Chlorophenol production by anaerobic microorganisms: transformation of a biogenic chlorinated hydroquinone metabolite

A comprehensive review of chlorophenols: Fate, toxicology and its treatment

Chlorophenols represent one of the most abundant families of toxic pollutants emerging from various industrial manufacturing units. The toxicity of these chloroderivatives is proportional to the number and position of chlorine atoms on the benzene ring. In the aquatic environment, these pollutants accumulate in the tissues of living organisms, primarily in fishes, inducing mortality at an early embryonic stage. Contemplating the behaviour of such xenobiotics and their prevalence in different environmental components, it is crucial to understand the methods used to remove/degrade the chlorophenol from contaminated environment. The current review describes the different treatment methods and their mechanism towards the degradation of these pollutants. Both abiotic and biotic methods are investigated for the removal of chlorophenols. Chlorophenols are either degraded through photochemical reactions in the natural environment, or microbes, the most diverse communities on earth, perform various metabolic functions to detoxify the environment. Biological treatment is a slow process because of the more complex and stable structure of pollutants. Advanced Oxidation Processes are effective in degrading such organics with enhanced rate and efficiency. Based on their ability to generate hydroxyl radicals, source of energy, catalyst type, etc., different processes such as sonication, ozonation, photocatalysis, and Fenton's process are discussed for the treatment or remediation efficiency towards the degradation of chlorophenols. The review entails both advantages and limitations of treatment methods. The study also focuses on reclamation of chlorophenol-contaminated sites. Different remediation methods are discussed to restore the degraded ecosystem back in its natural condition.

Microbial Synthesis and Transformation of Inorganic and Organic Chlorine Compounds

Organic and inorganic chlorine compounds are formed by a broad range of natural geochemical, photochemical and biological processes. In addition, chlorine compounds are produced in large quantities for industrial, agricultural and pharmaceutical purposes, which has led to widespread environmental pollution. Abiotic transformations and microbial metabolism of inorganic and organic chlorine compounds combined with human activities constitute the chlorine cycle on Earth. Naturally occurring organochlorines compounds are synthesized and transformed by diverse groups of (micro)organisms in the presence or absence of oxygen. In turn, anthropogenic chlorine contaminants may be degraded under natural or stimulated conditions. Here, we review phylogeny, biochemistry and ecology of microorganisms mediating chlorination and dechlorination processes. In addition, the co-occurrence and potential interdependency of catabolic and anabolic transformations of natural and synthetic chlorine compounds are discussed for selected microorganisms and particular ecosystems.

Organohalide respiration in pristine environments: implications for the natural halogen cycle

Halogenated organic compounds, also termed organohalogens, were initially considered to be of almost exclusively anthropogenic origin. However, over 5000 naturally synthesized organohalogens are known today. This has also fuelled the hypothesis that the natural and ancient origin of organohalogens could have primed development of metabolic machineries for their degradation, especially in microorganisms. Among these, a special group of anaerobic microorganisms was discovered that could conserve energy by reducing organohalogens as terminal electron acceptor in a process termed organohalide respiration. Originally discovered in a quest for biodegradation of anthropogenic organohalogens, these organohalide-respiring bacteria (OHRB) were soon found to reside in pristine environments, such as the deep subseafloor and Arctic tundra soil with limited/no connections to anthropogenic activities. As such, accumulating evidence suggests an important role of OHRB in local natural halogen cycles, presumably taking advantage of natural organohalogens. In this minireview, we integrate current knowledge regarding the natural origin and occurrence of industrially important organohalogens and the evolution and spread of OHRB, and describe potential implications for natural halogen and carbon cycles.

Evaluation of the effect of 2,4-dichlorophenol on oxidative parameters and viability of human blood mononuclear cells (in vitro)

