Biodegradation of benzo(a)pyrene by two freshwater microalgae Selenastrum capricornutum and Scenedesmus acutus: a comparative study useful for bioremediation

How to deal with xenobiotic compounds through environment friendly approach?

Every year, a huge amount of lethal compounds, such as synthetic dyes, pesticides, pharmaceuticals, hydrocarbons, etc. are mass produced worldwide, which negatively affect soil, air, and water quality. At present, pesticides are used very frequently to meet the requirements of modernized agriculture. The Food and Agriculture Organization of the United Nations (FAO) estimates that food production will increase by 80% by 2050 to keep up with the growing population, consequently pesticides will continue to play a role in agriculture. However, improper handling of these highly persistent chemicals leads to pollution of the environment and accumulation in food chain. These effects necessitate the development of technologies to eliminate or degrade these pollutants. Degradation of these compounds by physical and chemical processes is expensive and usually results in secondary compounds with higher toxicity. The biological strategies proposed for the degradation of these compounds are both cost-effective and eco-friendly. Microbes play an imperative role in the degradation of xenobiotic compounds that have toxic effects on the environment. This review on the fate of xenobiotic compounds in the environment presents cutting-edge insights and novel contributions in different fields. Microbial community dynamics in water bodies, genetic modification for enhanced pesticide degradation and the use of fungi for pharmaceutical removal, white-rot fungi's versatile ligninolytic enzymes and biodegradation potential are highlighted. Here we emphasize the factors influencing bioremediation, such as microbial interactions and carbon catabolism repression, along with a nuanced view of challenges and limitations. Overall, this review provides a comprehensive perspective on the bioremediation strategies.

Benzo(b)fluoranthene and benzo(k)fluoranthene removal by microalgae in the presence of triazine herbicides: Matrix solid-phase dispersion and solid-phase extraction (MSPD/SPE) for HPLC-UV analysis of the different culture components

Over the last decade, human activities in the industrial and agricultural sectors have significantly increased the concentration of persistent and harmful pollutants in aquatic ecosystems. The use of microorganisms is a green strategy for the bio-removal of certain contaminants. However, other pollutants in the same ecosystems can reduce their degrading activity and even affect their survival. Therefore, this study aimed to evaluate the efficiency of benzo(b)fluoranthene (BbF) and benzo(k)fluoranthene (BkF) removal by Selenastrum capricornutum in the presence of triazine herbicides, compounds mainly used in broadleaf weeds. The interest of this work focused on identifying in which of the microalgal components the degrading activity is best evidenced and affected. For this purpose, the use of solid-phase extraction (SPE) and matrix solid-phase dispersion (MSPD) extraction procedures and HPLC-UV analysis allowed the BbF and BkF trace quantification in biomass, liquid medium, and cell lysate separately from cultures exposed to these polycyclic aromatic hydrocarbons (PAHs) alone or with herbicides. The recovery percentages were between 78 and 94 %, good linearity (r2 ≈ 0.99), precision values measured as RSD < 15 %, and limits of detection (LOQs) at levels of ng mL-1 and ng mg-1 were obtained. The individual PAH amounts measured in the components of microalgae cultures show similar removal kinetics (removal percentages: 82-89 %). Likewise, the analysis demonstrated that the removal of PAHs is not affected in the presence of triazine herbicides (atrazine and cyanazine) and with similar removal percentages (79-86 %) compared to those cultures exposed to individual PAHs (74-83 %). These results support the possible real-world applications of PAH removal by extracts from S. capricornutum in aquatic environments contaminated with PAHs and near agriculture areas where triazine herbicides are used.

Cohesive phycoremediation of pyrene by freshwater microalgae Selenastrum sp. and biodiesel production and its assessment

In this study, the freshwater microalgae Selenastrum sp. was assessed for the effective degradation of pyrene and simultaneous production of biodiesel from pyrene-tolerant biomass. The growth of algae was determined based on the cell dry weight, cell density, chlorophyll content, and biomass productivity under different pyrene concentrations. Further, lipids from pyrene tolerant culture were converted into biodiesel by acid-catalyzed transesterification, which was characterized for the total fatty acid profile by gas chromatography. Increased pyrene concentration revealed less biomass yield and productivity after 20 days of treatment, indicating potent pyrene biodegradation by Selenastrum sp. Biomass yield was unaffected till the 20 mg/L pyrene. A 95% of pyrene bioremediation was observed at 20 days of culturing. Lipid accumulation of 22.14%, as evident from the estimation of the total lipid content, indicated a marginal increase in corroborating pyrene stress in the culture. Fatty acid methyl esters yield of 63.06% (% per 100 g lipids) was noticed from the pyrene tolerant culture. Moreover, fatty acid profile analysis of biodiesel produced under 10 mg/L and 20 mg/L pyrene condition showed escalated levels of desirable fatty acids in Selenastrum sp., compared to the control. Further, Selenastrum sp. and other freshwater microalgae are catalogued for sustainable development goals attainment by 2030, as per the UNSDG (United Nations Sustainable Development Goals) agenda. Critical applications for the Selenastrum sp. in bioremediation of pyrene, along with biodiesel production, are enumerated for sustainable and renewable energy production and resource management.

