Isolation and partial characterization of aClostridium species transforming para-hydroxybenzoate and 3,4-dihydroxybenzoate and producing phenols as the final transformation products

Processes and electron flow in a microbial electrolysis cell bioanode fed with furanic and phenolic compounds

Furanic and phenolic compounds are problematic compounds resulting from the pretreatment of lignocellulosic biomass for biofuel production. Microbial electrolysis cell (MEC) is a promising technology to convert furanic and phenolic compounds to renewable H₂. The objective of the research presented here was to elucidate the processes and electron equivalents flow during the conversion of two furanic (furfural, FF; 5-hydroxymethyl furfural, HMF) and three phenolic (syringic acid, SA; vanillic acid, VA; 4-hydroxybenzoic acid, HBA) compounds in the MEC bioanode. Cyclic voltammograms of the bioanode demonstrated that purely electrochemical reactions in the biofilm attached to the electrode were negligible. Instead, microbial reactions related to the biotransformation of the five parent compounds (i.e., fermentation followed by exoelectrogenesis) were the primary processes resulting in the electron equivalents flow in the MEC bioanode. A mass-based framework of substrate utilization and electron flow was developed to quantify the distribution of the electron equivalents among the bioanode processes, including biomass growth for each of the five parent compounds. Using input parameters of anode efficiency and biomass observed yield coefficients, it was estimated that more than 50% of the SA, FF, and HMF electron equivalents were converted to current. In contrast, only 12 and 9% of VA and HBA electron equivalents, respectively, resulted in current production, while 76 and 79% remained as fermentation end products not further utilized in exoelectrogenesis. For all five compounds, it was estimated that 10% of the initially added electron equivalents were used for fermentative biomass synthesis, while 2 to 13% were used for exoelectrogenic biomass synthesis. The proposed mass-based framework provides a foundation for the simulation of bioanode processes to guide the optimization of MECs converting biomass-derived waste streams to renewable H₂.

The extent of fermentative transformation of phenolic compounds in the bioanode controls exoelectrogenic activity in a microbial electrolysis cell

Phenolic compounds in hydrolysate/pyrolysate and wastewater streams produced during the pretreatment of lignocellulosic biomass for biofuel production present a significant challenge in downstream processes. Bioelectrochemical systems are increasingly recognized as an alternative technology to handle biomass-derived streams and to promote water reuse in biofuel production. Thus, a thorough understanding of the fate of phenolic compounds in bioanodes is urgently needed. The present study investigated the biotransformation of three structurally similar phenolic compounds (syringic acid, SA; vanillic acid, VA; 4-hydroxybenzoic acid, HBA), and their individual contribution to exoelectrogenesis in a microbial electrolysis cell (MEC) bioanode. Fermentation of SA resulted in the highest exoelectrogenic activity among the three compounds tested, with 50% of the electron equivalents converted to current, compared to 12 and 9% for VA and HBA, respectively. The biotransformation of SA, VA and HBA was initiated by demethylation and decarboxylation reactions common to all three compounds, resulting in their corresponding hydroxylated analogs. SA was transformed to pyrogallol (1,2,3-trihydroxybenzene), whose aromatic ring was then cleaved via a phloroglucinol pathway, resulting in acetate production, which was then used in exoelectrogenesis. In contrast, more than 80% of VA and HBA was converted to catechol (1,2-dihydroxybenzene) and phenol (hydroxybenzene) as their respective dead-end products. The persistence of catechol and phenol is explained by the fact that the phloroglucinol pathway does not apply to di- or mono-hydroxylated benzenes. Previously reported, alternative ring-cleaving pathways were either absent in the bioanode microbial community or unfavorable due to high energy-demand reactions. With the exception of acetate oxidation, all biotransformation steps in the bioanode occurred via fermentation, independently of exoelectrogenesis. Therefore, the observed exoelectrogenic activity in batch runs conducted with SA, VA and HBA was controlled by the extent of fermentative transformation of the three phenolic compounds in the bioanode, which is related to the number and position of the methoxy and hydroxyl substituents.

Cryptanaerobacter phenolicus gen. nov., sp. nov., an anaerobe that transforms phenol into benzoate via 4-hydroxybenzoate

An anaerobic bacterium that transforms phenol and 4-hydroxybenzoate (4-OHB) into benzoate, strain LR7.2T, was isolated from a culture originating from a mixture of swamp water, sewage sludge, swine waste and soil. Cells of strain LR7.2T are Gram-positive short rods (1×2 μm) that are electron-dense when observed by electron microscopy. The optimum pH and temperature for growth and transformation activity of 4-OHB are 7·5–8·0 and 30–37 °C, respectively. The bacterium does not use sulphate, thiosulphate, nitrate, nitrite, FeCl3, fumarate or arsenate as an electron acceptor. It does not normally use sulphite, although stimulation of growth and 4-OHB transformation activity at a low concentration (up to 2 mM) has been reported previously under different culture conditions. The presence of 4-OHB or phenol is essential for growth; transformation of 4-OHB or phenol into benzoate is used to produce energy for growth. Using [6D]-phenol, 4-OHB was shown to be an intermediate in the transformation of phenol into benzoate. No spore was observed. The bacterium has a DNA G+C content of 51 mol% and its major membrane fatty acid is anteiso-C15 : 0. The 16S rRNA gene sequence of strain LR7.2T shows only 90 % similarity to its closest relative (Pelotomaculum thermopropionicum). From these results, a new taxon is proposed: Cryptanaerobacter phenolicus gen. nov., sp. nov. The type strain is LR7.2T (=ATCC BAA-820T=DSM 15808T).