2,4-Dichlorophenol (2,4-DCP) is formed in drinking water as a result of its chlorination, and it is created in the environment during transformation of various xenobiotics such as triclosan or herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). The molecular mechanism depicting the action of phenolic compounds on nucleated blood cells has been insufficiently studied, and therefore, we have assessed the effect of 2,4-DCP on the structure and viability of human peripheral blood mononuclear cells (PBMCs). We have evaluated necrotic, apoptotic, and morphological changes (alterations in the size and granulation) in PBMCs incubated with 2,4-DCP in the concentration ranging from 10 to 500 µg mL−1 for 4 h at 37°C. Moreover, we have estimated changes in reactive oxygen species (ROS) formation, lipid peroxidation, and protein carbonylation in the incubated cells. We have noted that 2,4-DCP increased ROS formation and lipid peroxidation (from 10 µg mL−1) and oxidized proteins (from 50 µg mL−1) in PBMCs. The compound studied also provoked apoptotic (from 50 µg mL−1), necrotic (from 100 µg mL−1) and alterations in the size and granulation (from 50 µg mL−1) in the incubated cells. The analysis of quinolinium 4-[(3-methyl-2(3 H)-benzoxazolylidene)methyl]-1-[3-(trimethyl-ammonio)-propyl]-diiodide/propidium iodide staining revealed that 2,4-DCP (50–250 µg mL−1) more strongly increased the number of apoptotic than necrotic cells, which suggests that this cell death type is mainly provoked by this compound in PBMCs. The observed changes were caused by relatively high concentrations of 2,4-DCP, which cannot influence human organism during environmental exposure and thus may only occur as a result of acute or subacute poisoning with this compound.

Consumption of 2-chlorophenol using anaerobic sludge: physiological and kinetic analysis

Chlorophenols are toxic and recalcitrant compounds produced by many industrial. Different strategies have been used to improve their biological consumption, but there is insufficient information to understand how the process is carried out. The objective of this study was to evaluate in batch tests the effect of the addition of phenol, acetate, or glucose as electron donors at different concentrations on the efficiencies, yields, and specific rates of 2-chlorophenol (2-CP) consumption. The addition of phenol (177.6 mg C/L), acetate (127.6 mg C/L), or glucose (77.6 mg C/L) increased the 2-CP consumption efficiency up to 54.6, 98.6, and 97.8 %, respectively. With respect to the control assay without electron donor, the specific rate of 2-CP consumption was up to 2.5 times higher with phenol (177.6 mg C/L), 8.4 times higher with acetate (127.6 mg C/L), and 3 times higher with glucose (127.6 mg C/L). The results showed that the type and concentration of electron donor determine the physiological behavior of the anaerobic sludge, modifying efficiency, yield, and specific rate values of the 2-CP consumption process. The addition of readily oxidable cosubstrates seems to be a good alternative and might be used for the biological treatment of industrial wastewater polluted with chlorinated phenols.

Hydroquinone: environmental pollution, toxicity, and microbial answers

Hydroquinone is a major benzene metabolite, which is a well-known haematotoxic and carcinogenic agent associated with malignancy in occupational environments. Human exposure to hydroquinone can occur by dietary, occupational, and environmental sources. In the environment, hydroquinone showed increased toxicity for aquatic organisms, being less harmful for bacteria and fungi. Recent pieces of evidence showed that hydroquinone is able to enhance carcinogenic risk by generating DNA damage and also to compromise the general immune responses which may contribute to the impaired triggering of the host immune reaction. Hydroquinone bioremediation from natural and contaminated sources can be achieved by the use of a diverse group of microorganisms, ranging from bacteria to fungi, which harbor very complex enzymatic systems able to metabolize hydroquinone either under aerobic or anaerobic conditions. Due to the recent research development on hydroquinone, this review underscores not only the mechanisms of hydroquinone biotransformation and the role of microorganisms and their enzymes in this process, but also its toxicity.

Layer of organic pine forest soil on top of chlorophenol-contaminated mineral soil enhances contaminant degradation

Chlorophenols, like many other synthetic compounds, are persistent problem in industrial areas. These compounds are easily degraded in certain natural environments where the top soil is organic. Some studies suggest that mineral soil contaminated with organic compounds is rapidly remediated if it is mixed with organic soil. We hypothesized that organic soil with a high degradation capacity even on top of the contaminated mineral soil enhances degradation of recalcitrant chlorophenols in the mineral soil below. We first compared chlorophenol degradation in different soils by spiking pristine and pentachlorophenol-contaminated soils with 2,4,6-trichlorophenol in 10-L buckets. In other experiments, we covered contaminated mineral soil with organic pine forest soil. We also monitored in situ degradation on an old sawmill site where mineral soil was either left intact or covered with organic pine forest soil. 2,4,6-Trichlorophenol was rapidly degraded in organic pine forest soil, but the degradation was slower in other soils. If a thin layer of the pine forest humus was added on top of mineral sawmill soil, the original chlorophenol concentrations (high, ca. 70 μg g(-1), or moderate, ca. 20 μg g(-1)) in sawmill soil decreased by >40 % in 24 days. No degradation was noticed if the mineral soil was kept bare or if the covering humus soil layer was sterilized beforehand. Our results suggest that covering mineral soil with an organic soil layer is an efficient way to remediate recalcitrant chlorophenol contamination in mineral soils. The results of the field experiment are promising.

