New Frontiers of Anaerobic Hydrocarbon Biodegradation in the Multi-Omics Era

Microbe-plant-nanoparticle interactions: role in bioremediation of petroleum hydrocarbons

Petroleum hydrocarbons (PHCs) are organic substances that occur naturally on earth. PHCs have emerged as one of the most prevalent and detrimental contaminants in regions comprising soil and water resources. The limitations of conventional physicochemical and biological remediation solutions could be solved by combining remediation techniques. An effective, affordable, and environmentally benign method of reducing petroleum toxins is provided by the advanced idea of bioremediation, which has evolved into nanobioremediation. Environments contaminated with PHCs have been restored through microbe-plant-nanoparticle (NP)-mediated remediation, this review emphasizes how various metallic NPs interact with microbes and plants changing both their activity and that of enzymes, therefore accelerating the remediation process. This work further examines the challenges and possible uses of nanobioremediation, as well as the application of novel technologies in the interactions between bacteria, plants, and NPs for the bioremediation of PHCs. Furthermore, it has been shown that the use of plant-based, microbe-based, microbe-plant-based, and microbe-plant-NP-based techniques to remediate contaminated soils or water bodies is economical and environmentally beneficial. Microbial consortia have been reported as the treasure houses for the cleaning and recovery of hydrocarbon-contaminated environments, and the development of technologies for bioremediation requires an understanding of hydrocarbon degradation mechanisms.

Methanogenesis coupled hydrocarbon biodegradation enhanced by ferric and sulphate ions

Bioremediation provides an environmentally sound solution for hydrocarbon removal. Although bioremediation under anoxic conditions is slow, it can be coupled with methanogenesis and is suitable for energy recovery. By altering conditions and supplementing alternative terminal electron acceptors to the system to induce syntrophic partners of the methanogens, this process can be enhanced. In this study, we investigated a hydrocarbon-degrading microbial community derived from chronically contaminated soil. Various hydrocarbon mixtures were used during our experiments in the presence of different electron acceptors. In addition, we performed whole metagenome sequencing to identify the main actors of hydrocarbon biodegradation in the samples. Our results showed that the addition of ferric ions or sulphate increased the methane yield. Furthermore, the addition of CO2, ferric ion or sulphate enhanced the biodegradation of alkanes. A significant increase in biodegradation was observed in the presence of ferric ions or sulphate in the case of all aromatic components, while naphthalene and phenanthrene degradation was also enhanced by CO2. Metagenome analysis revealed that Cellulomonas sp. is the most abundant in the presence of alkanes, while Ruminococcus and Faecalibacterium spp. are prevalent in aromatics-supplemented samples. From the recovery of 25 genomes, it was concluded that the main pathway of hydrocarbon activation was fumarate addition in both Cellulomonas, Ruminococcus and Faecalibacterium. Chloroflexota bacteria can utilise the central metabolites of aromatics biodegradation via ATP-independent benzoyl-CoA reduction. KEY POINTS: • Methanogenesis and hydrocarbon biodegradation were enhanced by Fe3+ or SO42- • Cellulomonas, Ruminococcus and Faecalibacterium can be candidates for the main hydrocarbon degraders • Chloroflexota bacteria can utilise the central metabolites of aromatics degradation.

Diversity of hydrocarbon-degrading bacteria in mangroves rhizosphere as an indicator of oil-pollution bioremediation in mangrove forests

Mangrove ecosystems, characterized by high levels of productivity, are susceptible to anthropogenic activities, notably oil pollution arising from diverse origins including spills, transportation, and industrial effluents. Owing to their role in climate regulation and economic significance, there is a growing interest in developing mangrove conservation strategies. In the Arabian Gulf, mangroves stand as the sole naturally occurring green vegetation due to the region's hot and arid climate. However, they have faced persistent oil pollution for decades. This review focuses on global mangrove distribution, with a specific emphasis on Qatar's mangroves. It highlights the ongoing challenges faced by mangroves, particularly in relation to the oil industry, and the impact of oil pollution on these vital ecosystems. It outlines major oil spill incidents worldwide and the diverse hydrocarbon-degrading bacterial communities within polluted areas, elucidating their potential for bioremediation. The use of symbiotic interactions between mangrove plants and bacteria offers a more sustainable, cost-effective and environmentally friendly alternative. However, the success of these bioremediation strategies depends on a deep understanding of the dynamics of bacterial communities, environmental factors and specific nature of the pollutants.