Thermophilic, reversible gamma-resorcylate decarboxylase from Rhizobium sp. strain MTP-10005: purification, molecular characterization, and expression

We found the occurrence of thermophilic reversible gamma-resorcylate decarboxylase (gamma-RDC) in the cell extract of a bacterium isolated from natural water, Rhizobium sp. strain MTP-10005, and purified the enzyme to homogeneity. The molecular mass of the enzyme was determined to be about 151 kDa by gel filtration, and that of the subunit was 37.5 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis; in other words, the enzyme was a homotetramer. The enzyme was induced specifically by the addition of gamma-resorcylate to the medium. The enzyme required no coenzyme and did not act on 2,4-dihydroxybenzoate, 2,5-dihydroxybenzoate, 3,4-dihydroxybenzoate, 3,5-dihydroxybenzoate, 2-hydroxybenzoate, or 3-hydroxybenzoate. It was relatively thermostable to heat treatment, and its half-life at 50 degrees C was estimated to be 122 min; furthermore, it catalyzed the reverse carboxylation of resorcinol. The values of k(cat)/K(m) (mMu(-1) . s(-1)) for gamma-resorcylate and resorcinol at 30 degrees C and pH 7 were 13.4 and 0.098, respectively. The enzyme contains 327 amino acid residues, and sequence identities were found with those of hypothetical protein AGR C 4595p from Agrobacterium tumefaciens strain C58 (96% identity), 5-carboxyvanillate decarboxylase from Sphingomonas paucimobilis (32%), and 2-amino-3-carboxymuconate-6-semialdehyde decarboxylases from Bacillus cereus ATCC 10987 (26%), Rattus norvegicus (26%), and Homo sapiens (25%). The genes (graA [1,230 bp], graB [888 bp], and graC [1,056 bp]) that are homologous to those in the resorcinol pathway also exist upstream and downstream of the gamma-RDC gene. Judging from these results, the resorcinol pathway also exists in Rhizobium sp. strain MTP-10005, and gamma-RDC probably catalyzes a reaction just before the hydroxylase in it does.

Biotransformation of the major fungal metabolite 3,5-dichloro- p-anisyl alcohol under anaerobic conditions and its role in formation of Bis(3,5-dichloro-4-Hydroxyphenyl)methane

Higher fungi have a widespread capacity for biosynthesis of organohalogens. Commonly occurring chloroaromatic fungal metabolites can end up in anaerobic microniches at the boundary of fungal colonies and wetland soils. The aim of this study was to investigate the environmental fate of a major fungal metabolite, 3, 5-dichloro-p-anisyl alcohol, under anaerobic conditions. This compound was incubated with methanogenic sludge to study its biotransformation reactions. Initially, 3,5-dichloro-p-anisyl alcohol was readily demethylated in stoichiometric quantities to 3, 5-dichloro-4-hydroxybenzyl alcohol. The demethylated product was converted further via two routes: a biotic route leading to the formation of 3,5-dichloro-4-hydroxybenzoate and 2,6-dichlorophenol, as well as an abiotic route leading to the formation of bis(3, 5-dichloro-4-hydroxyphenyl)methane. In the first route, the benzyl alcohol moiety on the aromatic ring was oxidized, giving 3, 5-dichloro-4-hydroxybenzoate as a transient or accumulating product, depending on the type of methanogenic sludge used. In sludge previously adapted to low-molecular-weight lignin from straw, a part of the 3,5-dichloro-4-hydroxybenzoate was decarboxylated, yielding detectable levels of 2,6-dichlorophenol. In the second route, 3, 5-dichloro-4-hydroxybenzyl alcohol dimerized, leading to the formation of a tetrachlorinated bisphenolic compound, which was identified as bis(3,5-dichloro-4-hydroxyphenyl)methane. Since formation of this dimer was also observed in incubations with autoclaved sludge spiked with 3,5-dichloro-4-hydroxybenzyl alcohol, it was concluded that its formation was due to an abiotic process. However, demethylation of the fungal metabolite by biological processes was a prerequisite for dimerization. The most probable reaction mechanism leading to the formation of the tetrachlorinated dimer in the absence of oxygen is presented, and the possible environmental implications of its natural occurrence are discussed.

