Degradation Of Organic Chemicals Research Articles

Remediation of Saline Wastewater Producing a Fuel Gas Containing Alkanes and Hydrogen Using Zero Valent Iron (Fe0)

Zero valent iron (Fe0) water remediation studies, over the last 40 years, have periodically reported the discovery of CnH2n+2 in the product water or product gas, where n = 1 to 20. Various theories have been proposed for the presence of these hydrocarbons. These include: (i) reductive transformation of a more complex organic chemical; (ii) hydrogenation of an organic chemical, as part of a degradation process; (iii) catalytic hydrogenation and polymerisation of carbonic acid; and (iv) redox transformation. This study uses wastewater (pyroligneous acid, (pH = 0.5 to 4.5)) from a carbonization reactor processing municipal waste to define the controls for the formation of CnH2n+2 (where n = 3 to 9), C3H4, and C3H6. A sealed, static diffusion, batch flow reactor, containing zero-valent metals [181 g m-Fe0 + 29 g m-Al0 + 27 g m-Cu0 + 40 g NaCl] L−1, was operated at two temperatures, 273–298 K and 348 K, respectively. The reactions, reactant quotients, and rate constants for the catalytic formation of H2(g), CO2(g), C3H4(g), C3H6(g), C3H8(g), C4H10(g), C5H12(g), C6H14(g,l), and C7H16(g,l), are defined as function of zero valent metal concentration (g L−1), reactor pressure (MPa), and reactor temperature (K). The produced fuel gas (422–1050 kJ mole−1) contained hydrogen + CnHy(gas), where n = 3 to 7. The gas production rate was: [1058 moles CnHy + 132 moles H2] m−3 liquid d−1 (operating pressure = 0.1 MPa; temperature = 348 K). Increasing the operating pressure to 1 MPa increased the fuel gas production rate to [2208 moles CnHy + 1071 moles H2] m−3 liquid d−1. In order to achieve these results, the Fe0, operated as a “Smart Material”, simultaneously multi-tasking to create self-assembly, auto-activated catalysts for hydrogen production, hydrocarbon formation, and organic chemical degradation (degrading carboxylic acids and phenolic species to CO2 and CO).

Critical evaluation of the microbial turnover to biomass approach for the estimation of biogenic non-extractable residues (NER)

BackgroundPersistence is a key criterion for the risk assessment of chemicals. In degradation tests, microbial biodegradation of labeled test chemicals leads to the incorporation of the label in microbial biomass, resulting in biogenic non-extractable residues (bioNER), which are not considered as harmful in persistence assessment. The amount of bioNER can be estimated using the microbial turnover to biomass (MTB) model. MTB estimates the biomass growth during productive degradation of a compound from theoretical growth yield and CO2-formation and gives an upper and a lower value for bioNER formation. The aim of this study is use available experimental data for bioNER to assess the validity, accuracy and precision of the MTB method as new tool in persistence assessment.ResultsWe collected experimental data in order to test accuracy and precision of this estimation method. In total, 16 experimental studies were found in literature where bioNER was experimentally quantified. Hereof, 13 studies used the amount of label recovered from total amino acid (tAA) content as proxy for bioNER. Unfortunately, the comparison with experimental data was difficult due to the variety of employed methods. A conversion factor is required to extrapolate from tAA on bioNER, and this factor may vary during the experiment and between experiments. The bioNER formation for all compounds tested was calculated with the MTB method, and the outcome was compared to measured tAA as proxy for bioNER. The relation between predicted and measured bioNER was significant, but no better correlation was obtained than with CO2 to tAA. The mean absolute error of the prediction (low MTB versus tAA) was 5% applied label (range 0.3 to 16%). Some deviation between measured results and calculated bioNER could be contributed to uncertainties in the experimental determination, as shown by variance in replicates (bromoxynil) or high background of label in sterile samples (sulfadiazine).ConclusionsMTB thus provides a robust model for determining of the potential amounts of biomass and bioNER formed from the degradation of organic chemicals.