Microbial degradation of the benzimidazole fungicide carbendazim by Bacillus velezensis HY-3479

Carbendazim (methyl benzimidazol-2-ylcarbamate: MBC) is a fungicide of the benzimidazole group that is widely used in the cultivation of pepper, ginseng, and many other crops. To remove the remnant carbendazim, many rhizobacteria are used as biodegradation agents. A bacterial strain of soil-isolated Bacillus velezensis HY-3479 was found to be capable of degrading MBC in M9 minimal medium supplemented with 250 mg/L carbendazim. The strain had a significantly higher degradation efficiency compared to the control strain Bacillus subtilis KACC 15590 in high-performance liquid chromatography (HPLC) analysis, and HY-3479 had the best degradation efficiency of 76.99% at 48 h. In gene expression analysis, upregulation of carbendazim-degrading genes (mheI, hdx) was observed in the strain. HY-3479 was able to use MBC as the sole source of carbon and nitrogen, but the addition of 12.5 mM NH4NO3 significantly increased the degradation efficiency. HPLC analysis showed that the degradation efficiency increased to 87.19% when NH4NO3 was added. Relative gene expression of mheI and hdx also increased for samples with NH4NO3 supplementation. The enzyme activity of the carbendazim-degrading enzyme and the 2-aminobenzimidazole-degrading enzyme was found to be highly present in the HY-3479 strain. It is the first reported B. velezensis strain to biodegrade carbendazim (MBC). The biodegradation activity of strain HY-3479 may be developed as a useful means for bioremediation and used as a potential microbial agent in sustainable agriculture.

Microbial community structure and metabolic profile of anthropized freshwater tributary channels from La Plata River, Argentina, to develop sustainable remediation strategies

Microbial communities from freshwater sediments are involved in biogeochemical cycles and they can be modified by physical and chemical changes in the environment. Linking the microbial community structure (MCS) with physicochemistry of freshwater courses allows a better understanding of its ecology and can be useful to assess the ecological impact generated by human activity. The MCS of tributary channels from La Plata River affected by oil refinery (C, D, and E) and one also by urban discharges (C) was studied. For this purpose, 16S rRNA metabarcoding analysis, in silico metagenome functional prediction, and the hydrocarbon degradation potential (in silico predictions of hydrocarbon-degrading genes and their quantification by qPCR) of the MCS were studied. Principal coordinate analysis revealed that the MCS was different between sites, and it was not structured by the hydrocarbon content. Site C showed physicochemical characteristics, bacterial taxa, and an in silico functional prediction related to fermentative/heterotrophic metabolism. Site D, despite having higher concentration of hydrocarbon, presented autotrophic, syntrophic, and methanogenic pathways commonly involved in natural processes in anoxic sediments. Site E showed and intermediate autotrophic/heterotrophic behavior. The hydrocarbon degradation potential showed no positive correlation between the hydrocarbon-degrading genes quantified and predicted. The results suggest that the hydrocarbon concentration in the sites was not enough selection pressure to structure the bacterial community composition. Understanding which is the variable that structures the bacterial community composition is essential for monitoring and designing of sustainable management strategies for contaminated freshwater ecosystems.

The bioremediation of the typical persistent organic pollutants (POPs) by microalgae-bacteria consortia: A systematic review

With industrialisation and the rapidly growing agricultural demand, many organic compounds have been leaked into the environment, causing serious damage to the biosphere. Persistent organic pollutants (POPs) are a type of toxic chemicals that are resistant to degradation through normal chemical, biological or photolytic approaches. With their stable chemical structures, POPs can be accumulated in the environment, and transported through wind and water, causing global environmental issues. Many researches have been conducted to remediate POPs contamination using various kinds of biological methods, and significant results have been seen. Microalgae-bacteria consortium is a newly developed concept for biological technology in contamination treatment, with the synergetic effects between microalgae and bacteria, their potential for pollutants degradation can be further released. In this review, two types of POPs (polychlorinated biphenyls and polycyclic aromatic hydrocarbons) are selected as the targeted pollutants to give a systematic analysis of the biodegradation through microalgae and bacteria, including the species selection, the identification of dominant enzymes, as well as the real application performance of the consortia. In the end, some outlooks and suggestions are given to further guide the development of applying microalgae-bacteria consortia in remediating POPs contamination. In general, the coculturing of microalgae and bacteria is a novel and efficient way to fulfil the advanced treatment of POPs in soil or liquid phase, and both monooxygenase and dioxygenase belonging to oxygenase play a vital role in the biodegradation of PCBs and PAHs. This review provides a general guide in the future investigation of biological treatment of POPs.

