation percentage of PAHs incubated anaerobically was greater than that in the aerobically incubated microcosm experiments. These data implicated a key role of the anaerobic pathway in PAH biodegradation. Co-occurrence network analysis suggested that anaerobic Anaerolineaceae, Dechloromonas, Bacteroidetes_vadin HA17 and Geobacter were key participants in the biodegradation of PAHs. The diversity analysis of functional bacteria based on the bamA gene and microcosm experiments further demonstrated that nitrate was the primary electron acceptor for PAH biodegradation. These findings provide a new perspective on the mechanism of PAH biodegradation in the TGR and knowledge that can be used to develop strategies for environmental management.

Polycyclic Aromatic Hydrocarbons: Sources, Toxicity, and Remediation Approaches

Polycyclic aromatic hydrocarbons (PAHs) are widespread across the globe mainly due to long-term anthropogenic sources of pollution. The inherent properties of PAHs such as heterocyclic aromatic ring structures, hydrophobicity, and thermostability have made them recalcitrant and highly persistent in the environment. PAH pollutants have been determined to be highly toxic, mutagenic, carcinogenic, teratogenic, and immunotoxicogenic to various life forms. Therefore, this review discusses the primary sources of PAH emissions, exposure routes, and toxic effects on humans, in particular. This review briefly summarizes the physical and chemical PAH remediation approaches such as membrane filtration, soil washing, adsorption, electrokinetic, thermal, oxidation, and photocatalytic treatments. This review provides a detailed systematic compilation of the eco-friendly biological treatment solutions for remediation of PAHs such as microbial remediation approaches using bacteria, archaea, fungi, algae, and co-cultures. In situ and ex situ biological treatments such as land farming, biostimulation, bioaugmentation, phytoremediation, bioreactor, and vermiremediation approaches are discussed in detail, and a summary of the factors affecting and limiting PAH bioremediation is also discussed. An overview of emerging technologies employing multi-process combinatorial treatment approaches is given, and newer concepts on generation of value-added by-products during PAH remediation are highlighted in this review.