Variability in Assembly of Degradation Operons for Naphthalene and its derivative, Carbaryl, Suggests Mobilization through Horizontal Gene Transfer

Polycyclic aromatic hydrocarbon: underpinning the contribution of specialist microbial species to contaminant mitigation in the soil

The persistence of PAHs poses a significant challenge for conventional remediation approaches, necessitating the exploration of alternative, sustainable strategies for their mitigation. This review underscores the vital role of specialized microbial species (nitrogen-fixing, phosphate-solubilizing, and biosurfactant-producing bacteria) in tackling the environmental impact of polycyclic aromatic hydrocarbons (PAHs). These resistant compounds demand innovative remediation strategies. The study explores microbial metabolic capabilities for converting complex PAHs into less harmful byproducts, ensuring sustainable mitigation. Synthesizing literature from 2016 to 2023, it covers PAH characteristics, sources, and associated risks. Degradation mechanisms by bacteria and fungi, key species, and enzymatic processes are examined. Nitrogen-fixing and phosphate-solubilizing bacteria contributions in symbiotic relationships with plants are highlighted. Biosurfactant-producing bacteria enhance PAH solubility, expanding microbial accessibility for degradation. Cutting-edge trends in omics technologies, synthetic biology, genetic engineering, and nano-remediation offer promising avenues. Recommendations emphasize genetic regulation, field-scale studies, sustainability assessments, interdisciplinary collaboration, and knowledge dissemination. These insights pave the way for innovative, sustainable PAH-contaminated environment restoration.

Metabolic engineering of Pseudomonas bharatica CSV86T to degrade Carbaryl (1-naphthyl-N-methylcarbamate) via the salicylate-catechol route

Pseudomonas bharatica CSV86T displays the unique property of preferential utilization of aromatic compounds over simple carbon sources like glucose and glycerol and their co-metabolism with organic acids. Well-characterized growth conditions, aromatic compound metabolic pathways and their regulation, genome sequence, and advantageous eco-physiological traits (indole acetic acid production, alginate production, fusaric acid resistance, organic sulfur utilization, and siderophore production) make it an ideal host for metabolic engineering. Strain CSV86T was engineered for Carbaryl (1-naphthyl-N-methylcarbamate) degradation via salicylate-catechol route by expression of a Carbaryl hydrolase (CH) and a 1-naphthol 2-hydroxylase (1NH). Additionally, the engineered strain exhibited faster growth on Carbaryl upon expression of the McbT protein (encoded by the mcbT gene, a part of Carbaryl degradation upper operon of Pseudomonas sp. C5pp). Bioinformatic analyses predict McbT to be an outer membrane protein, and Carbaryl-dependent expression suggests its probable role in Carbaryl uptake. Enzyme activity and protein analyses suggested periplasmic localization of CH (carrying transmembrane domain plus signal peptide sequence at the N-terminus) and 1NH, enabling compartmentalization of the pathway. Enzyme activity, whole-cell oxygen uptake, spent media analyses, and qPCR results suggest that the engineered strain preferentially utilizes Carbaryl over glucose. The plasmid-encoded degradation property was stable for 75-90 generations even in the absence of selection pressure (kanamycin or Carbaryl). These results indicate the utility of P. bharatica CSV86T as a potential host for engineering various aromatic compound degradation pathways.IMPORTANCEThe current study describes engineering of Carbaryl metabolic pathway in Pseudomonas bharatica CSV86T. Carbaryl, a naphthalene-derived carbamate pesticide, is known to act as an endocrine disruptor, mutagen, cytotoxin, and carcinogen. Removal of xenobiotics from the environment using bioremediation faces challenges, such as slow degradation rates, instability of the degradation phenotype, and presence of simple carbon sources in the environment. The engineered CSV86-MEC2 overcomes these disadvantages as Carbaryl was degraded preferentially over glucose. Furthermore, the plasmid-borne degradation phenotype is stable, and presence of glucose and organic acids does not repress Carbaryl metabolism in the strain. The study suggests the role of outer membrane protein McbT in Carbaryl transport. This work highlights the suitability of P. bharatica CSV86T as an ideal host for engineering aromatic pollutant degradation pathways.

Broad-spectrum hydrocarbon-degrading microbes in the global ocean metagenomes

Understanding the diversity and functions of hydrocarbon-degrading microorganisms in marine environments is crucial for both advancing knowledge of biogeochemical processes and improving bioremediation methods. In this study, we leveraged nearly 20,000 metagenome-assembled genomes (MAGs), recovered from a wide array of marine samples across the global oceans, to map the diversity of aerobic hydrocarbon-degrading microorganisms. A broad bacterial diversity was uncovered, with a notable preference for degrading aliphatic hydrocarbons over aromatic ones, primarily within Proteobacteria and Actinobacteriota. Three types of broad-spectrum hydrocarbon-degrading bacteria were identified for their ability to degrade various hydrocarbons and possession of multiple copies of hydrocarbon biodegradation genes. These bacteria demonstrate extensive metabolic versatility, aiding their survival and adaptability in diverse environmental conditions. Evidence of gene duplication and horizontal gene transfer in these microbes suggested a potential enhancement in the diversity of hydrocarbon-degrading bacteria. Positive correlations were observed between the abundances of hydrocarbon-degrading genes and environmental parameters such as temperature (-5 to 35 °C) and salinity (20 to 42 PSU). Overall, our findings offer valuable insights into marine hydrocarbon-degrading microorganisms and suggest considerations for selecting microbial strains for oil pollution remediation.

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.

