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New Uses of Haloarchaeal Species in Bioremediation ProcessesMaria Jose Bonete1, Vanesa Bautista1, Julia Esclapez1, Maria Jose Garcia-Bonete1, Carmen Pire1, Monica Camacho1, Javier Torregrosa-Crespo1 and Rosa Maria Martinez-Espinosa1[1] Departamento de Agroquimica y Bioquimica. We will be provided with an authorization token (please note: passwords are not shared with us) and will sync your accounts for you. Investigators have observed genetic defects in individuals with the familial forms of the disease. The reviews by Bertram and Tanzi (2008) as well as Tanzi and Bertram (2005) have elucidated the genetic origins for the familial and the late onset forms of the disease. Indeed, biochemical and epidemiological studies now indicate that age is the dominant risk factor in sporadic forms of AD. The extrapolation of the model of early onset forms of AD to the late onset expressions is based on the assumption that age is primarily a measure of how long it takes for neurons to accumulate toxic moieties of beta amyloid.
These anomalies, and the consistent failure of clinical trials designed to assess the efficacy of therapeutic strategies, have stimulated various efforts to modify the premises of the model. The amyloid hypothesis considers the sporadic forms of AD, as a disease determined primarily by the instability of the nuclear genome. The efficiency of these two processes is highly dependent on the activity of the enzymes involved in the metabolic reactions.
The model for the sporadic forms of AD proposed in Demetrius and Simon (2012) as well as in Demetrius and Driver (2013), implicates these two factors, energy and age, as the critical elements in the origin of neurodegenerative diseases. This neuroenergetic perspective posits that the primary cause of sporadic forms of AD is an age-induced energy deficit in the mitochondrial activity of neurons, and the up-regulation of oxidative phosphorylation as a compensatory mechanism of energy production to maintain the viability of the impaired cells. Oxidative phosphorylation and glycolysis are complementary mechanisms which cells utilize to meet their energy demands.
The cornerstone of the neuroenergetic perspective is the competition for oxidative energy substrates between healthy neurons, that is neurons with normal OxPhos activity, and impaired neurons, cells with up-regulated OxPhos activity.
This paper will review the theoretical and empirical support for the Inverse Warburg hypothesis. We will show that these three criteria are inconsistent with the amyloid hypothesis but concord with the predictions of the Inverse Warburg model. The article is organized as follows: Section Bioenergetics outlines in quantitative terms the two principal modes of energy production, oxidative phosphorylation and glycolysis, involved in brain metabolism. Our model for the origin of sporadic forms of AD postulates that energy and age are the two critical elements which drive the metabolic processes which culminate in neuronal loss, the histopathological hallmark of AD. The main energy currency in living organisms is ATP which must be continually available to maintain cell viability. Quantum metabolism (Demetrius et al., 2010), an analytic theory of bioenergetics, provides a framework for deriving expressions for the metabolic rate of cells, and the dependence of this rate on the mechanism of energy transduction, OxPhos, or glycolysis.
The proportionality constant α is contingent on the mechanism of energy transduction, OxPhos or glycolysis. Figure 3 shows the energy production associated with the conversion of glucose to pyruvate and with the complete oxidation of pyruvate to carbon dioxide and water as it occurs in aerobic cells relying on glucose as their main energy substrate. The degree of organization of the enzymes in the cytosol and the respiratory chain enzymes imposes constraints on the metabolic rate generated by glycolysis and oxidative phosphorylation, respectively. Previous (Citric acid)Next (Citrus)The citric acid cycle (also known as the tricarboxylic acid cycle, TCA cycle, and as the Krebs cycle) is a series of chemical reactions of central importance in all living cells that utilize oxygen to generate useful energy by cellular respiration.
In aerobic organisms, the citric acid cycle is a metabolic pathway that forms part of the breakdown of carbohydrates, fats and proteins into carbon dioxide and water in order to generate energy. The citric acid cycle also provides precursors for many compounds, such as certain amino acids, and some of its reactions are important in cells performing fermentation reactions in the absence of oxygen.
This key metabolic cycle was established very early in the unfolding plan of creation as the molecules involved, and the set of enzymes that run the cycle, are essentially the same in all bacteria, fungi, plants, and animals. The citric acid cycle is the focus of attention of both those advocating design by a supreme being and those opposing such design.
