Biodegradation of benzo(a)pyrene by Microbacterium sp. strain under denitrification: Degradation pathway and effects of limiting electron acceptors or carbon source

Anaerobic Microbial Degradation of Polycyclic Aromatic Hydrocarbons: A Comprehensive Review

Polycyclic aromatic hydrocarbons (PAHs) are a class of hazardous organic contaminants that are widely distributed in nature, and many of them are potentially toxic to humans and other living organisms. Biodegradation is the major route of detoxification and removal of PAHs from the environment. Aerobic biodegradation of PAHs has been the subject of extensive research; however, reports on anaerobic biodegradation of PAHs are so far limited. Microbial degradation of PAHs under anaerobic conditions is difficult because of the slow growth rate of anaerobes and low energy yield in the metabolic processes. Despite the limitations, some anaerobic bacteria degrade PAHs under nitrate-reducing, sulfate-reducing, iron-reducing, and methanogenic conditions. Anaerobic biodegradation, though relatively slow, is a significant process of natural attenuation of PAHs from the impacted anoxic environments such as sediments, subsurface soils, and aquifers. This review is intended to provide comprehensive details on microbial degradation of PAHs under various reducing conditions, to describe the degradation mechanisms, and to identify the areas that should receive due attention in further investigations.

Effectiveness of the Zea mays-Streptomyces association for the phytoremediation of petroleum hydrocarbons impacted soils

Restoring polluted sites by petroleum hydrocarbons is a challenge because of their complexity and persistence in the environment. The main objective of the present study was to investigate the performance of plant-actinobacteria system for the remediation of crude petroleum and pure-polycyclic aromatic hydrocarbons (PAHs) contaminated soils. The endophytic strain Streptomyces sp. Hlh1 was tested for its ability to degrade model PAHs (phenanthrene, pyrene and anthracene) in liquid minimal medium. Streptomyces sp. Hlh1 demonstrated the ability to grow on PAHs as sole carbon and energy source, reaching hydrocarbons removal of 63%, 93% and 83% for phenanthrene, pyrene and anthracene, respectively. Maize plant was chosen to study the impact of Streptomyces sp. Hlh1 inoculation on the dissipation of contaminants and plant growth. Thus, maize seedlings grown in soils contaminated with crude petroleum and pure-PAHs were inoculated with Streptomyces sp. Hlh1. Results showed that the endophyte inoculation increased contaminants removal. Maximum hydrocarbons removal (70%) was achieved in inoculated and planted soil contaminated with crude oil, while 61%, 59%, and 46% of hydrocarbons dissipation were registered for phenanthrene, pyrene and anthracene, respectively. These degradations rates were significantly higher compared to non-inoculated systems in all the treatments evaluated. Further, it was revealed that hydrocarbons (C₈-C₃₀) were efficiently degraded in plant-Streptomyces Hlh1 system. Moreover, the inoculation with the actinobacteria resulted significant plant development and enhanced photosynthetic pigments compared to plants grown in the other experimental conditions. The present study provide evidence that the inoculation of maize plants with Streptomyces sp. Hlh1 play a remarkable role in the removal of petroleum hydrocarbons, enhancing plant development in contaminated soils.

Comparison of bacterial community structure and function under different petroleum hydrocarbon degradation conditions

Bioremediation methods have been successfully applied to the removal of organic pollutants for decades, but the responses of the microbial community to environmental factors remain less well known. In this work, the degradation rates of petroleum hydrocarbons (PHs) reached up to 50.11% ± 2.74% after optimizing the degradation conditions. Under the influence of the optimized degradation conditions, the diversity of the bacterial community gradually increased. Meanwhile, the dominant bacterial genera, encompassing Burkholderia-Paraburkholderia, Luteibacter, and Acinetobacter, remained stable. Moreover, statistical analysis indicated that the genera Bacterium, Burkholderia-Paraburkholderia, Luteibacter, and Acinetobacter contributed the most to PHs degradation. Additionally, the functional modules of amino acid metabolism, carbohydrate metabolism, as well as global and overview maps played a vital role in the metabolization of PHs. Therefore, understanding the changes of the microbial community structure and function can provide valuable guidance to further improve the degradation rate of organic waste via bioremediation methods.