Biodegradation of naphthalene - Ecofriendly approach for soil pollution mitigation

Naphthalene (NPT), a widely used household pest repellent and insecticide obtained from crude oil, serves as a toxic pollutant to non-target living matter. The stable and resistant nature of NPT makes it difficult to degrade through the physiochemical processes. The present study investigated the bacterial degradation of NPT isolated from crude oil-contaminated soil. Initially, the potent bacteria, Bacillus sp. GN 3.4, were isolated by enrichment culture method and subsequently assessed for NPT biodegradation. The optimum conditions for NPT biodegradation were pH 7.0 at 37 °C, 80 mg/L (initial NPT), 3% v/v (inoculum dose), and 7 days of treatment which showed 100% biodegradation. Furthermore, GC-MS analysis revealed the presence of degradation metabolites, namely, salicylate and hydroquinone indicating potential metabolic pathways. Considering the water-solubility and non-toxic nature of these metabolites, the results imply that Bacillus sp. GN 3.4. could potentially play a role in bioremediation by aiding in eliminating NPT from the soil.

Bioremediation of polycyclic aromatic hydrocarbons: An updated microbiological review

A class of organic priority pollutants known as PAHs is of critical public health and environmental concern due to its carcinogenic properties as well as its genotoxic, mutagenic, and cytotoxic properties. Research to eliminate PAHs from the environment has increased significantly due to awareness about their negative effects on the environment and human health. Various environmental factors, including nutrients, microorganisms present and their abundance, and the nature and chemical properties of the PAH affect the biodegradation of PAHs. A large spectrum of bacteria, fungi, and algae have ability to degrade PAHs with the biodegradation capacity of bacteria and fungi receiving the most attention. A considerable amount of research has been conducted in the last few decades on analyzing microbial communities for their genomic organization, enzymatic and biochemical properties capable of degrading PAH. While it is true that PAH degrading microorganisms offer potential for recovering damaged ecosystems in a cost-efficient way, new advances are needed to make these microbes more robust and successful at eliminating toxic chemicals. By optimizing some factors like adsorption, bioavailability and mass transfer of PAHs, microorganisms in their natural habitat could be greatly improved to biodegrade PAHs. This review aims to comprehensively discuss the latest findings and address the current wealth of knowledge in the microbial bioremediation of PAHs. Additionally, recent breakthroughs in PAH degradation are discussed in order to facilitate a broader understanding of the bioremediation of PAHs in the environment.

Comparative genomics of Stutzerimonas balearica (Pseudomonas balearica): diversity, habitats, and biodegradation of aromatic compounds

Stutzerimonas balearica (Pseudomonas balearica) has been found principally in oil-polluted environments. The capability of S. balearica to thrive from the degradation of pollutant compounds makes it a species of interest for potential bioremediation applications. However, little has been reported about the diversity of S. balearica. In this study, genome sequences of S. balearica strains from different origins were analyzed, revealing that it is a diverse species with an open pan-genome that will continue revealing new genes and functionalities as the genomes of more strains are sequenced. The nucleotide signatures and intra- and inter-species variation of the 16S rRNA genes of S. balearica were reevaluated. A strategy of screening 16S rRNA gene sequences in public databases enabled the detection of 158 additional strains, of which only 23% were described as S. balearica. The species was detected from a wide range of environments, although mostly from aquatic and polluted environments, predominantly related to petroleum oil. Genomic and phenotypic analyses confirmed that S. balearica possesses varied inherent capabilities for aromatic compounds degradation. This study increases the knowledge of the biology and diversity of S. balearica and will serve as a basis for future work with the species.

Genome-resolved analyses show an extensive diversification in key aerobic hydrocarbon-degrading enzymes across bacteria and archaea

Background

Hydrocarbons (HCs) are organic compounds composed solely of carbon and hydrogen that are mainly accumulated in oil reservoirs. As the introduction of all classes of hydrocarbons including crude oil and oil products into the environment has increased significantly, oil pollution has become a global ecological problem. However, our perception of pathways for biotic degradation of major HCs and key enzymes in these bioconversion processes has mainly been based on cultured microbes and is biased by uneven taxonomic representation. Here we used Annotree to provide a gene-centric view of the aerobic degradation ability of aliphatic and aromatic HCs in 23,446 genomes from 123 bacterial and 14 archaeal phyla. 

Results

Apart from the widespread genetic potential for HC degradation in Proteobacteria, Actinobacteriota, Bacteroidota, and Firmicutes, genomes from an additional 18 bacterial and 3 archaeal phyla also hosted key HC degrading enzymes. Among these, such degradation potential has not been previously reported for representatives in the phyla UBA8248, Tectomicrobia, SAR324, and Eremiobacterota. Genomes containing whole pathways for complete degradation of HCs were only detected in Proteobacteria and Actinobacteriota. Except for several members of Crenarchaeota, Halobacterota, and Nanoarchaeota that have tmoA, ladA, and alkB/M key genes, respectively, representatives of archaeal genomes made a small contribution to HC degradation. None of the screened archaeal genomes coded for complete HC degradation pathways studied here; however, they contribute significantly to peripheral routes of HC degradation with bacteria.

Conclusion

Phylogeny reconstruction showed that the reservoir of key aerobic hydrocarbon-degrading enzymes in Bacteria and Archaea undergoes extensive diversification via gene duplication and horizontal gene transfer. This diversification could potentially enable microbes to rapidly adapt to novel and manufactured HCs that reach the environment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08906-w.

Exposure to polycyclic aromatic hydrocarbons and the associations with oxidative stress in waste incineration plant workers from South China

Waste incineration is one of the most common emission sources of polycyclic aromatic hydrocarbons (PAHs), causing potential occupational exposure in waste incineration workers. However, relative investigations among waste incineration plant workers are still very limited, particularly in China. Therefore, we collected urine specimens from 77 workers in a waste incineration plant as the exposed group, and 101 residents as the control group in Shenzhen, China. Nine mono-hydroxylated PAH metabolites (OH-PAHs) and 8-hydroxy-2'-deoxyguanosine (8-OHdG) were measured, and their internal relationships were explored. The urinary levels of most OH-PAHs and 8-OHdG in the exposed group exhibited high levels versus another group (p < 0.05). We found negative associations between OH-PAHs and 8-OHdG in the control group (p < 0.05), while most of OH-PAHs were not associated with 8-OHdG in the exposed group, which indicated that the exposure to waste incineration could enlarge the level of individual oxidative stress damage. Nevertheless, PAHs were less likely to trigger obvious health risks in exposed workers through estimation of human intake and exposure risks. This study provides a reference for occupational PAH exposure and strengthen the need of health monitoring among incineration workers.