CuO and TiO2 particles generated more stable and stronger EPFRs in dark than under UV-irradiation

Environmentally persistent free radicals (EFPRs) have recently attracted a great deal of research attention because of their significant toxicity and ubiquitous occurrence in the environment. The information is still very limited on how to estimate the intensity of EPFRs under ambient circumstances. This study is designed to specifically compare EPFRs generation during catechol degradation in dark and UV light irradiation. CuO and TiO2 were selected as model metal oxides to coat on silica at 1% CuO has a large electron exchange capacity, which may mediate catechol degradation in dark, while TiO2 possesses strong photocatalytic property and could accelerate catechol degradation under UV light. Under UV light irradiation, EPFRs were generated very quickly and reached the maximum value in 4 d, which was related to the photocatalytic property of the particle. However, these EPFRs dissipated quickly in 14 d. On the contrary, the intensities of EPFRs generated in dark were 2 times higher, and stabled for over 2 months. Therefore, the environmental impacts of EPFRs in dark may be widespread and long-lasting, which should be monitored more carefully. It should be noted that for CuO-coated silica, a significant amount of EPFRs (20% of the maximum) survived the UV-light irradiation and stabled during the experimental period (45 d). Stronger EPFRs were associated with more abundant dimer structures, suggesting the dimer structures were related to EPFRs formation during catechol degradation. Monitoring the generation of dimer structures in the degradation of organic chemicals may provide useful information to estimate EPFRs generation and risks.

Halogen Radicals Contribute to the Halogenation and Degradation of Chemical Additives Used in Hydraulic Fracturing.

In hydraulic fracturing fluids, the oxidant persulfate is used to generate sulfate radical to break down polymer-based gels. However, sulfate radical may be scavenged by high concentrations of halides in hydraulic fracturing fluids, producing halogen radicals (e.g., Cl•, Cl2•-, Br•, Br2•-, and BrCl•-). In this study, we investigated how halogen radicals alter the mechanisms and kinetics of the degradation of organic chemicals in hydraulic fracturing fluids. Using a radical scavenger (i.e., isopropanol), we determined that halogenated products of additives such as cinnamaldehyde (i.e., α-chlorocinnamaldehyde and α-bromocinnamaldehyde) and citrate (i.e., trihalomethanes) were generated via a pathway involving halogen radicals. We next investigated the impact of halogen radicals on cinnamaldehyde degradation rates. The conversion of sulfate radicals to halogen radicals may result in selective degradation of organic compounds. Surprisingly, we found that the addition of halides to convert sulfate radicals to halogen radicals did not result in selective degradation of cinnamaldehyde over other compounds (i.e., benzoate and guar), which may challenge the application of radical selectivity experiments to more complex molecules. Overall, we find that halogen radicals, known to react in advanced oxidative treatment and sunlight photochemistry, also contribute to the unintended degradation and halogenation of additives in hydraulic fracturing fluids.

Mediation of rhodamine B photodegradation by biochar

Significant degradation of organic contaminants in biochar sorption systems has recently been reported, demonstrating a promising potential application of biochar in pollution control. We hypothesized that the degradation of organic chemicals by biochar may be further enhanced under irradiation by UV light due to the photocatalytic activities of carbonaceous materials. Our results confirmed that UV conditions increased the degradation of rhodamine B (RhB) by up to three times compared to dark conditions. Washing biochar with ultrapure water further increased RhB photodegradation by up to ten times. This photodegradation increase was the highest for biochar produced at 1000 °C. HF treatment and the addition of biochar supernatant did not increase RhB photodegradation. Therefore, the biochar properties mediating RhB photodegradation may be related to solid particles, mostly the combination of graphite structures and organic functional groups. Based on XPS and FTIR analysis, we propose that the quinoid CO in biochar may play an essential role in RhB photodegradation. Further research on identifying the photoactive components of biochar will be fundamental for improved biochar manufacture and application.

Laboratory spectral induced polarisation signatures associated with iron and manganese oxide dissolution because of anaerobic degradation