Establishing simultaneous nitrification and denitrification under continuous aeration for the treatment of multi-electrolytes saline wastewater

Simultaneous nitrification and denitrification (SND) was established under continuous aeration (6 mgO₂ L^-1) aiming at achieving a feasible and simple operational strategy for treating multi-electrolyte saline wastewaters. Two Structured Fixed-Bed Reactors (SFBR) were used to assess SND performance with (Saline Reactor, SR) and without (Control Reactor, CR) salinity interference. Salinity was gradually increased (from 1.7 to 9 atm) based on the composition of water supplied in arid regions of Brazil. At 1.7 atm, N-NH₄⁺ oxidation and Total Nitrogen (TN) removal efficiencies of 95.9 ± 2.8 and 65.76 ± 7.5%, respectively, were obtained. At osmotic pressure (OP) of 9 atm, the system was severely affected by specific salt toxicity and OP. High chemical oxygen demand (COD) removal efficiency was achieved at all operational conditions (97.2 ± 1.6 to 78.5 ± 4.6%). Salinity did not affect microbial diversity, although it modified microbial structure. Halotolerant genera were identified (Prosthecobacter, Chlamydia, Microbacterium, and Paenibacillus).

Contamination, Sources, and Health Risks Associated with Soil PAHs in Rebuilt Land from a Coking Plant, Beijing, China

This study investigated the polycyclic aromatic hydrocarbon (PAH) pollution in the reconstructed land of an abandoned industrial site: a coking plant in Beijing. To meet the needs of urban development, many factories have had to be relocated from city centers, and abandoned industrial sites often need to be transformed into residential land or urban green space through a series of restoration measures. It is necessary to study the level of residual pollutants and potential risks associated with industrial reconstructed land. The concentration of 16 PAHs in the study area ranged from 314.7 to 1618.3 µg/kg, and the average concentration was still at a medium pollution level; the concentration of PAHs in the original coking workshop had the highest levels (1350.5 µg/kg). The PAHs in the soil were mainly low-ring aromatics, especially naphthalene and phenanthrene. The isomer method and principal component analysis indicated that PAHs in the topsoil were the result of coal and biomass combustion. The seven carcinogenic PAHs were the main contributors to the total toxicity equivalence. The genetic toxicity of benzo[a]pyrene was relatively low, and the results were related to the concentration level. There were potential carcinogenic risks for people of varying ages in this residential area. In total, gender differences were small, and the comprehensive lifetime cancer risk level was still acceptable. For the remaining plots at the study site, the daily intake of PAHs by construction workers was between 0.74⁻2.31 ng/kg bw/day, which requires further evaluation about ignored area occupational exposure to environmental pollutants.

Population characteristics and influential factors of nitrogen cycling functional genes in heavy metal contaminated soil remediated by biochar and compost

Sixteen treatments of soil contaminated by Cu, Pb, and Zn by the addition of a different percentage of biochar and compost were incubated for 120 days. The abundance of denitrifying genes such as narG, nirK, nirS and nosZ and the ammonia-oxidizing amoA genes of ammonia-oxidizing archaea/bacteria (AOA/AOB), soil nitrite reductase activity (S-NiR) and their shaping factors were also determined. The relationships between functional genes, S-NiR, and physico-chemical parameters were analyzed using the Pearson correlation method. The study found that the changes in physico-chemical parameters, including water-soluble organic carbon (WSC), nitrate (NO₃^-) and ammonium (NH₄⁺), were predominant in different treatments. The abundance of nirK and narG genes is most sensitive to the changes in the properties of the soil sample. Bacterial 16S rDNA gene abundance was significantly affected by NO₃^- and S-NiR (P < 0.05). Nitrifying genes were mainly correlated to WSC and S-NiR, while denitrifying genes were associated with pH, electrical conductivity, NO₃^- and S-NiR. The systematic study for the relationship between the genes and the environmental parameters will help us to deep understand the biological mechanisms of nitrogen cycle in heavy metal contaminated soils remediated by biochar and compost.

Biodegradation of high-molecular-weight polycyclic aromatic hydrocarbons under anaerobic conditions: Overview of studies, proposed pathways and future perspectives

The biodegradation of low- and high-molecular-weight polycyclic aromatic hydrocarbons (PAHs) (LWM-PAHs and HMW-PAHs, respectively) has been studied extensively under aerobic conditions. Molecular O₂ plays 2 critical roles in this biodegradation process. O₂ activates the aromatic rings through hydroxylation prior to ring opening and serves as a terminal electron acceptor (TEA). However, several microorganisms have devised ways of activating aromatic rings, leading to ring opening (and thus biodegradation) when TEAs other than O₂ are used (under anoxic conditions). These microorganisms belong to the sulfate-, nitrate-, and metal-ion-reducing bacteria and the methanogens. Although the anaerobic biodegradation of monocyclic aromatic hydrocarbons and LWM-PAH naphthalene have been studied, little information is available about the biodegradation of HMW-PAHs. This manuscript reviews studies of the anaerobic biodegradation of HMW-PAHs and identifies gaps that limit both our understanding and the efficiency of this biodegradation process. Strategies that can be employed to overcome these limitations are also discussed.