The citric acid cycle is also known as the Krebs Cycle in honor of Sir Hans Adolf Krebs (1900 - 1981), who proposed the key elements of this pathway in 1937, and was awarded the Nobel Prize in Medicine for its discovery in 1953. In essence, the citric acid cycle plays a central role in the manipulation of small carbon-oxygen-hydrogen molecules.
Running in one direction, the cycle constructs many basic molecules on which the rest of metabolism is based.
In practice, a cell runs billions of such cycles simultaneously, most in the energy-generating direction. In oxygen-using aerobic organisms, the citric acid cycle is the second step in the breakdown of carbohydrates, fats, and proteins into carbon dioxide and water in order to generate energy. In carbohydrate catabolism (the breakdown of sugars), the citric acid cycle follows glycolysis, which breaks down glucose (a six-carbon-molecule) into pyruvate (a three-carbon molecule). In protein catabolism, proteins are broken down by protease enzymes into their constituent amino acids. In fat catabolism, triglycerides are hydrolyzed to break them into fatty acids and glycerol. The total energy gained from the complete breakdown of one molecule of glucose by glycolysis, the citric acid cycle, and oxidative phosphorylation equals about 36 ATP molecules. The citric acid cycle takes place within the mitochondrial matrix in eukaryotes, and within the cytoplasm in prokaryotes. The color scheme is as follows: enzymes, coenzymes, substrate names, metal ions, inorganic molecules, inhibition, stimulation . Fuel molecule catabolism (including glycolysis) produces acetyl-CoA, a two-carbon acetyl group bound to coenzyme A.
A different enzyme catalyzes each of the eight stages in the citric acid cycle, meaning there are eight different enzymes used in the cycle. Two carbons are oxidized to CO2, and the energy from these reactions is stored in guanosine triphosphate (GTP), NADH and FADH2. A simplified view of the process: The process begins with pyruvate, producing one CO2, then one CoA.
Many of the enzymes in the TCA cycle are regulated by negative feedback from ATP when the energy charge of the cell is high. New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. Note: Some restrictions may apply to use of individual images which are separately licensed.
Primary, secondary (the tank contains the supernatant followed by primary treatment), and tertiary treatments used in wastewater treatment plants. The Haloferax, Halobacterium, and Halococcus strains isolated on the basis of crude oil bioremediation [51] also degraded n-alkanes and mono- and polyaromatic compounds. This means that you will not need to remember your user name and password in the future and you will be able to login with the account you choose to sync, with the click of a button. This page doesn't support Internet Explorer 6, 7 and 8.Please upgrade your browser or activate Google Chrome Frame to improve your experience. The present article appeals to these hallmarks to evaluate and contrast two competing models of AD: the amyloid hypothesis (a neuron-centric mechanism) and the Inverse Warburg hypothesis (a neuron-astrocytic mechanism).
The genes implicated are the amyloid precursor protein (APP), and the secretases, presenilin 1 and presenilin 2, enzymes involved with APP processing.
These articles have documented and analyzed genes that are considered potential risk factors for AD.
These investigations suggest that the amyloid plaques that are considered the biochemical hallmarks of the disease are also consistent with an age-related misfolding of the APP protein (Chiti and Dobson, 2009). Accordingly, an imbalance between amyloid production and clearance can also be considered as the primary cause of sporadic AD. However, the amendments proposed (see Hardy, 2009) for an evaluation, are all within the neuron-centric framework of amyloid production and clearance.
The gene centered model essentially ignores the effect of two factors, energy and age, which play critical roles in neurodegenerative diseases.
Defects in energy metabolism may lead to a failure in the maintenance and restoration of ion gradients associated with synaptic transmission. Cellular metabolism invokes not only oxidative phosphorylation, an electrochemical process, but also glycolysis, a chemical process. Enzyme activity will decline with age in view of the intrinsic thermodynamic instability of large biomolecules.
We have therefore called the up-regulation of OxPhos activity, which we claim underlies the origin of sporadic forms of AD, the Inverse Warburg effect. The outcome of this competition depends on the relative capacity of the two types of neurons to appropriate energy substrates, and to convert these substrates into ATP. We will re-evaluate the amyloid hypothesis by contrasting its tenets with the principles underlying the Inverse Warburg hypothesis. The difference in vulnerability reflects the usual course of the disease in which episodic memory is the function affected in the early stages of AD (Hof and Morrison, 2004).