Degradation of organic chemicals in natural soils depends on oxidation-reduction conditions. To protect our groundwater resources we need to understand the degradation processes under anaerobic conditions. Available iron and manganese oxides are used as electron acceptors for anaerobic degradation and are reduced to the dissolved form of metallic cations in pore water. To monitor this process is a challenge, because anaerobic conditions are difficult to sample directly without introducing oxygen. A few studies have shown an impact of iron reduction on spectral induced polarisation (SIP) signature, often associated with bacterial growth. Our objective is to study the impact of iron and manganese oxide dissolution, caused by degradation of an organic compound, with spectral induced polarisation signatures.Twenty-six vertical columns (30 cm high, inner diameter 4.6 cm) were filled with a sand rich in oxides (manganese and iron) with a static water table in the middle. In half of the columns, a 2 cm high contaminated layer was installed just above the water table. As the contaminant degrades, the initial oxygen is consumed and anaerobic conditions form Every three days over a period of one month, spectral induced polarisation (twenty frequencies between 5mHz and 10 kHz) data were collected on six columns: three contaminated replicates and three control replicates. Chemical analysis was done on twenty columns assigned for destructive water sampling, ten contaminated columns and ten control.The results show an increase of the real conductivity associated with the degradation processes, independent of frequency. Compared with the pore water electrical conductivity in the saturated zone, the real conductivity measurement revealed the formation of surface conductivity before iron was released in the pore water. In parallel, we also observed an evolution of the imaginary conductivity in both saturated and unsaturated zones at frequencies below 1 Hz. Overall, the anaerobic reduction of iron and manganese oxide during the organic degradation increased both the conductive and polarisation component of the complex conductivity.

Fabricating MoS2 nanoflakes photoanode with unprecedented high photoelectrochemical performance and multi-pollutants degradation test for water treatment

In this work, vertically aligned MoS2 nanoflakes on Ti foil have been successfully prepared by different hydrothermal methods (using water or ammonia solvent) towards fabricating photoanodes with high visible light photoelectrochemical performance. The few-layered MoS2 (4–10 nm) nanoflakes electrode grown in water exhibits unprecedented higher photocurrents under visible light irradiation, and the stability of the few-layered MoS2 electrode was markedly enhanced by annealing the sample at 300 °C. The underlying mechanisms accounting for the superior photoelectrochemical performance of the few-layered MoS2 nanoflakes are found to be correlated with their phase, crystallinity, length, number of layers and photon absorption efficiency etc. Furthermore, the photoelectrochemical degradation of organic chemicals including rhodamine B dye, 4-chlorophenol, chlorpyrifos and ciprofloxacin selected as different types of pollutants in wastewater was evaluated and compared utilizing the few-layered MoS2 nanoflakes photoanode. The photoelectrochemical degradation efficiency is strongly influenced by the molecular structures and properties of pollutants and experimental condition parameters such as solution pH, oxygen concentration and applied bias potential. The available high electron cloud density sites induced by the electron-withdrawing functional groups in different substrates might determine the substrate-specific PEC degradation efficiency. The present MoS2-based PEC system could be further optimized for efficient degradation of recalcitrant pollutant like ciprofloxacin.

Enhancement of Fe@porous carbon to be an efficient mediator for peroxymonosulfate activation for oxidation of organic contaminants: Incorporation NH2-group into structure of its MOF precursor

Metal-organic frameworks (MOFs) derived metal@porous carbon showed good performance in peroxymonosulfate (PMS) activation for refractory organic chemical degradation from aqueous. However, the effect of structure and physical-chemical properties of metal@porous carbon on PMS activation and its involved reaction mechanism were still unclear. Herein, Fe@porous carbon derived from MOF MIL-53(Fe) was used as target, to discuss the role of NH2-group incorporation on the development of structure and physical-chemical properties of obtained Fe@porous carbon, and reaction mechanism for PMS activation. The incorporation of NH2-group significantly decreased the synthesis temperature of Fe@porous carbon and increased the encapsulation of Fe0 in the porous carbon. Furthermore, the addition of nitrogen in porous carbon and rigid encapsulation structure reduced the defects of the Fe@porous carbon. These improvements of the structure and chemical properties were favored for enhancement of the catalytic activity and stability of the obtained Fe@porous carbon in the activation of PMS. Electron paramagnetic resonance (EPR) experiments indicated that SO4−, OH and 1O2 were involved. The radical pathway involving SO4− and OH was the prevailing pathway while the nonradical pathway involving 1O2 was the recessive pathway. Based on intermediate identification, the degradation pathway of acyclovir (ACV) was proposed as SO4− and OH derived process, and eight of intermediates were first reported. It was interesting to note that iron species, carbon structure, and nitrogen element in the catalysts derived from MIL-53(Fe) or NH2-MIL-53(Fe) clearly showed different role and reaction pathway. This work not only provided an efficient Fe@N-doped porous carbon for activation PMS to degrade refractory organic chemicals for water purification, but also suggested a valuable insight for the design of metal@porous carbon derived from MOF.