Progress of Crude Oil Gasification Technology Assisted by Microorganisms in Reservoirs

Crude oil gasification bacteria, including fermenting bacteria, hydrocarbon-oxidizing bacteria, reducing bacteria, and methanogenic bacteria, participate in multi-step reactions involving initial activation, intermediate metabolism, and the methanogenesis of crude oil hydrocarbons. These bacteria degrade crude oil into smaller molecules such as hydrogen, carbon dioxide, acetic acid, and formic acid. Ultimately, they convert it into methane, which can be utilized or stored as a strategic resource. However, the current challenges in crude oil gasification include long production cycles and low efficiency. This paper provides a summary of the microbial flora involved in crude oil gasification, the gasification metabolism pathways within reservoirs, and other relevant information. It specifically focuses on analyzing the factors that affect the efficiency of crude oil gasification metabolism and proposes suggestions for improving this efficiency. These studies deepen our understanding of the potential of reservoir ecosystems and provide valuable insights for future reservoir development and management.

Bacterial Diversity in Old Hydrocarbon Polluted Sediments of Ecuadorian Amazon River Basins

The Ecuadorian Amazon rainforest stands out as one of the world's most biodiverse regions, yet faces significant threats due to oil extraction activities dating back to the 1970s in the northeastern provinces. This research investigates the environmental and societal consequences of prolonged petroleum exploitation and oil spills in Ecuador's Amazon. Conducted in June 2015, the study involved a comprehensive analysis of freshwater sediment samples from 24 locations in the Rio Aguarico and Napo basins. Parameters such as water and air temperature, conductivity, soil pH, and hydrocarbon concentrations were examined. Total petroleum hydrocarbon (TPH) concentrations ranged from 9.4 to 847.4 mg kg-1, with polycyclic aromatic hydrocarbon (PAH) levels varying from 10.15 to 711.1 mg kg-1. The pristane/phytane ratio indicated historic hydrocarbon pollution in 8 of the 15 chemically analyzed sediments. Using non-culturable techniques (Illumina), bacterial analyses identified over 350 ASV, with prominent families including Comamonadaceae, Chitinophagaceae, Anaeromyxobacteraceae, Sphingomonadaceae, and Xanthobacteraceae. Bacterial diversity, assessed in eight samples, exhibited a positive correlation with PAH concentrations. The study provides insights into how microbial communities respond to varying levels of hydrocarbon pollution, shedding light on the enduring impact of oil exploitation in the Amazonian region. Its objective is to deepen our understanding of the environmental and human well-being in the affected area, underscoring the pressing need for remedial actions in the face of ongoing ecological challenges.

Soils in distress: The impacts and ecological risks of (micro)plastic pollution in the terrestrial environment

Plastics have revolutionised human industries, thanks to their versatility and durability. However, their extensive use, coupled with inadequate waste disposal, has resulted in plastic becoming ubiquitous in every environmental compartment, posing potential risks to the economy, human health and the environment. Additionally, under natural conditions, plastic waste breaks down into microplastics (MPs<5 mm). The increasing quantity of MPs exerts a significant burden on the soil environment, particularly in agroecosystems, presenting a new stressor for soil-dwelling organisms. In this review, we delve into the effects of MP pollution on soil ecosystems, with a specific attention to (a) MP transport to soils, (b) potential changes of MPs under environmental conditions, (c) and their interaction with the physical, chemical and biological components of the soil. We aim to shed light on the alterations in the distribution, activity, physiology and growth of soil flora, fauna and microorganisms in response to MPs, offering an ecotoxicological perspective for environmental risk assessment of plastics. The effects of MPs are strongly influenced by their intrinsic traits, including polymer type, shape, size and abundance. By exploring the multifaceted interactions between MPs and the soil environment, we provide critical insights into the consequences of plastic contamination. Despite the growing body of research, there remain substantial knowledge gaps regarding the long-term impact of MPs on the soil. Our work underscores the importance of continued research efforts and the adoption of standardised approaches to address plastic pollution and ensure a sustainable future for our planet.