Comprehensive review on toxicity of persistent organic pollutants from petroleum refinery waste and their degradation by microorganisms

Control and prevention of environmental pollution has become a worldwide issue of concern. Aromatic hydrocarbons including benzene, toluene, ethyl benzene, xylene (BTEX) and polyaromatic hydrocarbons (PAHs) are persistent organic pollutants (POPs), released into the environment mainly by exploration activities of petroleum industry. These pollutants are mutagenic, carcinogenic, immunotoxic and teratogenic to lower and higher forms of life i.e. microorganisms to humans. According to the International Agency for Research on Cancer (IARC) and United States Environmental Protection Agency (U.S. EPA), Benzo[a]pyrene (BaP) is carcinogenic in laboratory animals and humans. Aromatic hydrocarbons are highly lipid soluble and thus readily absorbed from environment in gastrointestinal tract of mammals. Treatment and remediation of petroleum refinery waste have been shown either to reduce or to eliminate genotoxicity of these pollutants. Bioremediation by using microorganisms to treat this waste is showing a promising technology as it is safe and cost-effective option among various technologies tested. The main aim of this review is to provide contemporary information on variety of aromatic hydrocarbons present in crude oil (with special focus to mono- and poly-aromatic hydrocarbons), exposure routes and their adverse effects on humans. This review also provides a synthesis of scientific literature on remediation technologies available for aromatic hydrocarbons, knowledge gaps and future research developments in this field.

Anaerobic biodegradation of benzo(a)pyrene by a novel Cellulosimicrobium cellulans CWS2 isolated from polycyclic aromatic hydrocarbon-contaminated soil

Cellulosimicrobium cellulans CWS2, a novel strain capable of utilizing benzo(a)pyrene (BaP) as the sole carbon and energy source under nitrate-reducing conditions, was isolated from PAH-contaminated soil. Temperature and pH significantly affected BaP biodegradation, and the strain exhibited enhanced biodegradation ability at temperatures above 30 °C and between pH 7 and 10. The highest BaP removal rate (78.8%) was observed in 13 days when the initial BaP concentration was 10 mg/L, and the strain degraded BaP at constant rate even at a higher concentration (50 mg/L). Metal exposure experimental results illustrated that Cd(II) was the only metal ion that significantly inhibited biodegradation of BaP. The addition of 0.5 and 1.0 g/L glucose enhanced BaP biodegradation, while the addition of low-molecular-weight organic acids with stronger acidity reduced BaP removal rates during co-metabolic biodegradation. The addition of phenanthrene and pyrene, which were degraded to some extent by the strain, showed no distinct effect on BaP biodegradation. Gas chromatography–mass spectrometry (GC-MS) analysis revealed that the five rings of BaP opened, producing compounds with one to four rings which were more bioavailable. Thus, the strain exhibited strong BaP degradation capability and has great potential in the remediation of BaP-/PAH-contaminated environments.

Comparative proteomic analysis and characterization of benzo(a)pyrene removal by Microbacterium sp. strain M.CSW3 under denitrifying conditions

High-molecular-weight polycyclic aromatic hydrocarbons are persistent organic pollutants with great environmental and human health risks and the associated bioremediation activities have always been hampered by the lack of powerful bacterial species under redox conditions. A Microbacterium sp. strain capable of using benzo(a)pyrene as sole carbon and energy sources under denitrifying conditions was isolated. The difference in protein expression during BaP removal and removal characterization were investigated. A total of 146 proteins were differentially expressed, 44 proteins were significantly up-regulated and 102 proteins were markedly down-regulated. GO and COG analysis showed that BaP removal inhibited the expression of proteins related to glucose metabolism at different levels and activated other metabolic pathway. The proteins associated with catalytic activity and metabolic process were altered significantly. Furthermore, the BaP removal might be occurred in certain organelle of M.CSW3. The strain removed BaP with a speed of 0.0657-1.0072 mg/L/day over the concentrations range 2.5-100 mg/L. High removal rates (>70%) were obtained over the range of pH 7-11 in 14 days. Carbohydrates and organic acids which could be utilized by the strain, as well as heavy metal ions, reduced BaP removal efficiency. However, phenanthrene or pyrene addition enhanced the removal capability of M.CSW3. The strain was proved to have practical potential for bioremediation of PAHs-contaminated soil and this study provided a powerful platform for further application by improving production of associated proteins.