Catechol degradation on hematite/silica-gas interface as affected by gas composition and the formation of environmentally persistent free radicals.

Environmentally persistent free radicals (EPFRs) formed on a solid particle surface have received increasing attention because of their toxic effects. However, organic chemical fate regulated by EPFRs has rarely been investigated, and this information may provide the missing link in understanding their environmental behavior. Previous studies have suggested that the reduction of transition metals is involved in EPFRs formation. We thus hypothesize that an oxidative environment may inhibit EPFRs formation in particle-gas interface, which will consequently release free radicals and accelerate organic chemical degradation. Our result indicates that a 1% hematite coating on a silica surface inhibited catechol degradation in N2, especially at low catechol loadings on solid particles (SCT). However, under an O2 environment, catechol degradation decreased when SCT was <1 μg/mg but increased when SCT was >1 μg/mg. Stable organic free radicals were observed in the N2 system with g factors in the 2.0035–2.0050 range, suggesting the dominance of oxygen-centered free radicals. The introduction of O2 into the catechol degradation system substantially decreased the free radical signals and decreased the Fe(II) content. These results were observed in both dark and light irradiation systems, indicating the ubiquitous presence of EPFRs in regulating the fate of organic chemicals.

Degradation of p-Nitrophenol on Biochars: Role of Persistent Free Radicals.

Generation of environmentally persistent free radicals (EPFRs) on solid particles has recently attracted increasing research interest. EPFRs potentially have high reactivity and toxicity. However, the impact of EPFRs on organic contaminant behavior is unclear. We hypothesized that EPFRs in biochars can degrade organic contaminants and play an important role in organic contaminant behavior. We observed obvious degradation of p-nitrophenol (PNP) in the presence of biochars, through the detection of NO3(-) as well as organic byproducts. The extent of PNP degradation was correlated to the intensity of EPR signals of biochar particles. tert-Butanol (a •OH scavenger) did not completely inhibit PNP degradation, indicating that •OH could not fully explain PNP degradation. The decreased PNP degradation after tert-butanol addition was better correlated with reduced PNP sorption on biochars. PNP degradation through the direct contact with EPFRs in biochar particles could be an important contribution to the PNP concentration reduction in the aqueous phase. The coating of natural organic matter analogue (tannic acid) on biochars did not considerably inhibit PNP degradation, suggesting the ability of biochars to degrade PNP in soil and natural water. Similar EPFR-promoted degradation was observed for five different types of biochars and one activated carbon, as well as one additional chemical (p-aminophenol). Therefore, organic chemical degradation by EPFRs in biochars can be a common process in the environment and should be incorporated in organic chemical fate and risk studies.

Sediment microbial fuel cell prefers to degrade organic chemicals with higher polarity

By operating a SMFC in heavily contaminated sediment and analyzing its global organic chemical degradation profile, this study showed a brief trend that SMFC prefers to stimulate the degradation of organic chemicals with higher polarity. As a comparison, adding nitrate as a microbial respiration-based sediment remediation strategy preferred lower polarity chemicals. Both SMFC and nitrate reactors showed high degradation capacity in benzene homologs. These results provide crucial information for the selective and proper application of SMFC in bioremediation.

Formation of environmentally persistent free radicals as the mechanism for reduced catechol degradation on hematite-silica surface under UV irradiation

Iron is rich in soils, and is recently reported to form stable complexes with organic free radicals, generating environmentally persistent free radicals (EPFRs). The observation may challenge the common viewpoint that iron is an effective catalyst to facilitate the degradation of various organic chemicals. But no study was specifically designed to investigate the possible inhibited degradation of organic chemicals because of the formation of EPFRs in dry environment. We observed that catechol degradation under UV irradiation was decreased over 20% in silica particles coated with 1% hematite in comparison to uncoated silica particles. Stabilized semiquinone or quinine and phenol radicals were involved in HMT-silica system. EPFR formation was thus the reason for the reduced catechol degradation on HMT-silica surface under UV irradiation at ambient temperature. EPFRs should be incorporated in the studies of organic contaminants geochemical behavior, and will be a new input in their environmental fate modeling.