Microbial diversity and authigenic siderite mediation in sediments surrounding the Kedr-1 mud volcano, Lake Baikal

The gas hydrate-bearing structure-mud volcano Kedr-1 (Lake Baikal, southern basin)-is located near the coal-bearing sediments of the Tankhoy formation of Oligocene-Miocene age and can be an ideal source of gas-saturated fluid. A significant amount of siderite minerals (FeCO3 ) were collected from sediments at depths ranging from 0.5 to 327 cm below the lake floor (cmblf). An important feature of these carbonate minerals is the extremely strong enrichment in the heavy 13 C isotope, reaching values of +33.3‰ VPDB. The δ13 C of the siderite minerals, as well as their morphology and elemental composition, and the δ13 CDIC of the co-existing pore water, differed across layers of the core, which implies at least two generations of siderite formation. Here, we leverage mineralogical and geochemical data with 16S rRNA data from the microbial communities in sediments surrounding layers containing siderite minerals. Statistical data reveal the formation of three clusters of microbial communities based on taxonomical composition, key taxa among bacteria and archaea, and environmental parameters. Diversity and richness estimators decrease with sediment depth, with several similar prevailing clades located at the bottom of the core. Most of the taxa in the deep sediments could be associated with putative metabolisms involving organotrophic fermentation (Bathyarchaeia, Caldatribacteriota, and Chloroflexota). Various groups of methanogens (Methanoregulaceae, Methanosaetaceae, and Methanomassiliicoccales) and methanotrophic (Methanoperedenaceae) archaea are present in the sediment at variable relative abundances throughout the sampled depth. Based on the physicochemical characteristics of the sediment, carbon isotope analysis of carbonate minerals and DIC, and phylogenetic analysis of individual taxa and their metabolic potential, we present several models for subsurface siderite precipitation in Lake Baikal sediments.

Microbially mediated fossil concretions and their characterization by the latest methodologies: a review

The study of well-preserved organic matter (OM) within mineral concretions has provided key insights into depositional and environmental conditions in deep time. Concretions of varied compositions, including carbonate, phosphate, and iron-based minerals, have been found to host exceptionally preserved fossils. Organic geochemical characterization of concretion-encapsulated OM promises valuable new information of fossil preservation, paleoenvironments, and even direct taxonomic information to further illuminate the evolutionary dynamics of our planet and its biota. Full exploitation of this largely untapped geochemical archive, however, requires a sophisticated understanding of the prevalence, formation controls and OM sequestration properties of mineral concretions. Past research has led to the proposal of different models of concretion formation and OM preservation. Nevertheless, the formation mechanisms and controls on OM preservation in concretions remain poorly understood. Here we provide a detailed review of the main types of concretions and formation pathways with a focus on the role of microbes and their metabolic activities. In addition, we provide a comprehensive account of organic geochemical, and complimentary inorganic geochemical, morphological, microbial and paleontological, analytical methods, including recent advancements, relevant to the characterization of concretions and sequestered OM. The application and outcome of several early organic geochemical studies of concretion-impregnated OM are included to demonstrate how this underexploited geo-biological record can provide new insights into the Earth's evolutionary record. This paper also attempts to shed light on the current status of this research and major challenges that lie ahead in the further application of geo-paleo-microbial and organic geochemical research of concretions and their host fossils. Recent efforts to bridge the knowledge and communication gaps in this multidisciplinary research area are also discussed, with particular emphasis on research with significance for interpreting the molecular record in extraordinarily preserved fossils.