Removal of polycyclic aromatic hydrocarbons (PAHs) from produced water using the microalgae Chlorella vulgaris cultivated in mixotrophic and heterotrophic conditions

Chlorella vulgaris was cultivated for 15 days in 10 different treatments under mixotrophic and heterotrophic conditions, using wastewater from oil and poultry industries as the culture medium. The blends were made with produced water (PW), sterilized produced water (PWs), sterilized poultry wastewater (PoWs), sterilized seawater (SWs), and the addition of sodium nitrate to evaluate cell growth in treatments and the removal of PAHs. The heterotrophic condition showed more effective removal, having an initial concentration of 3.93 μg L-1 and a final concentration of 0.57 μg L-1 of total PAHs reporting 83%, during phycoremediation of (PW) than the mixotrophic condition, with an initial concentration of 3.93 μg L-1 and a final concentration of 1.96 and 43% removal for the PAHs. In the heterotrophic condition, the blend with (PWs + SWs) with an initial concentration of 0.90 μg L-1 and a final concentration of 0.32 μg L-1 had 64% removal of total PAHs compared to the mixotrophic condition with 37% removal having an initial concentration of 0.90 μg L-1 and a final concentration of 0.56 μg L-1. However, the best result in the mixotrophic condition was obtained using a blend of (PWs + PoWs) that had an initial cell concentration of 1.18 × 105 cells mL-1 and reached a final cell concentration of 4.39 × 105 cells mL-1, an initial concentration of 4.76 μg L-1 and a final concentration of 0.37 μg L-1 having a 92% total removal of PAHs. The biostimulation process increased the percentage of PAHs removal by 45% (PW) in the mixotrophic condition. This study showed that it is possible to allow an environmental remediation strategy that significantly reduces effluent toxicity and generates high value-added biomass in contaminated effluents rich in nutrients and carbon, based on a circular bioeconomy model.

Emerging organic contaminants in the soil-plant-receptor continuum: transport, fate, health risks, and removal mechanisms

There is a lack of comprehensive reviews tracking emerging organic contaminants (EOCs) within the soil-plant continuum using the source-pathway-receptor-impact-mitigation (SPRIM) framework. Therefore, this review examines existing literature to gain insights into the occurrence, behaviour, fate, health hazards, and strategies for mitigating EOCs within the soil-plant system. EOCs identified in the soil-plant system encompass endocrine-disrupting chemicals, surfactants, pharmaceuticals, personal care products, plasticizers, gasoline additives, flame retardants, and per- and poly-fluoroalkyl substances (PFAS). Sources of EOCs in the soil-plant system include the land application of biosolids, wastewater, and solid wastes rich in EOCs. However, less-studied sources encompass plastics and atmospheric deposition. EOCs are transported from their sources to the soil-plant system and other receptors through human activities, wind-driven processes, and hydrological pathways. The behaviour, persistence, and fate of EOCs within the soil-plant system are discussed, including sorption, degradation, phase partitioning, (bio)transformation, biouptake, translocation, and bioaccumulation in plants. Factors governing the behaviour, persistence, and fate of EOCs in the soil-plant system include pH, redox potential, texture, temperature, and soil organic matter content. The review also discusses the environmental receptors of EOCs, including their exchange with other environmental compartments (aquatic and atmospheric), and interactions with soil organisms. The ecological health risks, human exposure via inhalation of particulate matter and consumption of contaminated food, and hazards associated with various EOCs in the soil-plant system are discussed. Various mitigation measures including removal technologies of EOCs in the soil are discussed. Finally, future research directions are presented.

Benzo(k)fluoranthene and benzo(b)fluoranthene degradation by Selenastrum capricornutum and identification of metabolites using HPLC-FD and HPLC-ESI-QqQ-MS/MS

Selenastrum capricornutum efficiently degrades high molecular weight polycyclic aromatic hydrocarbons (HMW PAHs). Until now, there are few studies on the benzo(k)fluoranthene (BkF) and benzo(b)fluoranthene (BbF) biodegradation by this microalga. For this reason, in the present work, extracts obtained from cultures of S. capricornutum were incubated with BkF and BbF individually, and analyzed by HPLC with fluorescence and different mass spectrometry detection modes: i) the HPLC-ESI(+)-MS/MS (MRM mode) analysis that confirmed the formation of monohydroxylated and dihydrodiol metabolites indicating that these PAHs could be simultaneously degraded through the monooxygenase and dioxygenase; ii) HPLC-ESI(+)-MS (full scan mode) that showed the formation of key metabolites containing four and two aromatic rings possibly resulting from aromatic ring-opening oxygenases, not known until now in microalgae; iii) HPLC-FD analysis that confirmed the individual BkF and BbF degradation occurring in extra- and intra-cellular extracts, indicating that an oxygenase enzyme complex is released by microalgae cells to the external environment to perform HMW PAHs biodegradation. So, this work presents new insights into the metabolic pathways of BkF and BbF biodegradation by S. capricornutum; likewise, the intra- and extra-cellular extracts of this microalgae have great potential to be applied in environmental procedures.

Microbes and microbial strategies in carcinogenic polycyclic aromatic hydrocarbons remediation: a systematic review

Degradation, detoxification, or removal of the omnipresent polycyclic aromatic hydrocarbons (PAHs) from the ecosphere as well as their prevention from entering into food chain has never appeared simple. In this context, cost-effective, eco-friendly, and sustainable solutions like microbe-mediated strategies have been adopted worldwide. With this connection, measures have been taken by multifarious modes of microbial remedial strategies, i.e., enzymatic degradation, biofilm and biosurfactant production, application of biochar-immobilized microbes, lactic acid bacteria, rhizospheric-phyllospheric-endophytic microorganisms, genetically engineered microorganisms, and bioelectrochemical techniques like microbial fuel cell. In this review, a nine-way directional approach which is based on the microbial resources reported over the last couple of decades has been described. Fungi were found to be the most dominant taxa among the CPAH-degrading microbial community constituting 52.2%, while bacteria, algae, and yeasts occupied 37.4%, 9.1%, and 1.3%, respectively. In addition to these, category-wise CPAH degrading efficiencies of each microbial taxon, consortium-based applications, CPAH degradation-related molecular tools, and factors affecting CPAH degradation are the other important aspects of this review in light of their appropriate selection and application in the PAH-contaminated environment for better human-health management in order to achieve a sustainable ecosystem.