Zn–Cr layered double hydroxide: Visible light responsive photocatalyst for photocatalytic degradation of organic pollutants

Heterogeneous photocatalysis, one of the important steps in advanced oxidative process (AOPs) that was found to be promising for the elimination of toxic organic and inorganic compounds from wastewater. Herein, we reported Zn–Cr-CO3, a novel layered double hydroxide (LDH) for visible-light-driven photocatalytic degradation of organic chemicals. The physicochemical properties of the synthesized Zn/Cr-CO3 LDH, prepared by co-precipitation method at constant pH, were determined by several techniques such as powder X-ray diffraction (PXRD), Fourier Transform Infrared (FT-IR), thermo gravimetric/differential thermal analysis (TG/DTA), BET surface area, scanning electronic microscope (SEM), transmission electron microscope (TEM) and optical absorption spectra. The photocatalytic degradation studies on xanthene dyes (Rhodamine B, Rhodamine 6G) and substituted phenol (4-chloro 2-nitro phenol) using Zn–Cr-CO3 were reported here for the first time. The kinetic study demonstrated that Zn–Cr-CO3 LDH was an efficient photocatalyst for degradation of organic pollutants. The formation of intermediate oxidative species (OH, O2- and 1O2) during the photocatalytic reactions were ascertained by using different quenchers and the possible mechanism was proposed.

Design and Properties of Multifunctional Lubricant

A MoS2/TiO2 nanocomposite was synthesized and characterized. Its lubricating properties and photocatalytic activity were also investigated, respectively. The as-prepared MoS2/TiO2 nanocomposite presented anti-friction properties better than those of the pure nanoMoS2. However, the difference in the anti-wear properties was not very notable between the pure nanoMoS2 and the MoS2/TiO2 nanocomposite. Moreover, the MoS2/TiO2 nanocomposite was also an excellent photocatalyst for the degradation of organic chemicals.

Fenton's oxidation of pentachlorophenol

The combination of H 2O 2 and Fe(II) (Fenton's reaction) has been demonstrated to rapidly degrade many organics via hydroxyl radicals. However, few studies have related hydroxyl radical generation rates with measured organic chemical degradation data. The goals of this work were to investigate the kinetics, stoichiometry, and intermediates of pentachlorophenol (PCP) degradation in the Fenton's reaction and to develop a mathematical model of this reaction system. Batch reaction experiments were performed to assess both initial transients and steady states, and special attention was given to the analysis of intermediates. Solutions of PCP (55 μM) and Fe(II) (200 μM) were treated with variable levels of H 2O 2 (<850 μM), and the concentrations of these reactants and their products were measured. Partial PCP degradation and near stoichiometric dechlorination were observed at low initial H 2O 2 concentrations. Higher H 2O 2 doses achieved at most 70% dechlorination even though nearly all of the PCP was degraded. The reaction intermediates tetrachlorohydroquinone and dichloromaleic acid accounted for up to 5% of the PCP degraded. Organic carbon mineralization (transformation to CO 2) was not observed. The OH scavenging effects of the PCP-by-products mixture were characterized as a lumped parameter in the reaction kinetics model, which provided reasonable predictions of experimental results at different oxidant concentrations and reaction time.

Plant uptake of organic chemicals: Current developments and recommendations for future research

Plant uptake of organic chemicals: Current developments and recommendations for future research

Accelerated aromatic compounds degradation in aquatic environment by use of interaction between Spirodela polyrrhiza and bacteria in its rhizosphere

Accelerated degradation of organic chemicals by aquatic plant-bacterial associations was reported for the first time with elucidation of the role and contribution of aquatic plant and bacteria in its rhizosphere using a fast-growing giant duckweed, Spirodela polyrrhiza. The results clearly showed the accelerated degradation of all the three aromatic compounds (phenol, aniline and 2,4-dichlorophenol [2,4-DCP]) tested by aquatic plant-bacterial associations. In phenol degradation system, phenol-degrading bacteria indigenous to the rhizosphere fraction of S. polyrrhiza mainly contributed, while in aniline degradation system S. polyrrhiza mainly contributed by stimulating aniline-degrading bacteria both in the rhizosphere and balk water fraction. On the other hand in 2,4-DCP degradation system, S. polyrrhiza itself mainly contributed to its removal by uptake and degradation. Thus, the mechanisms for accelerated removal of aromatic compounds were quite different depending on the substrates. S. polyrrhiza showed selective accumulation of phenol-degrading bacteria in its rhizosphere fraction, while aniline- and 2,4-DCP-degrading bacteria were not much accumulated. S. polyrrhiza secreted peroxidase and laccase. However, both of the enzymatic activities increased with the addition of aromatic compounds, degrading ability of S. polyrrhiza itself should be owing to the production of peroxidase rather than laccase because the change of peroxidase activity and concentration of each aromatic compound well concurred. From the results obtained in the present study, it can be concluded that the feasibility of the use of aquatic plant-bacterial associations to accelerate the degradation of organic chemicals especially recalcitrant compounds in aquatic environment was shown.