Role of Candida in the bioremediation of pollutants: a review

The population and modernization of society have increased dramatically from past few decades. In order to meet societal expectations, there has been a massive industrialization and resource exploitation. Anthropogenic practices like disposal of hazardous waste, large carbon footprint release variety of xenobiotic substances into the environment, which endanger the health of the natural ecosystem. Therefore, discovering proper long-term treatment approaches is a global concern. Various physical and chemical approaches are employed to remove contaminants. However, these technologies possess limitations like high cost and low efficacy. Consequently, bioremediation is regarded as one of the most promising remedies to these problems. It creates the option of either totally removing pollutants or transforming them into nonhazardous compounds with the use of natural biological agents. Several microorganisms are being utilized for bioremediation among which yeasts possess benefits such as high biodegradability, ease of cultivation etc. The yeast of Candida genus has the capability to effectively eliminate heavy metal ions, as well as to degrade and emulsify hydrocarbons which makes it a promising candidate for this purpose. The review highlights many potential uses of Candida in various remediation strategies and discusses future directions for research in this field.

Microaerobic degradation of crude oil and long chain alkanes by a new Rhodococcus strain from Gulf of Mexico

Bacterial degradation of crude oil is a promising strategy for reducing the concentration of hydrocarbons in contaminated environments. In the first part of this study, we report the enrichment of two bacterial consortia from deep sediments of the Gulf of Mexico with crude oil as the sole carbon and energy source. We conducted a comparative analysis of the bacterial community in the original sediment, assessing its diversity, and compared it to the enrichment observed after exposure to crude oil in defined cultures. The consortium exhibiting the highest hydrocarbon degradation was predominantly enriched with Rhodococcus (75%). Bacterial community analysis revealed the presence of other hydrocarbonoclastic members in both consortia. In the second part, we report the isolation of the strain Rhodococcus sp. GOMB7 with crude oil as a unique carbon source under microaerobic conditions and its characterization. This strain demonstrated the ability to degrade long-chain alkanes, including eicosane, tetracosane, and octacosane. We named this new strain Rhodococcus qingshengii GOMB7. Genome analysis revealed the presence of several genes related to aromatic compound degradation, such as benA, benB, benC, catA, catB, and catC; and five alkB genes related to alkane degradation. Although members of the genus Rhodococcus are well known for their great metabolic versatility, including the aerobic degradation of recalcitrant organic compounds such as petroleum hydrocarbons, this is the first report of a novel strain of Rhodococcus capable of degrading long-chain alkanes under microaerobic conditions. The potential of R. qingshengii GOMB7 for applications in bioreactors or controlled systems with low oxygen levels offers an energy-efficient approach for treating crude oil-contaminated water and sediments.

Bacterial enzymatic degradation of recalcitrant organic pollutants: catabolic pathways and genetic regulations

Contamination of soil and natural water bodies driven by increased organic pollutants remains a universal concern. Naturally, organic pollutants contain carcinogenic and toxic properties threatening all known life forms. The conventional physical and chemical methods employed to remove these organic pollutants ironically produce toxic and non-ecofriendly end-products. Whereas microbial-based degradation of organic pollutants provides an edge, they are usually cost-effective and take an eco-friendly approach towards remediation. Bacterial species, including Pseudomonas, Comamonas, Burkholderia, and Xanthomonas, have the unique genetic makeup to metabolically degrade toxic pollutants, conferring their survival in toxic environments. Several catabolic genes, such as alkB, xylE, catA, and nahAc, that encode enzymes and allow bacteria to degrade organic pollutants have been identified, characterized, and even engineered for better efficacy. Aerobic and anaerobic processes are followed by bacteria to metabolize aliphatic saturated and unsaturated hydrocarbons such as alkanes, cycloalkanes, aldehydes, and ethers. Bacteria use a variety of degrading pathways, including catechol, protocatechuate, gentisate, benzoate, and biphenyl, to remove aromatic organic contaminants such as polychlorinated biphenyls, polycyclic aromatic hydrocarbons, and pesticides from the environment. A better understanding of the principle, mechanisms, and genetics would be beneficial for improving the metabolic efficacy of bacteria to such ends. With a focus on comprehending the mechanisms involved in various catabolic pathways and the genetics of the biotransformation of these xenobiotic compounds, the present review offers insight into the various sources and types of known organic pollutants and their toxic effects on health and the environment.