A unique global metabolic trait of Pseudomonas bharatica CSV86T: metabolism of aromatics over simple carbon sources and co-metabolism with organic acids

Hierarchical utilization of substrate by microbes (utilization of simple carbon sources prior to complex ones) poses a major limitation to the efficient remediation of aromatic pollutants. Aromatic compounds, being complex and reduced in nature, appear to be a deferred choice as the carbon source in the presence of a plethora of simple organic compounds in the environment. The soil bacterium Pseudomonas bharatica CSV86^T displays a unique carbon source utilization hierarchy. It preferentially utilizes aromatics over glucose and co-metabolizes them with succinate or pyruvate (Basu et al., 2006, Applied and Environmental Microbiology, 72 : 22226-2230). In the present study, the substrate utilization hierarchy for strain CSV86^T was tested for additional simple carbon sources such as glycerol, acetate, and tri-carboxylic acid (TCA) cycle intermediates like α-ketoglutarate and fumarate. When grown on a mixture of aromatics (benzoate or naphthalene) plus glycerol, the strain displayed a diauxic growth profile with significantly high activity of aromatic utilization enzymes (catechol 1,2- or 2,3-dioxygenase, respectively) in the first-log phase. This suggests utilization of aromatics in the first-log phase followed by glycerol in the second-log phase. On a mixture of an aromatic plus organic acid (acetate, α-ketoglutarate or fumarate), the strain displayed a monoauxic growth profile, indicating co-metabolism. Interestingly, the presence of glycerol, acetate, α-ketoglutarate or fumarate does not repress metabolism/utilization of the aromatic. Thus, the substrate utilization hierarchy of strain CSV86^T is aromatics=organic acids>glucose/glycerol, which is unique as compared to other Pseudomonas species, where degradation of aromatics is repressed by glycerol, glucose, acetate or organic acids, including TCA cycle intermediates. This novel substrate utilization hierarchy appears to be a global metabolic phenomenon in strain CSV86^T, thus implying it to be an ideal host for metabolic engineering as well as for its potential application in bioremediation.

Life Within a Contaminated Niche: Comparative Genomic Analyses of an Integrative Conjugative Element ICEnahCSV86 and Two Genomic Islands From Pseudomonas bharatica CSV86T Suggest Probable Role in Colonization and Adaptation

Comparative genomic and functional analyses revealed the presence of three genomic islands (GIs, >50 Kb size): ICEnahCSV86, Pseudomonas bharatica genomic island-1 (PBGI-1), and PBGI-2 in the preferentially aromatic-degrading soil bacterium, Pseudomonas bharatica CSV86^T. Site-specific genomic integration at or near specific transfer RNAs (tRNAs), near-syntenic structural modules, and phylogenetic relatedness indicated their evolutionary lineage to the type-4 secretion system (T4SS) ICEclc family, thus predicting these elements to be integrative conjugative elements (ICEs). These GIs were found to be present as a single copy in the genome and the encoded phenotypic traits were found to be stable, even in the absence of selection pressure. ICEnahCSV86 harbors naphthalene catabolic (nah-sal) cluster, while PBGI-1 harbors Co-Zn-Cd (czc) efflux genes as cargo modules, whereas PBGI-2 was attributed to as a mixed-function element. The ICEnahCSV86 has been reported to be conjugatively transferred (frequency of 7 × 10^–8/donor cell) to Stenotrophomonas maltophilia CSV89. Genome-wide comparative analyses of aromatic-degrading bacteria revealed nah-sal clusters from several Pseudomonas spp. as part of probable ICEs, syntenic to conjugatively transferable ICEnahCSV86 of strain CSV86^T, suggesting it to be a prototypical element for naphthalene degradation. It was observed that the plasmids harboring nah-sal clusters were phylogenetically incongruent with predicted ICEs, suggesting genetic divergence of naphthalene metabolic clusters in the Pseudomonas population. Gene synteny, divergence estimates, and codon-based Z-test indicated that ICEnahCSV86 is probably derived from PBGI-2, while multiple recombination events masked the ancestral lineage of PBGI-1. Diversifying selection pressure (dN-dS = 2.27–4.31) imposed by aromatics and heavy metals implied the modular exchange-fusion of various cargo clusters through events like recombination, rearrangement, domain reshuffling, and active site optimization, thus allowing the strain to evolve, adapt, and maximize the metabolic efficiency in a contaminated niche. The promoters (Pnah and Psal) of naphthalene cargo modules (nah, sal) on ICEnahCSV86 were proved to be efficient for heterologous protein expression in Escherichia coli. GI-based genomic plasticity expands the metabolic spectrum and versatility of CSV86^T, rendering efficient adaptation to the contaminated niche. Such isolate(s) are of utmost importance for their application in bioremediation and are the probable ideal host(s) for metabolic engineering.