Biodegradation of benzo[a]pyrene by a marine Chlorella vulgaris LH-1 with heterotrophic ability

In this study, a microalga, Chlorella vulgaris LH-1, with heterotrophic ability to degrade BaP was explored. The effect of BaP concentration on microalga growth was investigated, and the possible biodegradation mechanism of BaP was proposed. Results showed that low BaP concentration (<5 mg/L) had less negative influence on the growth of this microalga under mixotrophic condition, but high BaP concentration (>5 mg/L) had a significant inhibitory effect on its growth. During heterotrophic cultivation, low BaP concentration (<20 mg/L) promoted the growth of C. vulgaris LH-1, whereas high BaP concentration (>20 mg/L) inhibited its growth significantly. The degradation rates of mixotrophic and heterotrophic C. vulgaris LH-1 were 62.56 %-74.13 % and 52.07 %-71.67 %, respectively, when the BaP concentration ranged from 0.5 mg/L to 2 mg/L. The expression of functional enzyme genes of C. vulgaris LH-1 such as phenol 2-monooxygenase activity, protocatechuate 3,4-dioxygenase activity, catechol 1,2-dioxygenase activity, styrene degradation, and benzoate degradation were upregulated in the process of BaP degradation. C. vulgaris LH-1 may degrade BaP by monooxygenase and dioxygenase simultaneously. The degradation of BaP by this microalga under mixotrophic condition goes through the degradation pathway of phthalic acid, whereas it goes through the degradation pathway of benzoic acid under heterotrophic condition.

Exploring the efficacy of metabarcoding and non-target screening for detecting treated wastewater

Wastewater treatment processes can eliminate many pollutants, yet remainder pollutants contain organic compounds and microorganisms released into ecosystems. These remainder pollutants have the potential to adversely impact downstream ecosystem processes, but their presence is currently not being monitored. This study was set out with the aim of investigating the effectiveness and sensitivity of non-target screening of chemical compounds, 18S V9 rRNA gene, and full-length 16S rRNA gene metabarcoding techniques for detecting treated wastewater in receiving waters. We aimed at assessing the impact of introducing 33 % treated wastewater into a triplicated large-scale mesocosm setup during a 10-day exposure period. Discharge of treated wastewater significantly altered the chemical signature as well as the microeukaryotic and prokaryotic diversity of the mesocosms. Non-target screening, 18S V9 rRNA gene, and full-length 16S rRNA gene metabarcoding detected these changes with significant covariation of the detected pattern between methods. The 18S V9 rRNA gene metabarcoding exhibited superior sensitivity immediately following the introduction of treated wastewater and remained one of the top-performing methods throughout the study. Full-length 16S rRNA gene metabarcoding demonstrated sensitivity only in the initial hour, but became insignificant thereafter. The non-target screening approach was effective throughout the experiment and in contrast to the metabarcoding methods the signal to noise ratio remained similar during the experiment resulting in an increasing relative strength of this method. Based on our findings, we conclude that all methods employed for monitoring environmental disturbances from various sources are suitable. The distinguishing factor of these methods is their ability to detect unknown pollutants and organisms, which sets them apart from previously utilized approaches and allows for a more comprehensive perspective. Given their diverse strengths, particularly in terms of temporal resolution, these methods are best suited as complementary approaches.

Degradation of polycyclic aromatic hydrocarbons in aquatic environments by a symbiotic system consisting of algae and bacteria: green and sustainable technology

Polycyclic aromatic hydrocarbons (PAHs) are genotoxic, carcinogenic, and persistent in the environment and are therefore of great concern in the environmental protection field. Due to the inherent recalcitrance, persistence and nonreactivity of PAHs, they are difficult to remediate via traditional water treatment methods. In recent years, microbial remediation has been widely used as an economical and environmentally friendly degradation technology for the treatment of PAH-contaminated water. Various bacterial and microalgal strains are capable of potentially degrading or transforming PAHs through intrinsic metabolic pathways. However, their biodegradation potential is limited by the cytotoxic effects of petroleum hydrocarbons, unfavourable environmental conditions, and biometabolic limitations. To address this limitation, microbial communities, biochemical pathways, enzyme systems, gene organization, and genetic regulation related to PAH degradation have been intensively investigated. The advantages of algal-bacterial cocultivation have been explored, and the limitations of PAHs degradation by monocultures of algae or bacteria have been overcome by algal-bacterial interactions. Therefore, a new model consisting of a "microalgal-bacterial consortium" is becoming a new management strategy for the effective degradation and removal of PAHs. This review first describes PAH pollution control technologies (physical remediation, chemical remediation, bioremediation, etc.) and proposes an algal-bacterial symbiotic system for the degradation of PAHs by analysing the advantages, disadvantages, and PAH degradation performance in this system to fill existing research gaps. Additionally, an algal-bacterial system is systematically developed, and the effects of environmental conditions are explored to optimize the degradation process and improve its technical feasibility. The aim of this paper is to provide readers with an effective green and sustainable remediation technology for removing PAHs from aquatic environments.

Microbiome based approaches for the degradation of polycyclic aromatic hydrocarbons (PAHs): A current perception

Globally, polycyclic aromatic hydrocarbons (PAHs) pollution is primarily driven by their release into the air through various combustion processes, including burning fossil fuels such as coal, oil, and gas in motor vehicles, power plants, and industries, as well as burning organic matter like wood, tobacco, and food in fireplaces, cigarettes, and grills. Apart from anthropogenic pollution sources, PAHs also occur naturally in crude oil, and their potential release during oil extraction, refining processes, and combustion further contributes to contamination and pollution concerns. PAHs are resistant and persistent in the environment because of their inherent features, viz., heterocyclic aromatic ring configurations, hydrophobicity, and thermostability. A wide range of microorganisms have been found to be effective degraders of these recalcitrant contaminants. The presence of hydrocarbons as a result of numerous anthropogenic activities is one of the primary environmental concerns. PAHs are found in soil, water, and the air, making them ubiquitous in nature. The presence of PAHs in the environment creates a problem, as their presence has a detrimental effect on humans and animals. For a variety of life forms, PAH pollutants are reported to be toxic, carcinogenic, mutation-inducing, teratogenic, and immune toxicogenics. Degradation of PAHs via biological activity is an extensively used approach in which diverse microorganisms (fungal, algal, clitellate, and protozoan) and plant species and their derived composites are utilized as biocatalysts and biosurfactants. Some microbes have the ability to transform and degrade these PAHs, allowing them to be removed from the environment. The goal of this review is to provide a critical overview of the existing understanding of PAH biodegradation. It also examines current advances in diverse methodologies for PAH degradation in order to shed light on fundamental challenges and future potential.