Development of an in vitro method for activation of X-succinate synthases for fumarate hydroalkylation

Anaerobic microbial degradation of hydrocarbons is often initiated through addition of the hydrocarbon to fumarate by enzymes known as X-succinate synthases (XSSs). XSSs use a glycyl radical cofactor, which is installed by an activating enzyme (XSS-AE), to catalyze this carbon-carbon coupling reaction. The activation step, although crucial for catalysis, has not previously been possible in vitro because of insolubility of XSS-AEs. Here, we take a genome mining approach to find an XSS-AE, a 4-isopropylbenzylsuccinate synthase (IBSS)-AE (IbsAE) that can be solubly expressed in Escherichia coli. This soluble XSS-AE can activate both IBSS and the well-studied benzylsuccinate synthase (BSS) in vitro, allowing us to explore XSSs biochemically. To start, we examine the role of BSS subunits and find that the beta subunit accelerates the rate of hydrocarbon addition. Looking forward, the methodology and insight gathered here can be used more broadly to understand and engineer XSSs as synthetically useful biocatalysts.

Bioengineering for the Microbial Degradation of Petroleum Hydrocarbon Contaminants

Petroleum hydrocarbons are relatively recalcitrant compounds, and as contaminants, they are one of the most serious environmental problems. n-Alkanes are important constituents of petroleum hydrocarbons. Advances in synthetic biology and metabolic engineering strategies have made n-alkane biodegradation more designable and maneuverable for solving environmental pollution problems. In the microbial degradation of n-alkanes, more and more degradation pathways, related genes, microbes, and alkane hydroxylases have been discovered, which provide a theoretical basis for the further construction of degrading strains and microbial communities. In this review, the current advances in the microbial degradation of n-alkanes under aerobic condition are summarized in four aspects, including the biodegradation pathways and related genes, alkane hydroxylases, engineered microbial chassis, and microbial community. Especially, the microbial communities of “Alkane-degrader and Alkane-degrader” and “Alkane-degrader and Helper” provide new ideas for the degradation of petroleum hydrocarbons. Surfactant producers and nitrogen providers as a “Helper” are discussed in depth. This review will be helpful to further achieve bioremediation of oil-polluted environments rapidly.

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.

Molecular response of Anoxybacillus sp. PDR2 under azo dye stress: An integrated analysis of proteomics and metabolomics

Treating azo dye wastewater using thermophilic bacteria is considered a more efficient bioremediation strategy. In this study, a thermophilic bacterial strain, Anoxybacillus sp. PDR2, was regarded as the research target. This strain was characterized at different stages of azo dye degradation by using TMT quantitative proteomic and non-targeted metabolome technology. A total of 165 differentially expressed proteins (DEPs) and 439 differentially metabolites (DMs) were detected in comparisons between bacteria with and without azo dye. It was found that Anoxybacillus sp. PDR2 can degrade azo dye Direct Black G (DBG) through extracellular electron transfer with glucose serving as electron donors. Most proteins related to carbohydrate metabolism, including acetoacetate synthase, and malate synthase G, were overexpressed to provide energy. The bacterium can also self-synthesize riboflavin as a redox mediator of in vitro electron transport. These results lay a theoretical basis for industrial bioremediation of azo dye wastewater.

Evolution in mitigation approaches for petroleum oil-polluted environment: recent advances and future directions

Increasing petroleum consumption and a rise in incidental oil spillages have become global concerns owing to their aquatic and terrestrial toxicity. Various physicochemical and biological treatment strategies have been studied to tackle them and their impact on environment. One of such approaches in this regard is the use of microbial processes due to their being "green" and also apparent low cost and high effectiveness. This review presents the advancement in the physical and biological remediation methods and their progressive efficacy if employed in combination of hybrid modes. The use of biosurfactants and/or biochar along with microbes seems to be a more effective bioremediation approach as compared to their individual effects. The lacuna in research at community or molecular level has been overcome by the recent introduction of "-omics" technology in hydrocarbon degradation. Thus, the review further focuses on presenting the state-of-art information on the advancement of petroleum bioremediation strategies and identifies the research gaps for achieving total mitigation of petroleum oil.