Functional genome mining and taxono-genomics reveal eco-physiological traits and species distinctiveness of aromatic-degrading Pseudomonas bharatica sp. nov

Assistive eco-physiological traits are necessary for microbes to adapt and colonize at polluted niches, enabling efficient clean-up. To demarcate species distinctiveness and eco-physiological traits of aromatic compounds metabolizing Pseudomonas sp. CSV86^T (earlier identified as Pseudomonas putida), an Indian isolate from a petrol station soil, comparative genome mining, taxono-genomic, and physiological analyses were performed. A 6.79 Mbp genome (62.72 G + C mol%) of CSV86^T encodes 6798 CDS and 238 unique genes. Naphthalene metabolism and Co-Zn-Cd resistance gene clusters were part of distinct genomic islands. Abundance of transporters (aromatics, organic acids, amino acids, and metals) and mobile elements (integrases, transposases, conjugative proteins) differentiated CSV86^T from its closest relatives. Enhanced siderophore production for Fe-uptake during aromatic metabolism, indole acetic acid production, and fusaric acid resistance wasvalidated by genomic attributes. Full-length 16S-rRNA phylogeny revealed Pseudomonas japonica WL^T as a closest relative of CSV86^T . However, lower genomic indices (<97% gyrB-rpoB-rpoD homology, <90% ANI, <50% DNA-DNA relatedness) and taxonomic differences (assimilation of organic acids, amino acids, fatty acids composition) substantially differentiated CSV86^T from its closest relatives, indicating it to be a novel species as Pseudomonas bharatica. Preferential metabolism of aromatics with advantageous eco-physiological traits renders CSV86^T an ideal candidate for bioremediation and host for metabolic engineering.

Shifts in microbial community structure and function in polycyclic aromatic hydrocarbon contaminated soils at petrochemical landfill sites revealed by metagenomics

Investigations of the microbial community structures, potential functions and polycyclic aromatic hydrocarbon (PAH) degradation-related genes in PAH-polluted soils are useful for risk assessments, microbial monitoring, and the potential bioremediation of soils polluted by PAHs. In this study, five soil sampling sites were selected at a petrochemical landfill in Beijing, China, to analyze the contamination characteristics of PAHs and their impact on microorganisms. The concentrations of 16 PAHs were detected by gas chromatography-mass spectrometry. The total concentrations of the PAHs ranged from ND to 3166.52 μg/kg, while phenanthrene, pyrene, fluoranthene and benzo [ghi]perylene were the main components in the soil samples. According to the specific PAH ratios, the PAHs mostly originated from petrochemical wastes in the landfill. The levels of the total toxic benzo [a]pyrene equivalent (1.63-107.73 μg/kg) suggested that PAHs might result in adverse effects on soil ecosystems. The metagenomic analysis showed that the most abundant phyla in the soils were Proteobacteria and Actinobacteria, and Solirubrobacter was the most important genus. At the genus level, Bradyrhizobium, Mycobacterium and Anaeromyxobacter significantly increased under PAH stress. Based on the Kyoto Encyclopedia of Genes and Genomes (KEGG) annotations, the most abundant category of functions that are involved in adapting to contaminant pressures was identified. Ten PAH degradation-related genes were significantly influenced by PAH pressure and showed correlations with PAH concentrations. All of the results suggested that the PAHs from the petrochemical landfill could be harmful to soil environments and impact the soil microbial community structures, while microorganisms would change their physiological functions to resist pollutant stress.

Ecological selection of bacterial taxa with larger genome sizes in response to polycyclic aromatic hydrocarbons stress

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous priority pollutants that cause great damage to the natural environment and health. Average genome size in a community is critical for shedding light on microbiome's functional response to pollution stress within an environment. Here, microcosms under different concentrations were performed to evaluate the selection of PAHs stress on the average genome size in a community. We found the distinct communities of significantly larger genome size with the increase of PAHs concentration gradients in soils, and consistent trends were discovered in soils at different latitudes. The abundance of Proteobacteria and Deinococcus-Thermus with relatively larger genomes increased along with PAHs stress and well adapted to polluted environments. In contrast, the abundance of Patescibacteria with a highly streamlined and smaller genome decreased, implying complex interactions between environmental selection and functional fitness resulted in bacteria with larger genomes becoming more abundant. Moreover, we confirmed the increased capacity for horizontal transfer of degrading genes between communities by showing an increased connection number per node positively related to the nidA gene along the concentration gradients in the co-occurrence network. Our findings suggest PAHs tend to select bacterial taxa with larger genome sizes, with significant consequences for community stability and potential biodegradation strategies.

Applications of Metagenomics for Unrevealing the Extended Horizons of Microbiota Prevalence from Soil to Human Health

Phylogenetic analysis of different ecosystems has shown that the number of microbial communities in a single sample exceeds their cultured counterparts. Microbes have been found throughout nature and can thrive in adverse conditions. Besides inhabiting diverse environments, they also play a key role in the maintenance of the ecosystem. Most of these microbes are either unculturable or difficult to culture with conventional culturing methods. Metagenomics is an emerging field of science that has been in the light for a decade and offers a potential way to assess microbial diversity. The development of metagenomics opens new ways to study genetic material directly from the environmental samples. DNA sequencing and synthesis technologies are making it possible to read and write entire genomes. The huge amount of data obtained from genome sequencing inevitably requires bioinformatics tools to handle and further process them for analysis. Advances in DNA sequencing and high-performance computing have brought about exemplar improvement in metagenomics, allowing in-depth study of the largely unexplored frontier of microbial life. This culture-independent method provides extensive information regarding the structure, composition, and function of the diverse assemblages of the environmental microbes. The current review presents an overview of the technical aspects of metagenomics along with its diverse applications.

Not All That Glitters Is Gold: The Paradox of CO-dependent Hydrogenogenesis in Parageobacillus thermoglucosidasius

The thermophilic bacterium Parageobacillus thermoglucosidasius has recently gained interest due to its ability to catalyze the water gas shift reaction, where the oxidation of carbon monoxide (CO) is linked to the evolution of hydrogen (H₂) gas. This phenotype is largely predictable based on the presence of a genomic region coding for a carbon monoxide dehydrogenase (CODH—Coo) and hydrogen evolving hydrogenase (Phc). In this work, seven previously uncharacterized strains were cultivated under 50% CO and 50% air atmosphere. Despite the presence of the coo—phc genes in all seven strains, only one strain, Kp1013, oxidizes CO and yields H₂. The genomes of the H₂ producing strains contain unique genomic regions that code for proteins involved in nickel transport and the detoxification of catechol, a by-product of a siderophore-mediated iron acquisition system. Combined, the presence of these genomic regions could potentially drive biological water gas shift (WGS) reaction in P. thermoglucosidasius.