Efficiency, mechanism, influencing factors, and integrated technology of biodegradation for aromatic compounds by microalgae: A review

Aromatic compounds have received widespread attention because of their threat to ecosystem and human health. However, traditional physical and chemical methods are criticized due to secondary pollution and high cost. As a result of ecological security and the ability of carbon sequestration, biodegradation approach based on microalgae has emerged as a promising alternative treatment for aromatic pollutants. In light of the current researches, the degradation efficiency of BTEX (benzene, toluene, ethylbenzene, and xylene), polycyclic aromatic hydrocarbons (PAHs), and phenolic compounds by microalgae was reviewed in this study. We summarized the degradation pathways and metabolites of p-xylene, benzo [a]pyrene, fluorene, phenol, bisphenol A, and nonylphenol by microalgae. The influence factors on the degradation of aromatic compounds by microalgae were also discussed. The integrated technologies based on microalgae for degradation of aromatic compounds were reviewed. Finally, this study discussed the limitations and future research needs of the degradation of these compounds by microalgae.

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

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

Harnessing Diesel-Degrading Potential of an Antarctic Microalga from Greenwich Island and Its Physiological Adaptation

Microalgae are well known for their metal sorption capacities, but their potential in the remediation of hydrophobic organic compounds has received little attention in polar regions. We evaluated in the laboratory the ability of an Antarctic microalga to remediate diesel hydrocarbons and also investigated physiological changes consequent upon diesel exposure. Using a polyphasic taxonomic approach, the microalgal isolate, WCY_AQ5_1, originally sampled from Greenwich Island (South Shetland Islands, maritime Antarctica) was identified as Tritostichococcus sp. (OQ225631), a recently erected lineage within the redefined Stichococcus clade. Over a nine-day experimental incubation, 57.6% of diesel (~3.47 g/L) was removed via biosorption and biodegradation, demonstrating the strain's potential for phytoremediation. Fourier transform infrared spectroscopy confirmed the adsorption of oil in accordance with its hydrophobic characteristics. Overall, degradation predominated over sorption of diesel. Chromatographic analysis confirmed that the strain efficiently metabolised medium-chain length n-alkanes (C-7 to C-21), particularly n-heneicosane. Mixotrophic cultivation using diesel as the organic carbon source under a constant light regime altered the car/chl-a ratio and triggered vacuolar activities. A small number of intracellular lipid droplets were observed on the seventh day of cultivation in transmission electron microscopic imaging. This is the first confirmation of diesel remediation ability in an Antarctic green microalga.

The Utilisation of Antarctic Microalgae Isolated from Paradise Bay (Antarctic Peninsula) in the Bioremediation of Diesel

Research has confirmed that the utilisation of Antarctic microorganisms, such as bacteria, yeasts and fungi, in the bioremediation of diesel may provide practical alternative approaches. However, to date there has been very little attention towards Antarctic microalgae as potential hydrocarbon degraders. Therefore, this study focused on the utilisation of an Antarctic microalga in the bioremediation of diesel. The studied microalgal strain was originally obtained from a freshwater ecosystem in Paradise Bay, western Antarctic Peninsula. When analysed in systems with and without aeration, this microalgal strain achieved a higher growth rate under aeration. To maintain the growth of this microalga optimally, a conventional one-factor-at a-time (OFAT) analysis was also conducted. Based on the optimized parameters, algal growth and diesel degradation performance was highest at pH 7.5 with 0.5 mg/L NaCl concentration and 0.5 g/L of NaNO₃ as a nitrogen source. This currently unidentified microalga flourished in the presence of diesel, with maximum algal cell numbers on day 7 of incubation in the presence of 1% v/v diesel. Chlorophyll a, b and carotenoid contents of the culture were greatest on day 9 of incubation. The diesel degradation achieved was 64.5% of the original concentration after 9 days. Gas chromatography analysis showed the complete mineralisation of C₇-C₁₃ hydrocarbon chains. Fourier transform infrared spectroscopy analysis confirmed that strain WCY_AQ5_3 fully degraded the hydrocarbon with bioabsorption of the products. Morphological and molecular analyses suggested that this spherical, single-celled green microalga was a member of the genus Micractinium. The data obtained confirm that this microalga is a suitable candidate for further research into the degradation of diesel in Antarctica.

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.

Electrophoretic characterization of cellular and extracellular proteins from Selenastrum capricornutum cultures degrading benzo(a)pyrene and their identification by UPLC-ESI-TOF mass spectrometry

Selenastrum capricornutum efficiently degrades benzo(a)pyrene (BaP) but few proteins related to BaP degradation have been identified in this microalgae. So far, it has only been suggested that it could degrade BaP via the monooxygenase and/or dioxygenase pathways. To know more about this fact, in this work, cultures of S. capricornutum incubated with BaP were used to obtain the molecular weights (MWs) of proteins existing in its extra- and cellular extracts by electrophoresis and UPLC-ESI(+)-TOF MS analysis. The results of this proteomic approach indicated that BaP markedly induces the MWs: 6-20, 30, 45, and 65 kDa in cells; 6-20, 30.3, 38-45, and 55 kDa in liquid medium. So, these proteins could be related to BaP biodegradation. An identified protein with monooxygenase activity and rubredoxins (Rds) show to be related to BaP degradation: Rds could participate, together with the monooxygenase in the electron transfer during the formation of monohydroxylated-BaP metabolites. Rds may be also associated with a dioxygenase system that degrades BaP to form dihydrodiol-BaP metabolites. A multi-pass membrane protein was identified too, and it can regulate the transport of molecules like enzymes from inside the cell to the outside environment. At the same time, the presence of a dihydrolipoamide acetyltransferase validated the stress caused by the exposure to BaP. It is noteworthy that these findings provide valuable and original information on the characterization of the proteins of S. capricornutum cultures degrading BaP, whose enzymes have so far not been known. It is important to highlight that the functions of the identified proteins can help in understanding the metabolic and environmental behavior of this microalgae, and the extracts containing the degrading enzymes could be utilized in bioremediation applications.