Sustainable strategies for combating hydrocarbon pollution: Special emphasis on mobil oil bioremediation

The global rise in industrialization and vehicularization has led to the increasing trend in the use of different crude oil types. Among these mobil oil has major application in automobiles and different machines. The combustion of mobil oil renders a non-usable form that ultimately enters the environment thereby causing problems to environmental health. The aliphatic and aromatic hydrocarbon fraction of mobil oil has serious human and environmental health hazards. These components upon interaction with soil affect its fertility and microbial diversity. The recent advancement in the omics approach viz. metagenomics, metatranscriptomics and metaproteomics has led to increased efficiency for the use of microbial based remediation strategy. Additionally, the use of biosurfactants further aids in increasing the bioavailability and thus biodegradation of crude oil constituents. The combination of more than one approach could serve as an effective tool for efficient reduction of oil contamination from diverse ecosystems. To the best of our knowledge only a few publications on mobil oil have been published in the last decade. This systematic review could be extremely useful in designing a micro-bioremediation strategy for aquatic and terrestrial ecosystems contaminated with mobil oil or petroleum hydrocarbons that is both efficient and feasible. The state-of-art information and future research directions have been discussed to address the issue efficiently.

Anaerobic oxidation of petroleum hydrocarbons in enrichment cultures from sediments of the Gorevoy Utes natural oil seep under methanogenic and sulfate-reducing conditions

This article presents the first experimental data on the ability of microbial communities from sediments of the Gorevoy Utes natural oil seep to degrade petroleum hydrocarbons under anaerobic conditions. Like in marine ecosystems associated with oil discharge, available electron acceptors, in particular sulfate ions, affect the composition of the microbial community and the degree of hydrocarbon conversion. The cultivation of the surface sediments under sulfate-reducing conditions led to the formation of a more diverse bacterial community and greater loss of n-alkanes (28%) in comparison to methanogenic conditions (6%). Microbial communities of both surface and deep sediments are more oriented to degrade polycyclic aromatic hydrocarbons (PAHs), to which the degree of the PAH conversion testifies (up to 46%) irrespective of the present electron acceptors. Microorganisms with the uncultured closest homologues from thermal habitats, sediments of mud volcanoes, and environments contaminated with hydrocarbons mainly represented microbial communities of enrichment cultures. The members of the phyla Firmicutes, Chloroflexi, and Caldiserica (OP5), as well as the class Deltaproteobacteria and Methanomicrobia, were mostly found in enrichment cultures. The influence of gas-saturated fluids may be responsible for the presence in the bacterial 16S rRNA gene libraries of the sequences of "rare taxa": Planctomycetes, Ca. Atribacteria (OP9), Ca. Armatimonadetes (OP10), Ca. Latescibacteria (WS3), Ca. division (AC1), Ca. division (OP11), and Ca. Parcubacteria (OD1), which can be involved in hydrocarbon oxidation.

Bacterial community profile of the crude oil-contaminated saline soil in the Yellow River Delta Natural Reserve, China

Crude oil contamination greatly influence soil bacterial community. Proliferative microbes in the crude oil-contaminated soil are closely related to the living conditions. Oil wells in the Yellow River Delta Natural Reserve (YRDNR) region is an ideal site for investigating the bacterial community of crude oil-contaminated saline soil. In the present study, 18 soil samples were collected from the depths of 0-20 cm and 20-40 cm around the oil wells in the YRDNR. The bacterial community profile was analyzed through high-throughput sequencing to trace the oil-degrading aerobic and anaerobic bacteria. The results indicated that C₁₅-C₂₈ and C₂₉-C₃₈ were the main fractions of total petroleum hydrocarbon (TPH) in the sampled soil. These TPH fractions had a significant negative effect on bacterial biodiversity (Shannon, Simpson, and Chao1 indices), which led to the proliferation of hydrocarbon-degrading bacteria. A comprehensive analysis between the environmental factors and soil microbial community structure showed that Streptococcus, Bacillus, Sphingomonas, and Arthrobacter were the aerobic hydrocarbon-degrading bacteria; unidentified Rhodobacteraceae and Porticoccus were considered to be the possible facultative anaerobic bacteria with hydrocarbon biodegradation ability; Acidithiobacillus, SAR324 clade, and Nitrosarchaeum were predicted to be the anaerobic hydrocarbon-degrading bacteria in the sub-surface soil. Furthermore, large amount of carbon sources derived from TPH was found to cause depletion of bioavailable nitrogen in the soil. The bacteria associated with nitrogen transformation, such as Solirubrobacter, Candidatus Udaeobacter, Lysinibacillus, Bradyrhizobium, Sphingomonas, Mycobacterium, and Acidithiobacillus, were highly abundant; these bacteria may possess the ability to increase nitrogen availability in the crude oil-contaminated soil. The bacterial community functions were significantly different between the surface and the sub-surface soil, and the dissolved oxygen concentration in soil was considered to be potential influencing factor. Our results could provide useful information for the bioremediation of crude oil-contaminated saline soil.

