Anaerobic Microbial Degradation of Polycyclic Aromatic Hydrocarbons: A Comprehensive Review

Differential degradation of petroleum hydrocarbons by Shewanella putrefaciens under aerobic and anaerobic conditions

The complexity of crude oil composition, combined with the fluctuating oxygen level in contaminated environments, poses challenges for the bioremediation of oil pollutants, because of compound-specific microbial degradation of petroleum hydrocarbons under certain conditions. As a result, facultative bacteria capable of breaking down petroleum hydrocarbons under both aerobic and anaerobic conditions are presumably effective, however, this hypothesis has not been directly tested. In the current investigation, Shewanella putrefaciens CN32, a facultative anaerobic bacterium, was used to degrade petroleum hydrocarbons aerobically (using O2 as an electron acceptor) and anaerobically (using Fe(III) as an electron acceptor). Under aerobic conditions, CN32 degraded more saturates (65.65 ± 0.01%) than aromatics (43.86 ± 0.03%), with the following order of degradation: dibenzofurans > n-alkanes > biphenyls > fluorenes > naphthalenes > alkylcyclohexanes > dibenzothiophenes > phenanthrenes. In contrast, under anaerobic conditions, CN32 exhibited a higher degradation of aromatics (53.94 ± 0.02%) than saturates (23.36 ± 0.01%), with the following order of degradation: dibenzofurans > fluorenes > biphenyls > naphthalenes > dibenzothiophenes > phenanthrenes > n-alkanes > alkylcyclohexanes. The upregulation of 4-hydroxy-3-polyprenylbenzoate decarboxylase (ubiD), which plays a crucial role in breaking down resistant aromatic compounds, was correlated with the anaerobic degradation of aromatics. At the molecular level, CN32 exhibited a higher efficiency in degrading n-alkanes with low and high carbon numbers relative to those with medium carbon chain lengths. In addition, the degradation of polycyclic aromatic hydrocarbons (PAHs) under both aerobic and anaerobic conditions became increasingly difficult with increased numbers of benzene rings and methyl groups. This study offers a potential solution for the development of targeted remediation of pollutants under oscillating redox conditions.

Spatial distribution, source, and ecological risk of PAHs in the sediment of the Fenhe River Basin, China

Polycyclic aromatic hydrocarbons (PAHs) are persistent toxic substances that have ubiquitous presence in water, air, soil, and sediment environments. The growth of PAH toxicities and related ecotoxicology risk in sediment has been a serious concern. Present study examined the PAH concentration, sources, and ecological risk from the selected sites in sediment of Fenhe River. The characteristic molecular ratio (CMR) and principal component analysis (PCA) were applied to analyze the sources. The ecological risk assessment was conducted based on the sediment quality guidelines, the mean effects range median quotient, as well as the toxic equivalent quantity values. The results showed that the mean values of total contents of the 16 individual PAHs were 3.66 mg/kg and 3.16 mg/kg in wet and dry seasons, which were relatively high when compared with other rivers worldwide. Their spatial distribution presented the lower contents in the upstream, while higher concentrations in the middle and down streams of the river. The low molecular weight PAHs were major constituents, and 3-ring PAHs have the highest contents. The results of source analysis indicated that PAHs were primarily from the burning of oil, coal, and biomass. The ecological risk evaluations suggested that the possible adverse biological effects, the low to medium comprehensive risks, and the minor carcinogenic risks existed in the study area. This investigation might provide useful baseline data and technical support for policy-makers and researchers.

Enhanced biodegradation of phenanthrene and anthracene using a microalgal-bacterial consortium

Polycyclic aromatic hydrocarbons (PAHs) are chemicals that are released into the environment during activities of the petroleum industry. The bioaccumulation, carcinogenic and mutagenic potential of PAHs necessitates the bioremediation of these contaminants. However, bioremediation of PAHs has a number of limitations including the inability of a single microbe to degrade all of the PAH fraction's environmental constituents. Therefore, a different paradigm, employing microalgal-bacterial consortium (MBC), may be used to effectively remove PAHs contaminants. In this type of interaction, the microalgae and bacteria species in the consortium work together in a way that enhances the overall performance of the MBC. Bacterial species in the consortium provide essential nutrients or growth factors by degrading toxic substances and provide these to microalgae, while the microalgae species provide organic carbon for the bacterial species to grow. For the first time, the ability of Gonium pectorale (G. pectorale) microalgae to break down phenanthrene (PHE) and anthracene (ANT) was investigated. Phenanthrene was shown to be more effectively degraded by G. pectorale (98%) as compared to Bacillus licheniformis (B. licheniformis) 19%. Similarly, G. pectorale has effectively degrade anthracene (98%) as compared with B. licheniformis (45%). The consortia of G. pectorale and B. licheniformis has shown a slight increase in the degradation of PHE (96%) and ANT (99%). Our findings show that B. licheniformis did not inhibit the growth of G. pectorale and in the consortia has effectively eliminated the PAHs from the media. Therefore G. pectorale has a tremendous potential to remove PAHs from the polluted environment. Future research will be conducted to assess Gonium's capacity to eliminate PAHs that exhibit high molar masses than that of PHE and ANT.