Up to the age of reproductive maturity, there will be intense selection to maintain the viability of the organism. This notion refers to a lower than expected probability of disease occurring in individuals diagnosed with other medical disorders. This observation will be invoked as the rationale for abandoning the amyloid hypothesis, and for advocating the processes involving metabolic reprogramming and natural selection as the effective model for the origin and progression of sporadic forms of AD. Here we also describe how energetics in the brain is based on the integration of these two modes of energy production. Oxidative phosphorylation (OxPhos) and substrate phosphorylation are the principal mechanisms of energy production in cells.
The energy is derived from two types of processes: OxPhos, which provides about 88% of the total energy in most eukaryotic cells, including neurons, and substrate phosphorylation (mainly glycolysis) which contributes the remaining 12%. The glycolytic enzymes occur in relatively stable multienzyme complexes with metabolites passed on from one active site to the next without exchanging with the bulk cytoplasm. A cornerstone of the theory is the allometric relation between metabolic rate, P, and cell size, W.


In the case of OxPhos, the proportionality constant α is determined by the proton gradient across the mitochondrial membrane.
The primitive nature of the process is indicated by the fact that the enzymes which catalyze the sequence of reactions exist free in solution in the cytosol. In the case of glycolysis the metabolic rate will be determined primarily by the kinetic activity of the enzymes, as shown by Equation (2.2).
Essentially, the cycle involves converting the potential energy of a variety of nutrients into the readily available energy of adenosine triphosphate (ATP). It is one of three metabolic pathways that are involved in fuel molecule catabolism and adenosine triphosphate production, the other two being glycolysis and oxidative phosphorylation. The implication is that the cycle was well established well before the last universal ancestor of all life.
Biochemist Michael Behe, in his 1996 book Darwin's Black Box, made the claim that Darwinian evolution cannot account for the biochemical complexity of the living cell, which thus must be the products of intelligent design.
In essence, the citric acid cycle has food molecules fed into it by a preprocessing pathway. These amino acids are brought into the cells and can be a source of energy by being funnelled into the citric acid cycle.
In the liver, the glycerol can be converted into glucose via dihydroxyacetone phosphate and glyceraldehyde-3-phosphate by way of gluconeogenesis (carbohydrate catabolism of the glucose can then take place, as above). This process extracts the energy from NADH and FADH2, recreating NAD+ and FAD, so that the cycle can continue.
One turn of the cycle turns the glucose fragment into carbon dioxide and water, just as if it had burnt in a flame.
NADH and FADH2 are coenzymes (molecules that enable or enhance enzymes) that store energy and are utilized in oxidative phosphorylation.
Such enzymes include the pyruvate dehydrogenase complex that synthesises the acetyl-CoA needed for the first reaction of the TCA cycle.
This mechanism for regulation is due to substrate inhibition by NADH of the enzymes that use NAD+ as a substrate. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution.
General characteristics of archaeal species and their potential availabilities to support bioremediation strategies2.1. We show that these three hallmarks of AD conflict with the amyloid hypothesis, but are consistent with the Inverse Warburg hypothesis, a bioenergetic model which postulates that AD is the result of a cascade of three events—mitochondrial dysregulation, metabolic reprogramming (the Inverse Warburg effect), and natural selection.
Mutant forms of these genes induce an overproduction of beta amyloid due to an alteration in APP processing, creating an imbalance between production and clearance, and the clinical and histopathological phenotype associated with AD. Accordingly, the amyloid model emerged as the organizing element in studies of both familial and sporadic forms of AD. These studies, however, indicate that none of the genes implicated in the familial form of AD consistently influences disease risk in the late onset form.