Advancement in algal bioremediation for organic, inorganic, and emerging pollutants

Rapidly changing bioremediation prospects are key drive to develop sustainable options that can offer extra benefits rather than only environmental remediation. Algal remediating is gaining utmost attention due to its mesmerising sustainable features, removing odour and toxicity, co-remediating numerous common and emerging inorganic and organic pollutants from gaseous and aqueous environments, and yielding biomass for a range of valuable products refining. Moreover, it also improves carbon footprint via carbon-capturing offers a better option than any other non-algal process for several high CO₂-emitting industries. Bio-uptake, bioadsorption, photodegradation, and biodegradation are the main mechanisms to remediate a range of common and emerging pollutants by various algae species. Bioadsorption was a dominant remediation mechanism among others implicating surface properties of pollutants and algal cell walls. Photodegradable pollutants were photodegraded by microalgae by adsorbing photons on the surface and intracellularly via stepwise photodissociation and breakdown. Biodegradation involves the transportation of selective pollutants intracellularly, and enzymes help to convert them into simpler non-toxic forms. Robust models are from the green microalgae group and are dominated by Chlorella species. This article compiles the advancements in microalgae-assisted pollutants remediation and value-addition under sustainable biorefinery prospects. Moreover, filling the knowledge gaps, and recommendations for developing an effective platform for emerging pollutants remediation and realization of commercial-scale algal bioremediation.

Transformation and toxicity dynamics of polycyclic aromatic hydrocarbons in a novel biological-constructed wetland-microalgal wastewater treatment process

In this study, a novel wastewater treatment process combining sequencing batch reactor, constructed wetland and microalgal membrane photobioreactor (BCM process) was proposed, and its performance on removal, transformation and toxicity reduction of polycyclic aromatic hydrocarbons (PAHs) was intensively explored. Satisfactory PAHs removal (90.58%-97.50%) was achieved and molecular weight had significant impact on the removal pathways of different PAHs. Adsorption dominated the removal of high molecular weight PAHs, while the contribution ratio of microbial degradation increased with the decrease of molecular weight of PAHs. More importantly, it was reported for the first time that substituted PAHs (SPAHs) produced by microbial degradation of PAHs would lead to increased toxicity during the BCM process. High PAHs (75.37%-88.52%) and SPAHs removal (99.56%-100.00%) were achieved in the microalgae unit due to its abundant cytochrome P450 enzyme, which decreased the bacterial toxicity by 90.93% and genotoxicity by 93.08%, indicating that microalgae played significance important role in ensuring water security. In addition, the high quantitative relationship (R² = 0.98) between PAHs, SPAHs and toxicity exhibited by regression model analysis proved that more attention should be paid to the ecotoxicity of derivatives of refractory organic matters in wastewater treatment plants.

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.

Phycoremediation of wastewater for pollutant removal: A green approach to environmental protection and long-term remediation

Surface and water bodies in many parts of the world are affected due to eutrophication, contamination and depletion. The approach of wastewater treatment using algae for eliminating nutrients and other pollutants from domestic wastewater is growing interest among the researchers. However, sustainable treatment of the wastewater is considered to be important in establishing more effective nutrient and pollutant reduction using algal systems. In comparison to the conventional method of remediation, there are opportunities to commercially viable businesses interest with phycoremediation, thus by achieving cost reductions and renewable bioenergy options. Phycoremediation is an intriguing stage for treating wastewater since it provides tertiary bio-treatment while producing potentially valuable biomass that may be used for a variety of applications. Furthermore, the phycoremediation provides the ability to remove heavy metals as well as harmful organic substances, without producing secondary contamination. In this review, the role of microalgae in treating different wastewaters and the process parameters affecting the treatment and future scope of research have been discussed. Though several algae are employed for wastewater treatment, species of the genera Chlamydomonas, Chlorella, and Scenedesmus are extensively utilized. Interestingly, there is a vast scope for employing algal species with high flocculation capacity and adsorption mechanisms for the elimination of microplastics. In addition, the algal biomass generated during phycoremediation has been found to possess high protein and lipid contents, promising their exploitation in biofuel, food and animal feed industries.

A review on the enzymes and metabolites identified by mass spectrometry from bacteria and microalgae involved in the degradation of high molecular weight PAHs

High molecular weight PAHs (HMW PAHs) are dangerous pollutants widely distributed in the environment. The use of microorganisms represents an important tool for HMW PAHs bioremediation, so, the understanding of their biochemical pathways facilitates the development of biodegradation strategies. For this reason, the potential role of species of microalgae, bacteria, and microalga-bacteria consortia in the degradation of HMW PAHs is discussed. The identification of their metabolites, mostly by GC-MS and LC-MS, allows a better approach to the enzymes involved in the key steps of the metabolic pathways of HMW PAHs biodegradation. So, this review intends to address the proteomic research on enzyme activities and their involvement in regulating essential biochemical functions that help bacteria and microalgae in the biodegradation processes of HMW PAHs. It is noteworthy that, given that to the best of our knowledge, this is the first review focused on the mass spectrometry identification of the HMW PAHs metabolites; whereby and due to the great concern of the presence of HMW PAHs in the environment, this material could help the urgency of developing new bioremediation methods. The elucidation of the metabolic pathways of persistent pollutant degrading microorganisms should lead to a better knowledge of the enzymes involved, which could contribute to a very ecological route to the control of environmental contamination in the future.