Plasmid-mediated catabolism for the removal of xenobiotics from the environment

The large-scale application of xenobiotics adversely affects the environment. The genes that are present in the chromosome of the bacteria are considered nonmobile, whereas the genes present on the plasmids are considered mobile genetic elements. Plasmids are considered indispensable for xenobiotic degradation into the contaminated environment. In the contaminated sites, bacteria with plasmids can transfer the mobile genetic element into another strain. This mechanism helps in spreading the catabolic genes into the bacterial population at the contaminated sites. The indigenous microbial strains with such degradative plasmids are important for the bioremediation of xenobiotics. Environmental factors play a critical role in the conjugation efficiency, which is involved in the bioremediation of the xenobiotics at the contaminated sites. However, there is still a need for more research to fill in the gaps regarding plasmids and their impact on bioremediation. This review explores the role of bacterial plasmids in the bioremediation of xenobiotics from contaminated environments.

Wood‑feeding termites as an obscure yet promising source of bacteria for biodegradation and detoxification of creosote-treated wood along with methane production enhancement

This study aims to explore distinct bacterial strains from wood-feeding termites and to construct novel bacterial consortium for improving the methane yield during anaerobic digestion by degrading birchwood sawdust (BSD) and removing creosote (CRO) compounds simultaneously. A novel bacterial consortium CTB-4 which stands for the molecularly identified species Burkholderia sp., Xanthomonas sp., Shewanella sp., and Pseudomonas mosselii was successfully developed. The CTB-4 consortium showed high efficiency in the removal of naphthalene and phenol. It also revealed reduction in lignin, hemicellulose, and cellulose by 19.4, 52.5, and 76.8%, respectively. The main metabolites after the CRO degradation were acetic acid, succinate, pyruvate, and acetaldehyde. Pretreatment of treated BSD mixed with CRO enhanced the total methane yield (162 L/kg VS) by 82.7% and biomass reduction by 54.7% compared to the untreated substrate. CRO showed a toxicity decrease of >90%, suggesting the efficiency of constructed bacterial consortia in bioremediation and biofuel production.

Construction of a novel microbial consortium valued for the effective degradation and detoxification of creosote-treated sawdust along with enhanced methane production

Lignocellulosic biomass represents an unlimited and ubiquitous energy source, which can effectively address current global challenges, including climate change, greenhouse gas emissions, and increased energy demand. However, lignocellulose recalcitrance hinders microbial degradation, especially in case of contaminated materials such as creosote (CRO)-treated wood, which necessitates appropriate processing in order to eliminate pollution. This study might be the first to explore a novel bacterial consortium SST-4, for decomposing birchwood sawdust, capable of concurrently degrading lignocellulose and CRO compounds. Afterwards, SST-4 which stands for molecularly identified bacterial strains Acinetobacter calcoaceticus BSW-11, Shewanella putrefaciens BSW-18, Bacillus cereus BSW-23, and Novosphingobium taihuense BSW-25 was evaluated in terms of biological sawdust pre-treatment, resulting in effective lignocellulose degradation and 100% removal of phenol and naphthalene. Subsequently, the maximum biogas production observed was 18.7 L/kg VS, while cumulative methane production was 162.8 L/kg VS, compared to 88.5 without microbial pre-treatment. The cumulative energy production from AD-I and AD-II through biomethanation was calculated as 3177.1 and 5843.6 KJ/kg, respectively. The pretreatment process exhibited a significant increase in the energy yield by 83.9%. Lastly, effective CRO detoxification was achieved with EC₅₀ values exceeding 90%, showing the potential for an integrated process of effective contaminated wood management and bioenergy production.

Insights into the microbial degradation and biochemical mechanisms of carbamates

Carbamate compounds are commonly applied in agricultural sectors as alternative options to the recalcitrant organochlorine pesticides due to their easier breakdown and less persistent nature. However, the large-scale use of carbamates also leads to toxic environmental residues, causing severe toxicity in various living systems. The toxic effects of carbamates are due to their inhibitor activity against the acetylchlolinesterase enzyme. This enzyme is crucial for neurotransmission signaling in living beings. Hence, from the environmental point of view, the elimination of carbamates is a worldwide concern and priority. Microbial technology can be deliberated as a potential tool that can work efficiently and as an ecofriendly option for the dissipation of carbamate insecticides from contaminated environments by improving biodegradation processes via metabolic activities of microorganisms. A variety of bacterial and fungal species have been isolated and characterized and are capable of degrading a broad range of carbamates in soil and water environments. In addition, microbial carbamate hydrolase genes (mcd, cehA, cahA, cfdJ, and mcbA) were strongly implicated in the evolution of new metabolic functions and carbamate hydrolase enzymes. However, the accurate localization and appropriate functions of carbamate hydrolase enzymes/genes are very limited. To explore the information on the degradation routes of carbamates and promote the application of biodegradation, a study of molecular techniques is required to unlock insights regarding the degradation specific genes and enzymes. Hence, this review discusses the deep understanding of carbamate degradation mechanisms with microbial strains, metabolic pathways, molecular mechanisms, and their genetic basis in degradation.