However, in spite of these studies, proponents of the amyloid hypothesis have maintained that a mutation induced overproduction of beta amyloid underlies both the early onset and the late onset forms of AD, and consequently, both forms of the disease can be ascribed similar etiologies (Hardy, 2009; Selkoe, 2012). This argument is the rationale for therapeutic strategies of late onset AD based on targeting amyloid pathways, either through passive immunotherapy against beta amyloid, or active inhibition of beta amyloid generation (Hardy and Selkoe, 2002; Hardy, 2009). This stems primarily from numerous conflicts between empirical observations and the predictions of the model. Hence, they do not represent a significant advance in our understanding of the molecular basis of the sporadic forms of AD. There is no single gene for either of these processes, although genes do encode the individual enzymes involved in both metabolic pathways. This instability will result in a loss of molecular fidelity and an impairment in the capacity of the cells to appropriate energy from the external environment and to convert this energy into biosynthetic work.
The metabolic alteration proposed in this model is the up-regulation of glycolytic activity. This capacity is quantitatively described by the statistical measure, evolutionary entropy, a measure of the number of pathways of energy flow within a metabolic network (Demetrius, 1997, 2013).
The contrast between the two classes of models rests on the following three criteria which characterize certain cellular, demographic and epidemiological features of sporadic forms of AD. Such a maintenance increases the long term contribution of the organism to the ancestry of successive generations. Section The Origin and Progression of AD describes the bioenergetic model of the origin of AD. Coupled to the citric acid cycle, oxidative phophorylation allows the oxidative degradation and energy production from various energy substrates which include carbohydrates (in particular glucose after its conversion into pyruvate via glycolysis), lactate, ketone bodies or fatty acids. Glycolysis is the non-oxidative part of the metabolic pathway that allows the use of carbohydrates by eukaryotic cells. In eukaryotic cells that rely essentially on carbohydrates for their energy production, the conjonction of glycolysis, the citric acid cycle and oxidative phosphorylation is responsible for the production of energy as ATP. However, the rate of energy production, in the case of oxidative phosphorylation, will depend on quantities such as the proton motive force, and the phospholipid composition of the membrane, as indicated in Equation (2.1). The current consensus is that this cycle predated the advent of free oxygen where it was "run in reverse" (energy was put into the cycle) to assemble important molecules.
The essence of the argument is that aspects of cellular machinery (bacterial flagellum, blood clotting, cellular transport and immune systems, and metabolic pathways, etc.) are irreducibly complex, so that removal of any one part causes the system to break down. Running in the opposite direction, the cycle combines small molecules with oxygen and captures the liberated energy to run all of metabolism, breaking down molecules into smaller units in the process.
In eukaryote cells, such as in humans, this energy-generating cellular respiration is constrained to within the mitochondria, the bacteria-like powerhouses of the cell.
A basic food molecule, such as glucose, is first broken down, without oxygen, by a series of steps, into smaller molecules. In many tissues, especially heart tissue, fatty acids are broken down through a process known as beta oxidation, which results in acetyl-CoA that can be used in the citric acid cycle.
Citrate is both the first and the last product of the cycle, and is regenerated by the condensation of oxaloacetate and acetyl-CoA. Also the enzymes citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase, which regulate the first three steps of the TCA cycle, are inhibited by high concentrations of ATP.
Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. New advances in the understanding of haloarchaea metabolism, biochemistry, and molecular biology suggest that general biochemical pathways related to nitrogen (Nitrogen cycle), metals (iron, mercury), hydrocarbons, or phenols can be used for bioremediation proposals.The main goal of the chapter is to present a review about the main characteristics of the archaeal species and their possible uses for bioremediation processes paying special attention to the Halobacteriaceae family.
Tapilatu et al., [54] reassigned it to Haloarcula vallismortis based on 16 rRNARecently, the effect of vitamin and organic nitrogen on hydrocarbon removal was assessed by using halophilic bacteria and archaea from the Arabian Gulf.
We also provide an explanation for the failures of the clinical trials based on amyloid immunization, and we propose a new class of therapeutic strategies consistent with the neuroenergetic selection model. This correlation between beta amyloid and the various histological and clinical hallmarks of the disease is the basis for the Amyloid hypothesis as a model for the familial forms of AD. In the case of one genetic variant, an allele of the apolipoprotein E gene (APOE) called APOE4, it was shown that it represents a risk factor, but it accounts for very little in the heritability of the disease and cannot be considered as a cause of the early onset form of the disease. Energy transduction by means of glycolysis is determined by a sequence of chemical reactions which are localized in the cytosol.
Consequently, the capacity of neurons to convert substrates such as glucose and lactate into ATP and to use this energy to maintain neuronal viability will also decline with age.