A novel analytical approach for visualizing and tracking organic chemicals in plants.

Vegetation plays a key role in the environmental fate of many organic chemicals, from pesticides applied to plants, to the air-vegetation exchange and global cycling of atmospheric organic contaminants. Our ability to locate such compounds in plants has traditionally relied on inferences being made from destructive chemical extraction techniques or methods with potential artifacts. Here, for the first time, two-photon excitation microscopy (TPEM) is coupled with plant autofluorescence to visualize and track trace levels of an organic contaminant in living plant tissue, without any form of sample modification or manipulation. Anthracene-a polynuclear aromatic hydrocarbon (PAH)-was selected for study in living maize (Zea mays) leaves. Anthracene was tracked over 96 h, where amounts as low as approximately 0.1-10 pg were visible, as it moved through the epicuticular wax and plant cuticle, and was observed reaching the cytoplasm of the epidermal cells. By this stage, anthracene was identifiable in five separate locations within the leaf: (1) as a thin (approximately 5 microm) diffuse layer, in the upper surface of the epicuticular wax; (2) as thick (approximately 28 microm) diffuse bands extending from the epicuticular wax through the cuticle, to the cell walls of the epidermal cells; (3) on the external surface of epidermal cell walls; (4) on the internal surface of epidermal cell walls; and (5) within the cytoplasm of the epidermal cells. This technique provides a powerful nonintrusive tool for visualizing and tracking the movement, storage locations, and degradation of organic chemicals within vegetation using only plant and compound autofluorescence. Many other applications are envisaged for TPEM, in visualizing organic chemicals within different matrixes.

Promoters for Tocopherols as Antioxidants

Many kinds of additives are used to prevent the degradation of organic chemicals, including macromolecular materials. Tocopherols are natural products known to be effective antioxidants. Now, on the basis of the anti-oxidation mechanism of tocopherols proposed by us, we have studied amines as promoters, which may enhance the antioxidant activity of a tocopherol, and studied 4-n-butoxyphenol, too, as model compounds as tocopherols. It was found that amines exhibit an antagonism to 4-alkylphenols, such as 2,6-di-t-butyl-4-methylphenol, whereas they exhibit certain synergism to 4-alkoxyphenols. In addition, it is clarified that an amine having a low ionization potential improves the antioxidant activity of α-tocopherol.

Global features of the Pseudomonas putida KT2440 genome sequence.

The compositional bias of the G+C, di- and tetranucleotide contents in the 6 181 862 bp Pseudomonas putida KT2440 genome was analysed in sliding windows of 4000 bp in steps of 1000 bp. The genome has a low GC skew (mean 0.066) between the leading and lagging strand. The values of GC contents (mean 61.6%) and of dinucleotide relative abundance exhibit skewed Gaussian distributions. The variance of tetranucleotide frequencies, which increases linearly with increasing GC content, shows two overlapping Gaussian distributions of genome sections with low (minor fraction) or high variance (major fraction). Eighty per cent of the chromosome shares similar GC contents and oligonucleotide bias, but 105 islands of 4000 bp or more show atypical GC contents and/or oligonucleotide signature. Almost all islands provide added value to the metabolic proficiency of P. putida as a saprophytic omnivore. Major features are the uptake and degradation of organic chemicals, ion transport and the synthesis and secretion of secondary metabolites. Other islands endow P. putida with determinants of resistance and defenceor with constituents and appendages of the cell wall. A total of 29 islands carry the signature of mobile elements such as phage, transposons, insertion sequence (IS) elements and group II introns, indicating recent acquisition by horizontal gene transfer. The largest gene carries the most unusual sequence that encodes a multirepeat threonine-rich surface adhesion protein. Among the housekeeping genes, only genes of the translational apparatus were located in segments with an atypical signature, suggesting that the synthesis of ribosomal proteins is uncoupled from the rapidly changing translational demands of the cell by the separate utilization of tRNA pools.