Sources of Polycyclic Aromatic Hydrocarbons in Fecal Pellets of a Marphysa Species (Annelida: Eunicidae) in the Yoro Tidal Flat, Japan

The fecal pellets of Marphysa sp. E sensu Abe et al. (2019) (Annelida, Eunicidae) living in the Yoro tidal flat (Ichihara, Chiba, Japan) contain high levels of polycyclic aromatic hydrocarbons (PAHs), and the concentrations rapidly decrease over time. To investigate the origin of the high-concentration PAHs in the fecal pellets and food sources of the worms, the PAH concentrations, carbon and nitrogen stable isotope ratios (δ13C and δ15N), total organic carbon, and total nitrogen for two types of sediment (sands and reduced muds), fecal pellets, and the body of the worms were determined. The PAH concentrations and chemical properties of the fecal pellets were similar to those of the reduced muds (20-30 cm sediment depth). The δ13C, δ15N, and C/N values of reduced muds were the same as the typical values of terrestrial C3 plants, suggesting that reduced muds were derived from terrestrial plants. These data indicated that the worms selectively take up reduced muds containing high levels of PAHs. The δ13C and δ15N values of the worm bodies indicated that the worms did not use the organic carbon derived from terrestrial C3 plants as primary nutrition. Taking into consideration their selective uptake of reduced muds, excretion, and subsequent rapid decrease of PAHs in the fecal pellets, the worms could contribute to the remediation of chemical pollutants in the tidal flat sediments.

Hydrocarbons in the water and bottom sediments of Sivash Bay (the Azov Sea) during its salinization

Sivash Bay is a unique hypersaline lagoon located in the northern part of the Crimean Peninsula. In 2014, due to political events in connection with the closure of the North Crimean Canal, the inflow of fresh water to Sivash Bay has been significantly reduced. As a result, there has been a steady increase in salinity since 2014 to the present. The main purpose of this work was to determine the spatial distribution and qualitative composition of hydrocarbons (aliphatic hydrocarbons, n-alkanes, polycyclic aromatic hydrocarbons) in the water and bottom sediments of the hypersaline Sivash Bay under increasing water salinity. The analysis of the physico-chemical parameters of Sivash Bay in 2020 showed the continued salinization and change of physico-chemical conditions of the lagoon. At the same time, spatially, the change in salinity affected only the total content and qualitative composition of hydrocarbons in the water. The content of the studied classes of hydrocarbons in the bottom sediments did not demonstrate a reliable correlation with the concentration of salts. There was also no statistically significant dependence of Eh and pH of bottom sediments on salinity. In accordance with the composition of n-alkanes and polyaromatic hydrocarbons, as well as on the basis of PCA analysis, it is possible to make a conclusion on natural, mainly autochthonous, sources of this class of substances and low toxicity of bottom sediments of the bay. Low concentrations and composition of hydrocarbons indicate an insignificant input of pollutants of anthropogenic origin in Sivash Bay during its salinization.

Evaluation of Biodegradation of BTEX in the Subsurface of a Petrochemical Site near the Yangtze River, China

The contamination of soil and groundwater with BTEX (benzene, toluene, ethyl benzene, and xylenes) is a common issue at petrochemical sites, posing a threat to the ecosystems and human health. The goal of this study was to evaluate the biodegradation of BTEX in the subsurface of a petrochemical site near the Yangtze River, thus providing scientific basis for bioremediation of the contaminated site. Both molecular analysis of field samples and microcosm study in the laboratory were performed for the evaluation. Soil and groundwater samples were collected from the site. Microcosms were constructed with inoculum from the soil and incubated anaerobically in the presence of nitrate, ferric oxide, manganese oxide, sulfate, and sodium bicarbonate, respectively. The initial concentration of each component of BTEX (benzene, toluene, ethyl benzene, o-xylene) was 4-5 mg/L. Actinobacteria was dominant in the highly contaminated soil, while Proteobacteria was dominant in the slightly contaminated soil and the groundwater. The relative abundances of Firmicutes, Spirochaetes, and Caldiserica were higher in the highly contaminated soil and groundwater samples compared to those in the corresponding slightly contaminated samples. The relative abundances of predicted functions, such as carbohydrate transport and metabolism, nucleotide transport and metabolism, coenzyme transport and metabolism, amino acid transport and metabolism, etc., in the highly contaminated soil and groundwater samples were higher than those in the corresponding slightly contaminated samples. In microcosms, biodegradations of BTEX occurred, and the first-order rate constants in the presence of various electron acceptors had the following order: sulfate (0.08-0.10/d) > sodium bicarbonate (0.07-0.09/d) > ferric oxide (0.04-0.06/d) > nitrate (0.03-0.05/d) > manganese oxide (0.01-0.04/d).