Semi-continuous monitoring of HMWPAH in microalgae cultures by PT-SPE/HPLC/FD-UV: Estimation of the degradation constant

A degradation study has been performed with Selenastrum capricornutum incubated with benzo[a]anthracene and benzo[a]pyrene at 50, 100 and 266 ng mL^-1 in liquid cultures. After incubation, these high molecular weight polycyclic aromatic hydrocarbons (HMW PAH) were extracted from both, the medium and biomass in a single step, and then quantified by a sensitive and validated analytical methodology based on pipette-tip SPE and HPLC with fluorescence and UV detection (PT-SPE/HPLC/FD-UV). The methodology presented good linearity r² > 0.99, LOD of 0.9 and 0.7 ng mL^-1 for BaA and BaP, respectively. A fast and semi-continuous appreciation of the degradation behavior was achieved. The pollutants were monitored at different times (0.5-18 h) in the same culture flask, with sampling volume of 1 mL. Biodegradation percentages close to-90% were observed at 18 h. The degradation curves were fitted to the first order reaction (r² > 0.95) and the degradation rate constants were similar in all bioassays (0.1 h^-1) and independent of concentration and compound. The degradation pathways of HMW PAH by microalgae and their enzyme are poorly known but the hypothesis of the degrading enzyme proportionally activated according to the PAH concentration is supported by this result. The early emergence dihydrodiol-type metabolites were detected.

Bacteria, Fungi and Microalgae for the Bioremediation of Marine Sediments Contaminated by Petroleum Hydrocarbons in the Omics Era

Petroleum hydrocarbons (PHCs) are one of the most widespread and heterogeneous organic contaminants affecting marine ecosystems. The contamination of marine sediments or coastal areas by PHCs represents a major threat for the ecosystem and human health, calling for urgent, effective, and sustainable remediation solutions. Aside from some physical and chemical treatments that have been established over the years for marine sediment reclamation, bioremediation approaches based on the use of microorganisms are gaining increasing attention for their eco-compatibility, and lower costs. In this work, we review current knowledge concerning the bioremediation of PHCs in marine systems, presenting a synthesis of the most effective microbial taxa (i.e., bacteria, fungi, and microalgae) identified so far for hydrocarbon removal. We also discuss the challenges offered by innovative molecular approaches for the design of effective reclamation strategies based on these three microbial components of marine sediments contaminated by hydrocarbons.

Past, present, and future perspectives on the assessment of bioavailability/bioaccessibility of polycyclic aromatic hydrocarbons: A 20-year systemic review based on scientific econometrics

Bioaccessibility/bioavailability (bioac-bioav) is an important criterion in the risk assessment of polycyclic aromatic hydrocarbons (PAHs), especially in the restoration of contaminated sites. Although, the bioac-bioav concept is widely employed in PAH risk assessment for both humans and wildlife, their growth and integration in risk assessment models are seldom discussed. Consequently, the relevant literature listed on Web of Science (WOS)™ was retrieved and analyzed using the bibliometric software Citespace in order to gain a comprehensive understanding of this issue. Due to the limitations of the literature search software, we manually searched the articles about PAHs bioac-bioav that were published before 2000. This stage focuses on research on the distribution coefficient of PAHs between different environmental phases and laid the foundation for the adsorption-desorption of PAHs in subsequent studies of the bioac-bioav of PAHs. The research progress on PAH bioac-bioav from 2000 to the present was evaluated using the Citespace software based on country- and discipline-wise publication volumes and research hotspots. The development stages of PAH bioac-bioav after 2000 were divided into four time segments. The first three segments (2000-2005, 2006-2010, and 2011-2015) focused on the degradation of PAHs and their in vivo (bioavailability)-in vitro (bioaccessibility) evaluation method and risk assessment. Meanwhile, the current (2016-present) research focuses on the establishment of analytical methods for assessing PAH derivatives at environmental concentrations and the optimization of various in vitro digestion methods, including chemical optimization (sorptive sink) and biological optimization (Caco-2 cell). The contents are aimed at supplying researchers with a deeper understanding of the development of PAH bioac-bioav.

Chemo-metric engineering designs for deciphering the biodegradation of polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are non-polar organic compounds that are omnipresent in the environment and released due to anthropogenic activities through emissions and discharges. PAHs, being xenobiotic and exerts health impacts, thus they attract serious concern by the environmentalists. The stringent regulations and the need of sustainable development urges the hunt for a technically feasible and cost-effective wastewater treatment. Although the conventional physico-chemical treatment are widely preferred, they cause secondary pollution problems and demand subsequent treatment options. This comprehensive review intends to address the (a) different PAHs and their associated toxicity, (b) the remedial strategies, particularly biodegradation. The biological wastewater treatment techniques that involve microbial systems are highly influenced by the different physio-chemical and environmental parameters. Therefore, suitable optimization techniques are prerequisite for effective functioning of the biological treatment that sustains judiciously and interpreted in a lesser time. Here we have aimed to discuss (a) different chemo-metric tools involved in the design of experiments (DoE), (b) design equations and models, (c) tools for evaluating the model's adequacy and (d) plots for graphically interpreting the chemo-metric designs. However, to best of our knowledge, this is a first review to discuss the PAHs biodegradation that are tailored by chemo-metric designs. The associated challenges, available opportunities and techno-economic aspects of PAHs degradation using chemo-metric engineering designs are explained. Additionally, the review highlights how well these DoE tools can be suited for the sustainable socio-industrial sectors. Concomitantly, the futuristic scope and prospects to undertake new areas of research exploration were emphasized to unravel the least explored chemo-metric designs.