Bacterial diversity in petroleum coke based biofilters treating oil sands process water

Adopting nature-based solutions for the bioremediation of oil sands process water (OSPW) is of significant interest, which requires a thorough understanding of how bacterial communities behave within treatment systems operated under natural conditions. This study investigates the OSPW remediation potential of delayed petroleum-coke (PC), which is a byproduct of bitumen upgrading process and is readily available at oil refining sites, in fixed-bed biofilters particularly for the degradation of naphthenic acids (NAs) and aromatics. The biofilters were operated continuously and total and active bacterial communities were studied by DNA and RNA-based amplicon sequencing in a metataxonomic fashion to extrapolate the underlying degradation mechanisms. The results of total community structure indicated a high abundance of aerobic bacteria at all depths of the biofilter, e.g., Porphyrobacter, Legionella, Pseudomonas, Planctomyces. However, redox conditions within the biofilters were anoxic (-153 to -182 mV) that selected anaerobic bacteria to actively participate in the remediation of OSPW, i.e., Ruminicoccus, Eubacterium, Faecalibacterium, Dorea. After 15 days of operation, the removal of classical NAs was recorded up to 20% whereas oxidized NAs species were poorly removed, i.e., O₃-NAs: 4.8%, O₄-NAs: 1.2%, O₅-NAs: 1.7%, and O₆-NAs: 0.5%. Accordingly, monoaromatics, diaromatics, and triaromatics were removed up to 16%, 22%, and 15%, respectively. The physiology of the identified genera suggested that the degradation in the PC-based biofilters was most likely proceeded in a scheme similar to beta-oxidation during anaerobic digestion process. The presence of hydrogenotrophic methanogens namely Methanobrevibacter and Methanomassiliicoccus and quantification of mcrA gene (2.4 × 10² to 8.7 × 10² copies/mg of PC) revealed that methane production was likely occurring in a syntrophic mechanism during the OSPW remediation. A slight reduction in toxicity was also observed. This study suggests that PC-based biofilters may offer some advantages in the remediation of OSPW; however, the production of methane could be of future concerns if operated at field-scale.

A review on microbial diversity and genetic markers involved in methanogenic degradation of hydrocarbons: futuristic prospects of biofuel recovery from contaminated regions

Microbial activities within oil reservoirs have adversely impacted the world's majority of oil by lowering its quality, thereby increasing its recovery and refining cost. Moreover, conventional method of extraction leaves behind nearly two-thirds of the fossil fuels in the oil fields. This huge potential can be extracted if engineered methanogenic consortium is adapted to convert the hydrocarbons into natural gas. This process involves conversion of crude oil hydrocarbons into methanogenic substrates by syntrophic and fermentative bacteria, which are subsequently utilized by methanogens to produce methane. Microbial diversity of such environments supports the viability of this process. This review illuminates the potentials of abundant microbial groups such as Syntrophaceae, Anaerolineaceae, Clostridiales and Euryarchaeota in petroleum hydrocarbon-related environment, their genetic markers, biochemical process and omics-based bioengineering methods involved in methane generation. Increase in the copy numbers of catabolic genes during methanogenesis highlights the prospect of developing engineered biofuel recovery technology. Several lab-based methanogenic consortia from depleted petroleum reservoirs and microcosm studies so far would not be enough for field application without the advent of multi-omics-based technologies to trawl out the bottleneck parameters of the enhanced fuel recovery process. The adaptability of efficient consortium of versatile hydrocarbonoclastic and methanogenic microorganisms under environmental stress conditions is further needed to be investigated. The improved process might hold the potential of methane extraction from petroleum waste like oil tank and refinery sludge, oil field deposits, etc. What sounds as biodegradation could be a beginning of converting waste into wealth by recovery of stranded energy assets.