Construction of microbial consortia for microbial degradation of complex compounds

Increasingly complex synthetic environmental pollutants are prompting further research into bioremediation, which is one of the most economical and safest means of environmental restoration. From the current research, using microbial consortia to degrade complex compounds is more advantageous compared to using isolated bacteria, as the former is more adaptable and stable within the growth environment and can provide a suitable catalytic environment for each enzyme required by the biodegradation pathway. With the development of synthetic biology and gene-editing tools, artificial microbial consortia systems can be designed to be more efficient, stable, and robust, and they can be used to produce high-value-added products with their strong degradation ability. Furthermore, microbial consortia systems are shown to be promising in the degradation of complex compounds. In this review, the strategies for constructing stable and robust microbial consortia are discussed. The current advances in the degradation of complex compounds by microbial consortia are also classified and detailed, including plastics, petroleum, antibiotics, azo dyes, and some pollutants present in sewage. Thus, this paper aims to support some helps to those who focus on the degradation of complex compounds by microbial consortia.

A metagenomic portrait of the microbial community responsible for two decades of bioremediation of poly-contaminated groundwater

Biodegradation of pollutants is a sustainable and cost-effective solution to groundwater pollution. Here, we investigate microbial populations involved in biodegradation of poly-contaminants in a pipeline for heavily contaminated groundwater. Groundwater moves from a polluted park to a treatment plant, where an aerated bioreactor effectively removes the contaminants. While the biomass does not settle in the reactor, sediment is collected afterwards and used to seed the new polluted groundwater via a backwash cycle. The pipeline has successfully operated since 1999, but the biological components in the reactor and the contaminated park groundwater have never been described. We sampled seven points along the pipeline, representing the entire remediation process, and characterized the changing microbial communities using genome-resolved metagenomic analysis. We assembled 297 medium- and high-quality metagenome-assembled genome sequences representing on average 46.3% of the total DNA per sample. We found that the communities cluster into two distinct groups, separating the anaerobic communities in the park groundwater from the aerobic communities inside the plant. In the park, the community is dominated by members of the genus Sulfuricurvum, while the plant is dominated by generalists from the order Burkholderiales. Known aromatic compound biodegradation pathways are four times more abundant in the plant-side communities compared to the park-side. Our findings provide a genome-resolved portrait of the microbial community in a highly effective groundwater treatment system that has treated groundwater with a complex contamination profile for two decades.

Willow can be recommended as a strong candidate for the phytoremediation of cadmium and pyrene co-polluted soil under flooding condition

Soil cadmium (Cd) and pyrene (PYR) pollutions have gained worldwide attention due to their negative effects on the environment. Intermittent flooding in rain-rich areas may affect phytoremediation of Cd and PYR in soil. Therefore, a pot-culture experiment, with and without flooding, was conducted to study the effects of flooding on soil Cd and PYR phytoremediation. Concentrations of Cd, PYR, and nutrients in soils and plants, as well as plant physiological and biochemical responses, were examined. Under both flooding and non-flooding conditions, willow (Salix × aureo-pendula CL 'J1011') demonstrated a better ability to remove soil Cd and PYR. Flooding led to higher Cd accumulation in roots than that in shoots. Conversely, non-flooding resulted in higher Cd accumulation in shoots than that in roots. The maximum concentrations of Cd in shoots were 11.02 and 14.07 mg kg^-1 with and without flooding, respectively. The maximum dissipation rates of PYR in soil were 47.35% and 88.61% under flooding and non-flooding conditions, respectively. In addition, flooding significantly increased the photosynthetic pigment, photosynthetic fluorescence, and chlorophyll fluorescence parameters in leaves, compared with non-flooding treatment. Flooding also increased the concentrations of Mg, Mn, P, Fe, and K in roots and shoots. This study outlines an effective insight for the phytoremediation of Cd- and PYR-contaminated soil under flooding condition.