Identification of a fungi-derived terrestrial halogenated natural product in wild boar (Sus scrofa)

In this study, we identified and quantitated a tetrachlorinated compound found at high concentrations in some samples of the meat of free-ranging wild boar (Sus scrofa) from Southern Germany. Mass spectrometric analysis indicated that the compound was a tetrachloromethoxyphenol isomer, and the subsequently synthesized tetrachloro-p-methoxyphenol was identical with the unknown compound in wild boar. Tetrachloro-p-methoxyphenol is a known secondary metabolite of basidiomycetous fungi, which in turn are regular feed items of the wild boar. It is extremely likely that the wild boar have accumulated tetrachloro-p-methoxyphenol by exploiting basidiomycetes. The highest concentration in the samples (n = 22) was ~1 mg/kg lipids tetrachloro-p-methoxyphenol. This concentration was higher than that of polychlorinated biphenyls (PCBs) and dichlorodiphenyltrichloroethane (DDT) in any of the samples. Some samples did not contain tetrachloro-p-methoxyphenol, which indicates varied preferences in fungi by wild boars. Our data suggest that during their entire evolution, humans have been in contact with the natural product tetrachloro-p-methoxyphenol by consuming wild boars.

The Desulfitobacterium genus

Desulfitobacterium spp. are strictly anaerobic bacteria that were first isolated from environments contaminated by halogenated organic compounds. They are very versatile microorganisms that can use a wide variety of electron acceptors, such as nitrate, sulfite, metals, humic acids, and man-made or naturally occurring halogenated organic compounds. Most of the Desulfitobacterium strains can dehalogenate halogenated organic compounds by mechanisms of reductive dehalogenation, although the substrate spectrum of halogenated organic compounds varies substantially from one strain to another, even with strains belonging to the same species. A number of reductive dehalogenases and their corresponding gene loci have been isolated from these strains. Some of these loci are flanked by transposition sequences, suggesting that they can be transmitted by horizontal transfer via a catabolic transposon. Desulfitobacterium spp. can use H2 as electron donor below the threshold concentration that would allow sulfate reduction and methanogenesis. Furthermore, there is some evidence that syntrophic relationships occur between Desulfitobacterium spp. and sulfate-reducing bacteria, from which the Desulfitobacterium cells acquire their electrons by interspecies hydrogen transfer, and it is believed that this relationship also occurs in a methanogenic consortium. Because of their versatility, desulfitobacteria can be excellent candidates for the development of anaerobic bioremediation processes. The release of the complete genome of Desulfitobacterium hafniense strain Y51 and information from the partial genome sequence of D. hafniense strain DCB-2 will certainly help in predicting how desulfitobacteria interact with their environments and other microorganisms, and the mechanisms of actions related to reductive dehalogenation.

Occurrence and expression ofcrdAandcprA5encoding chloroaromatic reductive dehalogenases inDesulfitobacteriumstrains

Desulfitobacterium hafniense PCP-1 (formerly frappieri PCP-1) has two reductive dehalogenases (RDases) that have been characterized. One is a membrane-associated 2,4,6-trichlorophenol RDase, which is encoded by crdA, and the other is a 3,5-dichlorophenol RDase encoded by cprA5. In this report, we determined the occurrence of these two RDase genes in seven other Desulfitobacterium strains. The presence or absence of these two RDases may explain the differences in the spectrum of halogenated compounds by these Desulfitobacterium strains. crdA gene sequences were found in all of the tested strains. It was expressed in strain PCP-1 regardless of the absence or presence of chlorophenols in the culture medium. crdA was also expressed in D. hafniense strains DCB-2 and TCE-1. cprA5 was detected only in D. hafniense strains PCP-1, TCP-A, and DCB-2. In these strains, cprA5 transcripts were detected only in the presence of chlorophenols. We also examined the expression of putative cprA RDases (cprA2, cprA3, and cprA4) that were shown to exist in the D. hafniense DCB-2 genome. RT-PCR experiments showed that cprA2, cprA3, and cprA4 were expressed in D. hafniense strains PCP-1, DCB-2, and TCP-A in the presence of chlorophenols. However, contrary to cprA5, these three genes were also expressed in the absence of halogenated compounds in the culture medium.Key words: reductive dehalogenase, Desulfitobacterium, gene family, gene expression.