This mode of metabolic reprogramming, now known as the Warburg effect, is a well-established phenomenon in studies of the etiology and proliferation of cancer. After the age of reproductive maturity selection to maintain viability will be weak in view of its high metabolic cost and the low contribution to Darwinian fitness which such an investment in maintenance confers (Hayflick, 2007a; Demetrius, 2013). We apply the theory to distinguish between what we describe as normal aging and pathological aging, and the transition from normality to pathogenesis. The rate of energy production is now determined by the activity of the glycolytic enzymes in the cytosol. Both citric acid cycle and oxidative phosphorylation take place within mitochondria and give rise to carbon dioxide (CO2) and water (H2O) as waste products. Upon complete oxidation of energy substrates, both carbon dioxide (CO2) and water (H2O) are produced. The enzymes which catalyze the reactions of the respiratory chain are located in the inner membrane of the mitochondria, a complex molecular fabric of lipid and protein molecules. Sometimes beta oxidation can yield propionyl CoA, which can result in further glucose production by gluconeogenesis in liver. This regulation ensures that the TCA cycle will not oxidise excessive amount of pyruvate and acetyl-CoA when ATP in the cell is plentiful. The model, first proposed by Glenner and Wong (1984), contends that the neurodegenerative disease is due to an imbalance between the generation and clearance of beta amyloid.
The coherence of this process is determined by the coupling efficiency of oxidative phosphorylation. In the case of oxidative phosphorylation, energy transduction is contingent on two processes, the pumping of protons out of the mitochondrial inner membrane by the electron transport chain, and the conversion of proton flow into ATP (Lehninger, 1965). The age of reproductive maturity thus represents a critical point in the vulnerability of an organism to age-related diseases. In this process, protons are pumped from the matrix across the mitochondrial inner membrane through a set of respiratory complexes. In Figures 1, 2 are described the generation process of biological energy in the case of oxidative phosphorylation and glycolysis, respectively. In sharp contrast to the enzymes involved in glycolysis, the enzymes of the electron transport chain are located next to each other in the membrane in the exact sequence in which they interact (Lehninger, 1965; Harold, 2001). Most of the energy necessary to support brain activity is generated by the metabolism and oxidation of glucose via the tricarboxylic acid cycle coupled to oxidative phosphorylation in mitochondria (Magistretti, 2011). For a long time, it was considered that direct glucose utilization and oxidation by mitochondria should be the main source of ATP for neurons both at rest and during periods of activity.
The citric acid cycle is considered an amphibolic pathway because it participates in both catabolism and anabolism. In the absence of oxygen, no more energy can be extracted, and the waste is converted into molecules such as ethanol (alcohol) or lactic acid (involved in the cramp of a muscle cell).


IntroductionThe main benefit of using bioremediation is that microorganisms can destroy hazardous contaminants or turn them into less harmful forms.
Up to this age, the random age-related defects in the energy producing organelles will be repaired and hence the incidence of metabolic diseases will be rare.
Section Sporadic Forms of AD—Genetic and Metabolic contrasts the Amyloid Cascade model with the Energetic selection model.
When protons return to the mitochondrial matrix down their electrochemical gradient, ATP is synthesized via the enzyme ATP synthase. Changes in activity within specific brain areas lead to localized enhancement in blood flow as well as in glucose utilization. In aerobic organisms, the citric acid cycle and subsequent oxidative phosphorylation process generates a large number of ATP molecules. These microorganisms act against the contaminants if there are a variety of compounds aiding them to generate both energy and nutrients in order to grow more cells. Aging at the molecular level is associated with the increase in molecular disorder induced by random perturbations in the activity of large biomolecules (see below for a description). Empirical support for the existence of the Inverse Warburg effect is summarized in Section The Energetic Selection Model: Empirical Considerations.
Aerobic glycolysis describes the same metabolic production of lactate as end product from glucose despite adequate oxygen availability to normally carry on complete oxidation of pyruvate.
It was found that neurons exhibit a low level of expression of PFKFB3, a critical enzyme for the regulation of glycolysis (Herrero-Mendez et al., 2009).