A Recently Assembled Degradation Pathway for 2,3-Dichloronitrobenzene in Diaphorobacter sp. Strain JS3051

Diaphorobacter sp. strain JS3051 utilizes 2,3-dichloronitrobenzene (23DCNB), a toxic anthropogenic compound, as the sole carbon, nitrogen, and energy source for growth, but the metabolic pathway and its origins are unknown. Here, we establish that a gene cluster (dcb), encoding a Nag-like dioxygenase, is responsible for the initial oxidation of the 23DCNB molecule. The 2,3-dichloronitrobenzene dioxygenase system (DcbAaAbAcAd) catalyzes conversion of 23DCNB to 3,4-dichlorocatechol (34DCC). Site-directed mutagenesis studies indicated that residue 204 of DcbAc is crucial for the substrate specificity of 23DCNB dioxygenase. The presence of glutamic acid at position 204 of 23DCNB dioxygenase is unique among Nag-like dioxygenases. Genetic, biochemical, and structural evidence indicate that the 23DCNB dioxygenase is more closely related to 2-nitrotoluene dioxygenase from Acidovorax sp. strain JS42 than to the 34DCNB dioxygenase from Diaphorobacter sp. strain JS3050, which was isolated from the same site as strain JS3051. A gene cluster (dcc) encoding the enzymes for 34DCC catabolism, homologous to a clc operon in Pseudomonas knackmussii strain B13, is also on the chromosome at a distance of 2.5 Mb from the dcb genes. Heterologously expressed DccA catalyzed ring cleavage of 34DCC with high affinity and catalytic efficiency. This work not only establishes the molecular mechanism for 23DCNB mineralization, but also enhances the understanding of the recent evolution of the catabolic pathways for nitroarenes.

Eco-physiological portrait of a novel Pseudomonas sp. CSV86: an ideal host/candidate for metabolic engineering and bioremediation

Pseudomonas sp. CSV86, an Indian soil isolate, degrades wide range of aromatic compounds like naphthalene, benzoate and phenylpropanoids, amongst others. Isolate displays the unique and novel property of preferential utilization of aromatics over glucose and co-metabolizes them with organic acids. Interestingly, as compared to other Pseudomonads, strain CSV86 harbours only high-affinity glucokinase pathway (and absence of low-affinity oxidative route) for glucose metabolism. Such lack of gluconate loop might be responsible for the novel phenotype of preferential utilization of aromatics. The genome analysis and comparative functional mining indicated a large genome (6.79 Mb) with significant enrichment of regulators, transporters as well as presence of various secondary metabolite production clusters, suggesting its eco-physiological and metabolic versatility. Strain harbours various integrative conjugative elements (ICEs) and genomic islands, probably acquired through horizontal gene transfer events, leading to genome mosaicity and plasticity. Naphthalene degradation genes are arranged as regulonic clusters and found to be part of ICE_CSV86nah . Various eco-physiological properties and absence of major pathogenicity and virulence factors (risk group-1) in CSV86 suggest it to be an ideal candidate for bioremediation. Further, strain can serve as an ideal chassis for metabolic engineering to degrade various xenobiotics preferentially over simple carbon sources for efficient remediation.

Conserved Metabolic and Evolutionary Themes in Microbial Degradation of Carbamate Pesticides

Carbamate pesticides are widely used as insecticides, nematicides, acaricides, herbicides and fungicides in the agriculture, food and public health sector. However, only a minor fraction of the applied quantity reaches the target organisms. The majority of it persists in the environment, impacting the non-target biota, leading to ecological disturbance. The toxicity of these compounds to biota is mediated through cholinergic and non-cholinergic routes, thereby making their clean-up cardinal. Microbes, specifically bacteria, have adapted to the presence of these compounds by evolving degradation pathways and thus play a major role in their removal from the biosphere. Over the past few decades, various genetic, metabolic and biochemical analyses exploring carbamate degradation in bacteria have revealed certain conserved themes in metabolic pathways like the enzymatic hydrolysis of the carbamate ester or amide linkage, funnelling of aryl carbamates into respective dihydroxy aromatic intermediates, C1 metabolism and nitrogen assimilation. Further, genomic and functional analyses have provided insights on mechanisms like horizontal gene transfer and enzyme promiscuity, which drive the evolution of degradation phenotype. Compartmentalisation of metabolic pathway enzymes serves as an additional strategy that further aids in optimising the degradation efficiency. This review highlights and discusses the conclusions drawn from various analyses over the past few decades; and provides a comprehensive view of the environmental fate, toxicity, metabolic routes, related genes and enzymes as well as evolutionary mechanisms associated with the degradation of widely employed carbamate pesticides. Additionally, various strategies like application of consortia for efficient degradation, metabolic engineering and adaptive laboratory evolution, which aid in improvising remediation efficiency and overcoming the challenges associated with in situ bioremediation are discussed.

Microbial Degradation of Naphthalene and Substituted Naphthalenes: Metabolic Diversity and Genomic Insight for Bioremediation

Low molecular weight polycyclic aromatic hydrocarbons (PAHs) like naphthalene and substituted naphthalenes (methylnaphthalene, naphthoic acids, 1-naphthyl N-methylcarbamate, etc.) are used in various industries and exhibit genotoxic, mutagenic, and/or carcinogenic effects on living organisms. These synthetic organic compounds (SOCs) or xenobiotics are considered as priority pollutants that pose a critical environmental and public health concern worldwide. The extent of anthropogenic activities like emissions from coal gasification, petroleum refining, motor vehicle exhaust, and agricultural applications determine the concentration, fate, and transport of these ubiquitous and recalcitrant compounds. Besides physicochemical methods for cleanup/removal, a green and eco-friendly technology like bioremediation, using microbes with the ability to degrade SOCs completely or convert to non-toxic by-products, has been a safe, cost-effective, and promising alternative. Various bacterial species from soil flora belonging to Proteobacteria (Pseudomonas, Pseudoxanthomonas, Comamonas, Burkholderia, and Novosphingobium), Firmicutes (Bacillus and Paenibacillus), and Actinobacteria (Rhodococcus and Arthrobacter) displayed the ability to degrade various SOCs. Metabolic studies, genomic and metagenomics analyses have aided our understanding of the catabolic complexity and diversity present in these simple life forms which can be further applied for efficient biodegradation. The prolonged persistence of PAHs has led to the evolution of new degradative phenotypes through horizontal gene transfer using genetic elements like plasmids, transposons, phages, genomic islands, and integrative conjugative elements. Systems biology and genetic engineering of either specific isolates or mock community (consortia) might achieve complete, rapid, and efficient bioremediation of these PAHs through synergistic actions. In this review, we highlight various metabolic routes and diversity, genetic makeup and diversity, and cellular responses/adaptations by naphthalene and substituted naphthalene-degrading bacteria. This will provide insights into the ecological aspects of field application and strain optimization for efficient bioremediation.