Construction and Degradation Performance Study of Polycyclic Aromatic Hydrocarbons (PAHs) Degrading Bacterium Consortium

PAHs are widely distributed in the environment and pose a serious threat to ecological security and human health. The P&A (Pseudomonas aeruginosa and Alcaligenes faecalis) bacterium consortium obtained in this study comes from oily sludge and is reused for the degradation of PAHs mixture in oily sludge. Few articles pay attention to the PAHs mixture in oily sludge and reuse the bacterium consortium for its degradation. The PAHs solution degradation efficient of P&A bacterial consortium under different environmental conditions, bioaugmentations, and exogenous stimulations were studied by ultraviolet visible spectrophotometer and gas chromatography–mass spectrometry. The result shows that, after 8 days of degradation under 35 °C, pH 7, with 5% (volume percent) of the inoculation amount, the degradation rate of NAP, PHE, and PYR solution could higher than 90%, 80%, and 70%, respectively. The additional crude oil could further improve the NAP, PHE, and PYR degradation efficiency. The minimum inhibitory concentration of Cu2+, Zn2+, and Pb2+ to bacterium were 2.002, 17.388, and 9.435 mM, respectively. The addition of surfactants had negative or limited positive effect on the PAHs degradation rate. Furthermore, the average degradation rates of NAP, PHE, and PYR, in oily sludge of local petroleum polluted area by P&A bacterial consortium, could all reach above 80%. Based on gas chromatography–mass spectrometry test results before and after incubation, P&A bacterial consortium also provides more opportunities for other organic compounds’ degradation.

Bioremediation of Diesel Contaminated Marine Water by Bacteria: A Review and Bibliometric Analysis

Oil pollution can cause tremendous harm and risk to the water ecosystem and organisms due to the relatively recalcitrant hydrocarbon compounds. The current chemical method used to treat the ecosystem polluted with diesel is incompetent and expensive for a large-scale treatment. Thus, bioremediation technique seems urgent and requires more attention to solve the existing environmental problems. Biological agents, including microorganisms, carry out the biodegradation process where organic pollutants are mineralized into water, carbon dioxide, and less toxic compounds. Hydrocarbon-degrading bacteria are ubiquitous in the nature and often exploited for their specialty to bioremediate the oil-polluted area. The capability of these bacteria to utilize hydrocarbon compounds as a carbon source is the main reason behind their species exploitation. Recently, microbial remediation by halophilic bacteria has received many positive feedbacks as an efficient pollutant degrader. These halophilic bacteria are also considered as suitable candidates for bioremediation in hypersaline environments. However, only a few microbial species have been isolated with limited available information on the biodegradation of organic pollutants by halophilic bacteria. The fundamental aspect for successful bioremediation includes selecting appropriate microbes with a high capability of pollutant degradation. Therefore, high salinity bacteria are remarkable microbes for diesel degradation. This paper provides an updated overview of diesel hydrocarbon degradation, the effects of oil spills on the environment and living organisms, and the potential role of high salinity bacteria to decontaminate the organic pollutants in the water environment.

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.

Temporal Evolution of PAHs Bioaccessibility in an Aged-Contaminated Soil during the Growth of Two Fabaceae

Polycyclic aromatic hydrocarbons (PAHs) are health-concerning organic compounds that accumulate in the environment. Bioremediation and phytoremediation are studied to develop eco-friendly remediation techniques. In this study, the effects of two plants (Medicago sativa L. and Trifoliumpratense L.) on the PAHs’ bioaccessibility in an aged-contaminated soil throughout a long-term rhizoremediation trial was investigated. A bioaccessibility measurement protocol, using Tenax^® beads, was adapted to the studied soil. The aged-contaminated soil was cultured with each plant type and compared to unplanted soil. The bioaccessible and residual PAH contents were quantified after 3, 6 and 12 months. The PAHs’ desorption kinetics were established for 15 PAHs and described by a site distribution model. A common Tenax^® extraction time (24 h) was established as a comparison basis for PAHs bioaccessibility. The rhizoremediation results show that M. sativa developed better than T. pratense on the contaminated soil. When plants were absent (control) or small (T. pratense), the global PAHs’ residual contents dissipated from the rhizosphere to 8% and 10% of the total initial content, respectively. However, in the presence of M. sativa, dissipation after 12 months was only 50% of the total initial content. Finally, the PAHs’ bioaccessible content increased more significantly in the absence of plants. This one-year trial brought no evidence that the presence of M. sativa or T. pratense on this tested aged-contaminated soil was beneficial in the PAH remediation process, compared to unplanted soil.