In a few cases, the natural condition of the contaminated site provides all the essential material in sufficient quantities so that bioremediation can occur without the need for human intervention, which is known as “intrinsic bioremediation” [1]. These results were obtained not only for individual microorganisms in pure cultures but also for microbial consortia. These changes will ultimately result in a decline in the coupling efficiency of oxidative phosphorylation, and metabolic dysregulation (Hayflick, 2007a; Demetrius, 2013).
They will persist with highly cumulative deleterious effects on the metabolic integrity of the organism. The notion that the up-regulation of OxPhos activity in neurons induces the sequence of events that could ultimately lead to AD has important implications for diagnostic and therapeutic strategies. The rate of energy production is determined by the conductance of the biomembrane and the electromotive potential across the membrane (Nicholls and Ferguson, 2002). In these cases, cytosolic NADH is reoxidized within the cytosol by the conversion of pyruvate into lactate via the enzyme lactate dehydrogenase. The consequence is that neurons have a limited capacity to upregulate glycolysis to face enhanced energy demands. Often, bioremediation requires engineered systems to supply microbe-stimulating materials, which is called “engineered bioremediation” and relies on accelerating the desired biodegradation by encouraging growth of further organisms and optimizing the environment where detoxification takes place.
Therefore, the supplement of vitamins could be an effective practice to enhance bioremediation of oil-contaminated hypersaline environments [56]. Accordingly, the incidence of age-related diseases, such as Alzheimer's disease, will increase exponentially with age.
These strategies are discussed in Section Therapeutic Implications: Suppressing the Inverse Warburg Effect. In contrast, astrocytes express high levels of PFKFB3 but also low levels of an important component of the malate-aspartate shuttle, the aspartate-glutamate carrier aralar, which is important for shuttling cytosolic NADH within mitochondria and promoting glucose-derived pyruvate oxidation instead of lactate formation (Ramos et al., 2003). Engineered bioremediation may be chosen over intrinsic bioremediation due to the time factor and liability.
Moreover, astrocytes exhibit low levels of pyruvate dehydrogenase activity (Halim et al., 2010). Where an impending property transfer or potential impact of contamination calls for rapid pollutant removal engineered bioremediation maybe more appropriate as it accelerates biodegradation. Modeling studies have shown that these characteristics explain why neurons are essentially oxidative cells while astrocytes have such a high glycolytic capacity (Neves et al., 2012).
However, intrinsic bioremediation is an option where the natural occurrence of contaminant biodegradation is faster than contaminant migration. These rates depend on both, the type and concentration of contaminant, the microbial community, and the subsurface hydrogeochemical conditions [1].
Moreover, the lack of a sufficient microbial population can also hinder the cleanup rate.Terrestrial subsurface ecosystems constitute one of the largest habitats and represent an important resource of microbial diversity. Otherwise, surfactants and emulsifiers are used to solubilise and disperse hydrophobic compounds. Research in this area has intensified over the last two decades leading to significant discoveries in ecology, physiology, and phylogeny of subsurface microorganisms.
Despite considerable progress, the structure–function relationships remain largely uncharacterized. Halophilic archaea in bioremediation of aromatic hydrocarbonsStudies on aromatic hydrocarbons in earlier times were carried out with microorganisms isolated from samples of diverse hypersaline environments. Attempts to correlate microbial abundance and composition with variables likely to control metabolism have for the most part been unsuccessful. New technologies now give us the opportunity to gain further insights [2]A critical factor as to whether bioremediation is an appropriate remedy depends on if the contaminants are susceptible to biodegradation by the site organisms, or alternatively, if the relevant organisms can be added. While those already present can detoxify a vast array of contaminants, some are more easily degraded than others. Microorganisms are ideally suited to the task of contaminant destruction because they have enzymes that allow them to use environmental contaminants as food and because they are so small that they are able to contact contaminants easily [1].
Without the activity of microorganisms, the earth would literally be buried in wastes, and the nutrients necessary for life would be locked up in detritus.Coastal marine sediments subjected to high anthropogenic inputs can accumulate large amounts of contaminants, which represents a major concern for the potential detrimental consequences on the health of the ecosystem and the subsequent provision of goods and services. In particular, the contamination by metals, due to their persistence and toxicity even at low concentrations, represents a serious and widespread environmental problem.