Degradation of benzo[a]pyrene by halophilic bacterial strain Staphylococcus haemoliticus strain 10SBZ1A

The exploitation of petroleum oil generates a considerable amount of “produced water or petroleum waste effluent (PWE)” that is contaminated with polycyclic aromatic hydrocarbons (PAHs), including Benzo[a]pyrene (BaP). PWE is characterised by its high salinity, which can be as high as 30% NaCl, thus the exploitation of biodegradation to remove PAHs necessitates the use of active halophilic microbes. The strain 10SBZ1A was isolated from oil contaminated soils, by enrichment experiment in medium containing 10% NaCl (w/v). Homology analyses of 16S rRNA sequences identified 10SBZ1A as a Staphylococcus haemoliticus species, based on 99.99% homology (NCBI, accession number GI: MN388897). The strain could grow in the presence of 4–200 μmol l^-1 of BaP as the sole source of carbon, with a doubling time of 17–42 h. This strain optimum conditions for growth were 37 ^oC, 10% NaCl (w/v) and pH 7, and under these conditions, it degraded BaP at a rate of 0.8 μmol l^-1 per day. The strain 10SBZ1A actively degraded PAHs of lower molecular weights than that of BaP, including pyrene, phenanthrene, anthracene. This strain was also capable of removing 80% of BaP in the context of soil spiked with BaP (10 μmol l^-1 in 100 g of soil) within 30 days. Finally, a metabolic pathway of BaP was proposed, based on the identified metabolites using liquid chromatography-high resolution tandem mass spectrometry. To the best of our knowledge, this is the first report of a halophilic BaP degrading bacterial strain at salinity > 5% NaCl.

Polycyclic Aromatic Hydrocarbons: Sources, Toxicity, and Remediation Approaches

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

The potential assessment of green alga Chlamydomonas reinhardtii CC-503 in the biodegradation of benz(a)anthracene and the related mechanism analysis

Benz(a)anthracene (BaA) is a polycyclic aromatic hydrocarbons (PAHs), that belongs to a group of carcinogenic and mutagenic persistent organic pollutants found in a variety of ecological habitats. In this study, the efficient biodegradation of BaA by a green alga Chlamydomonas reinhardtii (C. reinhardtii) CC-503 was investigated. The results showed that the growth of C. reinhardtii was hardly affected with an initial concentration of 10 mg/L, but was inhibited significantly under higher concentrations of BaA (>30 mg/L) (p < 0.05). We demonstrated that the relatively high concentration of 10 mg/L BaA was degraded completely in 11 days, which indicated that C. reinhardtii had an efficient degradation system. During the degradation, the intermediate metabolites were determined to be isomeric phenanthrene or anthracene, 2,6-diisopropylnaphthalene, 1,3-diisopropylnaphthalene, 1,7-diisopropylnaphthalene, and cyclohexanol. The enzymes involved in the degradation included the homogentisate 1,2-dioxygenase (HGD), the carboxymethylenebutenolidase, the ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and the ubiquinol oxidase. The respective genes encoding these proteins were significantly up-regulated ranging from 3.17 fold to 13.03 fold and the activity of enzymes, such as HGD and Rubisco, was significantly induced up to 4.53 and 1.46 fold (p < 0.05), during the BaA metabolism. This efficient degradation ability suggests that the green alga C. reinhardtii CC-503 may be a sustainable candidate for PAHs remediation.

Bioremediation of Phenolic Waters using the Microalgae Chlamydomonas Reinhardtii

In this research, the capacity of biodegradation of phenolic compounds using the microalgae Chlamydomonas reinhardtii was evaluated, as well as the impact of the contaminant on the growth of the microalgae. The bioassays consisted on three phenol concentrations: 25 mgL/L, 50 mgL/L and 100 mg/L and a control solution. Bioremediation rates of approximately 99% were obtained, showing no significant difference between medium and high concentration bioassays. A good microalgae growth rate was obtained for the tests at low and medium concentration, they were above the control bioassay, but the high concentration bioassay showed an inhibition in the cell growth, this one presented growth below the control bioassay. The use of the microalgae Chlamydomonas reinhardtii in bioremediation processes of phenolic effluents, whose concentrations are below 100 mg/L, is proposed for future research.

Benzo(a)pyrene degradation by cytochrome P450 hydroxylase and the functional metabolism network of Bacillus thuringiensis

The relationship between benzo(a)pyrene biodegradation and certain target biomolecules has been investigated. To regulate the degradation process, the associated metabolism network must be clarified. To this end, benzo(a)pyrene degradation, carbon substrate metabolism and exometabolomic mechanism of Bacillus thuringiensis were analyzed. Benzo(a)pyrene was degraded through hydroxylation catalyzed by cytochrome P450 hydroxylase. After the treatment of 0.5 mg L^-1 of benzo(a)pyrene by 0.2 g L^-1 of cells for 9 d, biosorption and degradation efficiencies were measured at approximately 90% and 80%, respectively. During this process, phospholipid synthesis, glycogen, asparagine, arginine, itaconate and xylose metabolism were significantly downregulated, while glycolysis, pentose phosphate pathway, citrate cycle, amino sugar and nucleotide sugar metabolism were significantly upregulated. These findings offer insight into the biotransformation regulation of polycyclic aromatic hydrocarbons.

Removal and Biodegradation of 17β-Estradiol and Diethylstilbestrol by the Freshwater Microalgae Raphidocelis subcapitata

Natural steroidal and synthetic non-steroidal estrogens such as 17β-estradiol (E2) and diethylstilbestrol (DES) have been found in natural water, which can potentially endanger public health and aquatic ecosystems. The removal and biodegradation of E2 and DES by Raphidocelis subcapitata were studied in bacteria-free cultures exposed to single and mixture treatments at different concentrations for 96 h. The results showed that R. subcapitata exhibited a rapid and strong ability to remove E2 and DES in both single and mixture treatments by biodegradation. At the end of 96 h, the removal percentage of single E2 and DES achieved 82.0%, 80.4%, 74.6% and 89.9%, 73.4%, 54.1% in 0.1, 0.5, and 1.5 mg·L^-1, respectively. With the exception of the 0.1 mg·L^-1 treatment at 96 h, the removal capacity of E2 was more efficient than that of DES by R. subcapitata. Furthermore, the removal percentage of mixture E2 and DES achieved 88.5%, 82.9%, 84.3% and 87.2%, 71.8%, 51.1% in 0.1, 0.5, and 1.5 mg·L^-1, respectively. The removal percentage of mixed E2 was significantly higher than that of the single E2. The presence of DES could accelerate the removal of E2 from the mixture treatments in equal concentrations. In addition, the removal was mainly attributed to the biodegradation or biotransformation process by the microalgae cells rather than simple sorption and accumulation in the cells. The microalgae R. subcapitata demonstrated a high capability for the removal of the E2 and DES indicating future prospects for its application.