Substrate preference of carbamate hydrolase CehA reveals its environmental behavior

The cehA gene is the earliest reported and most widely found carbaryl hydrolase gene. CehA detoxifies carbaryl and other carbamate pesticides via de-esterification. Currently, there is no systematic research available on substrate preference or the mechanism of CehA action in different hosts. In this study, we found that CehA from different hosts is highly conserved, with more than 99% amino acid sequence similarity, and that transposable elements exist in both the upstream and downstream regions of cehA. By introducing point mutations into the cehA gene of Sphingobium sp. CFD-1, we obtained and heterologously expressed all reported CehA(CehAS) encoding genes. Assays to determine enzymatic properties and substrate profiles of CehAS showed that each CehA has a significant substrate preference for different carbamate insecticides. Specifically, CehA152Phe/Leu determines the catalytic preference for bicyclic carbamate substrates (carbofuran, carbaryl), while CehA494Thr/Ala and 570Thr/Ile determine the preference for monocyclic carbamate substrates (isoprocarb, propoxur) and linear carbamate substrates (oxamyl, aldicarb), respectively. Considering the existence of transposable elements in the flanking regions of cehA, we speculate that the cehA hosts may have acquired the hydrolysis ability, as well as substrate preference for carbamate pesticides, through horizontal gene transfer and genetic copying errors.

Co-exposure to polycyclic aromatic hydrocarbons and phthalates and their associations with oxidative stress damage in school children from South China

Monohydroxylated polycyclic aromatic hydrocarbons (OH-PAHs), phthalate metabolites (mPAEs), and 8-hydroxy-2'-deoxyguanosine (8-OHdG) in the urine of school children aged 8-11 years from Shenzhen, China were measured in order to investigate oxidative stress damage from co-exposure to PAHs and PAEs. The concentrations of OH-PAHs and mPAEs in urine were 0.36-36.5 (median: 3.86) and 9.48-1609 (median: 240) ng/mL respectively. Gender and age did not influence urinary concentrations of ΣOH-PAHs and ΣmPAEs, but geographical variations (i.e., urban versus suburban) were observed. Levels of 8-OHdG were positively correlated with urinary OH-PAHs and mPAEs, with correlation coefficients (r) varying between 0.160 and 0.365 (p < 0.05). OH-PAHs made a greater contribution to oxidative DNA damage than mPAEs when these two types of pollutants were present at the same concentrations. Human health risks were assessed using the hazard quotient and the hazard index for the cumulative risk of a complex of chemicals. The results demonstrated that risks from PAHs could be neglected, but that 29.5 % of school children may be subject to obvious health risks from PAEs, especially diethylhexyl phthalate.

Remarkable shift in structural and functional properties of an animal charcoal-polluted soil accentuated by inorganic nutrient amendment

Background

Soils polluted with animal charcoal from skin and hide cottage industries harbour extremely toxic and carcinogenic hydrocarbon pollutants and thus require a bio-based eco-friendly strategy for their depuration. The effects of carbon-free mineral medium (CFMM) amendment on hydrocarbon degradation and microbial community structure and function in an animal charcoal-polluted soil was monitored for 6 weeks in field moist microcosms consisting of CFMM-treated soil (FN4) and an untreated control (FN1). Hydrocarbon degradation was monitored using gas chromatography-flame ionization detector (GC-FID), and changes in microbial community structure were monitored using Kraken, while functional annotation of putative open reading frames (ORFs) was done using KEGG KofamKOALA and NCBI’s conserved domain database (CDD).

Results

Gas chromatographic analysis of hydrocarbon fractions revealed the removal of 84.02% and 82.38% aliphatic and 70.09% and 70.14% aromatic fractions in FN4 and FN1 microcosms in 42 days. Shotgun metagenomic analysis of the two metagenomes revealed a remarkable shift in the microbial community structure. In the FN4 metagenome, 92.97% of the population belong to the phylum Firmicutes and its dominant representative genera Anoxybacillus (64.58%), Bacillus (21.47%) and Solibacillus (2.39%). In untreated FN1 metagenome, the phyla Proteobacteria (56.12%), Actinobacteria (23.79%) and Firmicutes (11.20%), and the genera Xanthobacter (9.73%), Rhizobium (7.49%) and Corynebacterium (7.35%), were preponderant. Functional annotation of putative ORFs from the two metagenomes revealed the detection of degradation genes for aromatic hydrocarbons, benzoate, xylene, chlorocyclohexane/chlorobenzene, toluene and several others in FN1 metagenome. In the FN4 metagenome, only seven hydrocarbon degradation genes were detected.

Conclusion

This study revealed that though CFMM amendment slightly increases the rate of hydrocarbon degradation, it negatively impacts the structural and functional properties of the animal charcoal-polluted soil. It also revealed that intrinsic bioremediation of the polluted soil could be enhanced via addition of water and aeration.

Supplementary Information

The online version contains supplementary material available at 10.1186/s43141-020-00089-9.