The 4-hydroxybenzoic acid was changed to gentisate in the initial ring-cleavage reaction by the strain, although protocatechuic acid, hydroquinone or catechol is produced in case of the common pathways in aerobic bacteria, fungi, and yeast.
Their elimination from wastewater before being released into the environment is important for the maintenance of the ecosystem and from an economic point of view.
In order to isolate new halophilic archaea able to grow in aromatic compounds, Cuadros-Orellana et al. In this study, forty-four new halophilic archaea able to grow in 4-hydroxybenzoic acid as sole carbon and energy sources were isolated (Table 2). In contrast, autotrophic denitrifiers utilize inorganic carbon (carbon dioxide or bicarbonate) as a sole source of carbon.
Taxonomic characterization of these microorganisms revealed that the isolates represent at least four different groups of haloarchaea. Some advantages of autotrophic over heterotrophic denitrification are: avoiding of the poisoning effect of some organic carbon, low biomass buildup and less sludge production which results in reduction of reactor clogging and easier posttreatment. They concluded that the ability to metabolize 4-hydroxybenzoic acid is widespread in the Halobacteriaceae family, and thus, these haloarchaea microorganisms are excellent candidates to bioremediate aromatic compounds of hypersaline environments and treatment of saline effluents. Since some wastewaters have a very low concentration of biodegradable organic materials, autotrophic denitrification requires the addition of an electron donor substrate. These authors also determined biodegradation kinetics of strain L1 isolated from the Dead Sea [61], and suggested that the strain L1 could degrade benzoic acid more efficiently than Haloferax sp D1227 [58].
Extensive studies have been carried out on elemental sulfur and H2 as electron donors for autotrophic denitrification systems [5].Anaerobic ammonium oxidation (anammox) has received special attention because it is an efficient biological alternative to conventional nitrogen removal from wastewaters [6].
When the strain L1 was grown in the medium containing benzoic acid, gentisic acid was produced, which was not usual in other microorganisms. Therefore, gentisic acid is an intermediate in the degradation of benzoic acid, hydroxybenzoic acid, cinnamate, and phenylpropionate by the archaea Haloferax sp. D1, gdoA is expressed in the presence of 4-hydroxybenzoate but not benzoate; however, gdoA is expressed in Haloferax sp. The pattern of these genes expression is also different between the two species, obtaining only expression of acdB and tieA in Haloferax sp.
D1227 during growth on benzoate, cinnamate, and phenypropionate, but not on 3-hydroxybenzoate. D1227, while the gdoA genes encode part of a 4-hydroxybenzoate and 3- hydroxybenzoate pathways in Haloarcula sp. D1227, respectively [62].During the last four years, the number of studies which describe the degradation of aromatic compounds by halophilic archaea have increased. The MSNC 14, one of the strains (Table 2), was able to degrade 43% of phenanthrene after 30 days of incubation, although the degradation of anthracene and dibenzothiophene was not detected. The four strains were able to biodegraded not only aliphatic hydrocarbons but also aromatic hydrocarbons after three weeks of incubation.
In particular, Halobacterium and Haloccocus could grow in the presence of benzene, toluene, and p-hydroxybenzoic acid, and the two Haloferax strains could grow with toluene and phenanthrene, and one of them also with benzene, but both failed to grow on p-hydroxybenzoic acid. This study also revealed that the biodegradation rates increased in proportion to NaCl concentration in the medium, and thus supported the idea that extreme halophilic archaea are suitable biological material to bioremediate oil-polluted hypersaline environments. Aromatic compounds (p-hydroxybenzoic acid, naphthalene, phenanthrene, and pyrene) could be also degraded by nine halophilic archaea (Table 2) isolated from Camalti Saltern, Turkey [64]. This study broadens the understanding of metabolism of aromatic compounds, and the activities of catechol 1,2 dioxygenase and protocatechuate 3,4 dioxygenase were identified as the enzymes involved in ortho cleavage pathway. Ortho cleavage pathway is widely distributed in soil bacteria and fungi, constituting the major pathway for aromatic compounds catabolism in these organisms. Thus, the bioremediation process using those strains is promising, as the remediation processes using physical and chemical methods are complicated and expensive [65]. However, more precise understanding of the mechanism of carbon-cycling cleavage and their enzymes and genes may be necessary to achieve the bioremediation [66].



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