Remediation approaches for polycyclic aromatic hydrocarbons (PAHs) contaminated soils: Technological constraints, emerging trends and future directions

For more than a decade, the primary focus of environmental experts has been to adopt risk-based management approaches to cleanup PAH polluted sites that pose potentially destructive ecological consequences. This focus had led to the development of several physical, chemical, thermal and biological technologies that are widely implementable. Established remedial options available for treating PAH contaminated soils are incineration, thermal conduction, solvent extraction/soil washing, chemical oxidation, bioaugmentation, biostimulation, phytoremediation, composting/biopiles and bioreactors. Integrating physico-chemical and biological technologies is also widely practiced for better cleanup of PAH contaminated soils. Electrokinetic remediation, vermiremediation and biocatalyst assisted remediation are still at the development stage. Though several treatment methods to remediate PAH polluted soils currently exist, a comprehensive overview of all the available remediation technologies to date is necessary so that the right technology for field-level success is chosen. The objective of this review is to provide a critical overview in this respect, focusing only on the treatment options available for field soils and ignoring the spiked ones. The authors also propose the development of novel multifunctional green and sustainable systems like mixed cell culture system, biosurfactant flushing, transgenic approaches and nanoremediation in order to overcome the existing soil- contaminant- and microbial-associated technological limitations in tackling high molecular weight PAHs. The ultimate objective is to ensure the successful remediation of long-term PAH contaminated soils.

Monitoring dihydrodiol polyaromatic hydrocarbon metabolites produced by the freshwater microalgae Selenastrum capricornutum

We found that microalgae exposed to a mixture of polycyclic aromatic hydrocarbons (PAHs) did not show growth inhibition. Thus, we assumed that they could metabolize these compounds. In this study, the dihydrodiol-type PAH metabolites of benzo(a)pyrene (BaP), benzo(a)anthracene (BaA), benzo(b)fluoranthene (BbF) and benzo(k)fluoranthene (BkF) produced by the freshwater microalgae Selenastrum capricornutum were monitored and quantified using high-performance liquid chromatography with fluorescence detection (HPLC-FD) techniques. Exposure bioassays with S. capricornutum were performed using a 266 ng mL(-1) mixture of PAHs at different exposure times (0.75, 1, 3, 8, 16, 24 and 48 h) under controlled temperature (25 °C); the dihydrodiol metabolites formed in the liquid medium and the biomass were quantified. Metabolite identities were confirmed using HPLC-mass spectrometry; most of the metabolites formed were derived from BaA degradation. At 48 h after exposure 5,6-dBaA and 8,9-dBaA/10,11-dBaA were present in the liquid medium at 20% and 67% of the initial mass of BaA, respectively. Three metabolites of BaP were monitored in the liquid medium and biomass and, at 24 h, 4,5-dBaP accounted for 19%; , 7,8-dBaP, 5%; and 9,10-dBaP, 5% relative to the initial BaP mass. Microalgae exposed to BbF showed the presence of 1,2-dBbF and 9,10-dBbF (at 0.3% and 0.1% of the initial BbF mass, respectively) and those exposed to BkF produced 8,9-dBkF (6.5% of the initial BkF mass) in the liquid medium. Seven unknown compounds were formed after exposure; two compounds were identified as the metabolites of BaA and BaP. The results could facilitate the elucidation of the controversial biodegradation mechanism in microalgae.

Current State of Knowledge in Microbial Degradation of Polycyclic Aromatic Hydrocarbons (PAHs): A Review

Polycyclic aromatic hydrocarbons (PAHs) include a group of organic priority pollutants of critical environmental and public health concern due to their toxic, genotoxic, mutagenic and/or carcinogenic properties and their ubiquitous occurrence as well as recalcitrance. The increased awareness of their various adverse effects on ecosystem and human health has led to a dramatic increase in research aimed toward removing PAHs from the environment. PAHs may undergo adsorption, volatilization, photolysis, and chemical oxidation, although transformation by microorganisms is the major neutralization process of PAH-contaminated sites in an ecologically accepted manner. Microbial degradation of PAHs depends on various environmental conditions, such as nutrients, number and kind of the microorganisms, nature as well as chemical property of the PAH being degraded. A wide variety of bacterial, fungal and algal species have the potential to degrade/transform PAHs, among which bacteria and fungi mediated degradation has been studied most extensively. In last few decades microbial community analysis, biochemical pathway for PAHs degradation, gene organization, enzyme system, genetic regulation for PAH degradation have been explored in great detail. Although, xenobiotic-degrading microorganisms have incredible potential to restore contaminated environments inexpensively yet effectively, but new advancements are required to make such microbes effective and more powerful in removing those compounds, which were once thought to be recalcitrant. Recent analytical chemistry and genetic engineering tools might help to improve the efficiency of degradation of PAHs by microorganisms, and minimize uncertainties of successful bioremediation. However, appropriate implementation of the potential of naturally occurring microorganisms for field bioremediation could be considerably enhanced by optimizing certain factors such as bioavailability, adsorption and mass transfer of PAHs. The main purpose of this review is to provide an overview of current knowledge of bacteria, halophilic archaea, fungi and algae mediated degradation/transformation of PAHs. In addition, factors affecting PAHs degradation in the environment, recent advancement in genetic, genomic, proteomic and metabolomic techniques are also highlighted with an aim to facilitate the development of a new insight into the bioremediation of PAH in the environment.