A genomic view of trophic and metabolic diversity in clade-specific Lamellodysidea sponge microbiomes

BACKGROUND

Marine sponges and their microbiomes contribute significantly to carbon and nutrient cycling in global reefs, processing and remineralizing dissolved and particulate organic matter. Lamellodysidea herbacea sponges obtain additional energy from abundant photosynthetic Hormoscilla cyanobacterial symbionts, which also produce polybrominated diphenyl ethers (PBDEs) chemically similar to anthropogenic pollutants of environmental concern. Potential contributions of non-Hormoscilla bacteria to Lamellodysidea microbiome metabolism and the synthesis and degradation of additional secondary metabolites are currently unknown.

RESULTS

This study has determined relative abundance, taxonomic novelty, metabolic capacities, and secondary metabolite potential in 21 previously uncharacterized, uncultured Lamellodysidea-associated microbial populations by reconstructing near-complete metagenome-assembled genomes (MAGs) to complement 16S rRNA gene amplicon studies. Microbial community compositions aligned with sponge host subgroup phylogeny in 16 samples from four host clades collected from multiple sites in Guam over a 3-year period, including representatives of Alphaproteobacteria, Gammaproteobacteria, Oligoflexia, and Bacteroidetes as well as Cyanobacteria (Hormoscilla). Unexpectedly, microbiomes from one host clade also included Cyanobacteria from the prolific secondary metabolite-producer genus Prochloron, a common tunicate symbiont. Two novel Alphaproteobacteria MAGs encoded pathways diagnostic for methylotrophic metabolism as well as type III secretion systems, and have been provisionally assigned to a new order, designated Candidatus Methylospongiales. MAGs from other taxonomic groups encoded light-driven energy production pathways using not only chlorophyll, but also bacteriochlorophyll and proteorhodopsin. Diverse heterotrophic capabilities favoring aerobic versus anaerobic conditions included pathways for degrading chitin, eukaryotic extracellular matrix polymers, phosphonates, dimethylsulfoniopropionate, trimethylamine, and benzoate. Genetic evidence identified an aerobic catabolic pathway for halogenated aromatics that may enable endogenous PBDEs to be used as a carbon and energy source.

CONCLUSIONS

The reconstruction of high-quality MAGs from all microbial taxa comprising greater than 0.1% of the sponge microbiome enabled species-specific assignment of unique metabolic features that could not have been predicted from taxonomic data alone. This information will promote more representative models of marine invertebrate microbiome contributions to host bioenergetics, the identification of potential new sponge parasites and pathogens based on conserved metabolic and physiological markers, and a better understanding of biosynthetic and degradative pathways for secondary metabolites and halogenated compounds in sponge-associated microbiota. Video Abstract.

Special Issue: Genetics of Biodegradation and Bioremediation

Many different biodegradation pathways, both aerobic and anaerobic, have already been characterised, and the phylogenetic relationships among catabolic genes within the different types of pathways have been studied. However, new biodegradation activities and their coding genes are continuously being reported, including those involved in the catabolism of emerging contaminants or those generally regarded as non-biodegradable. Gene regulation is also an important issue for the efficient biodegradation of contaminants. Specific induction by the substrate and over-imposed global regulatory networks adjust the expression of the biodegradation genes to the bacterial physiological needs. New biodegradation pathways can be assembled in a particular strain or in a bacterial consortium by recruiting biodegradation genes from different origins through horizontal gene transfer. The abundance and diversity of biodegradation genes, analysed by either genomic or metagenomic approaches, constitute valuable indicators of the biodegradation potential of a particular environmental niche. This knowledge paves the way to systems metabolic engineering approaches to valorise biowaste for the production of value-added products.

Degradation strategies and associated regulatory mechanisms/features for aromatic compound metabolism in bacteria

As a result of anthropogenic activity, large number of recalcitrant aromatic compounds have been released into the environment. Consequently, microbial communities have adapted and evolved to utilize these compounds as sole carbon source, under both aerobic and anaerobic conditions. The constitutive expression of enzymes necessary for metabolism imposes a heavy energy load on the microbe which is overcome by arrangement of degradative genes as operons which are induced by specific inducers. The segmentation of pathways into upper, middle and/or lower operons has allowed microbes to funnel multiple compounds into common key aromatic intermediates which are further metabolized through central carbon pathway. Various proteins belonging to diverse families have evolved to regulate the transcription of individual operons participating in aromatic catabolism. These proteins, complemented with global regulatory mechanisms, carry out the regulation of aromatic compound metabolic pathways in a concerted manner. Additionally, characteristics like chemotaxis, preferential utilization, pathway compartmentalization and biosurfactant production confer an advantage to the microbe, thus making bioremediation of the aromatic pollutants more efficient and effective.

Jump around: transposons in and out of the laboratory

Since Barbara McClintock’s groundbreaking discovery of mobile DNA sequences some 70 years ago, transposable elements have come to be recognized as important mutagenic agents impacting genome composition, genome evolution, and human health. Transposable elements are a major constituent of prokaryotic and eukaryotic genomes, and the transposition mechanisms enabling transposon proliferation over evolutionary time remain engaging topics for study, suggesting complex interactions with the host, both antagonistic and mutualistic. The impact of transposition is profound, as over 100 human heritable diseases have been attributed to transposon insertions. Transposition can be highly mutagenic, perturbing genome integrity and gene expression in a wide range of organisms. This mutagenic potential has been exploited in the laboratory, where transposons have long been utilized for phenotypic screening and the generation of defined mutant libraries. More recently, barcoding applications and methods for RNA-directed transposition are being used towards new phenotypic screens and studies relevant for gene therapy. Thus, transposable elements are significant in affecting biology both in vivo and in the laboratory, and this review will survey advances in understanding the biological role of transposons and relevant laboratory applications of these powerful molecular tools.