Anaerobic metabolism of aromatic compounds

Aromatic compounds comprise a wide variety of low-molecular-mass natural compounds (amino acids, quinones, flavonoids, etc.) and biopolymers (lignin, melanin). They are almost exclusively degraded by microorganisms. Aerobic aromatic metabolism is characterised by the extensive use of molecular oxygen. Monoxygenases and dioxygenases are essential for the hydroxylation and cleavage of aromatic ring structures. Accordingly, the characteristic central intermediates of the aerobic pathways (e.g. catechol) are readily attacked oxidatively. Anaerobic aromatic catabolism requires, of necessity, a quite different strategy. The basic features of this metabolism have emerged from studies on bacteria that degrade soluble aromatic substrates to CO2 in the complete absence of molecular oxygen. Essential to anaerobic aromatic metabolism is the replacement of all the oxygen-dependent steps by an alternative set of novel reactions and the formation of different central intermediates (e.g. benzoyl-CoA) for breaking the aromaticity and cleaving the ring; notably, in anaerobic pathways, the aromatic ring is reduced rather than oxidised. The two-electron reduction of benzoyl-CoA to a cyclic diene requires the cleavage of two molecules of ATP to ADP and P1 and is catalysed by benzoyl-CoA reductase. After nitrogenase, this is the second enzyme known which overcomes the high activation energy required for reduction of a chemically stable bond by coupling electron transfer to the hydrolysis of ATP. The alicyclic product cyclohex-1,5-diene-1-carboxyl-CoA is oxidised to acetyl-CoA via a modified beta-oxidation pathway; the ring structure is opened hydrolytically. Some phenolic compounds are anaerobically transformed to resorcinol (1,3-dihydroxybenzene) or phloroglucinol (1,3,5-trihydroxybenzene). These intermediates are also first reduced and then as alicyclic products oxidised to acetyl-CoA. This review gives an outline of the anaerobic pathways which allow bacteria to utilize aromatics even in the absence of oxygen. We focus on previously unknown reactions and on the enzymes characteristic for such novel metabolism.

Isolation and characterization of a new bacterium carboxylating phenol to benzoic acid under anaerobic conditions

A consortium of spore-forming bacteria transforming phenol to benzoic acid under anaerobic conditions was treated with antibiotics to eliminate the four Clostridium strains which were shown to be unable to accomplish this reaction in pure culture and coculture. Clostridium ghonii was inhibited by chloramphenicol (10 micrograms/ml), whereas Clostridium hastiforme (strain 3) and Clostridium glycolicum were inhibited by clindamycin (20 micrograms/ml), without the transformation of phenol being affected. Electron microscopic observations of resulting liquid subcultures revealed the presence of two different bacilli: a dominant C hastiforme strain (strain 2) (width, 1 micron) and an unidentified strain 6 (width, 0.6 micron) which was not detected on solid medium. Bacitracin (0.5 U/ml) changed the ratio of the strains in favor of strain 6. C hastiforme 2 was eliminated from this culture by dilution. The isolated strain 6 transformed phenol to benzoic acid and 4-hydroxybenzoic acid to phenol and benzoic acid in the presence of proteose peptone. Both of these activities are inducible. This strain is a gram- variable, flagellated rod with a doubling time of 10 to 11 h in the presence of phenol. It has a cellular fatty acid composition like that of C. hastiforme. However, strain 6 does not hydrolyze gelatin or produce indole. The 16S rRNA sequence of strain 6 was found to be most similar to that of some Clostridium species, with homology ranging from 80 to 86%. Tbe evolutionary relationships of strain 6 to different groups of Clostridium and Clostridium-related species revealed that it does not emerge from any of these groups. Strain 6 most likely belongs to a new species closely related to Clostridium species.

Purification and characterization of an oxygen-sensitive reversible 4-hydroxybenzoate decarboxylase from Clostridium hydroxybenzoicum

A 4-hydroxybenzoate decarboxylase from the anaerobe Clostridium hydroxybenzoicum strain JW/Z-1T was purified and partially characterized. It had an apparent molecular mass of 350 kDa and consisted of six identical subunits of 57 kDa each. The temperature optimum for the decarboxylation was approximately 50 degrees C, the optimum pH 5.6-6.2. The pI of the enzyme was 5.1. The activation energy for decarboxylation of 4-hydroxybenzoate was 65 kJ.mol-1 (20-37 degrees C). The enzyme also catalyzed decarboxylation of 3,4-dihydroxybenzoate. The apparent Km and kcat values obtained for 4-hydroxybenzoate were 0.40 mM and 3.3 x 10(3) min-1, and for 3,4-dihydroxybenzoate 1.2 mM and 1.1 x 10(3) min-1, respectively, at pH 6.0 and 25 degrees C. The enzyme activity was not influenced by the addition of biotin or avidin to either the crude cell extracts or the purified enzyme. The p-hydroxyl group of hydroxybenzoate appears to be essential for binding by the enzyme. The N-terminal amino acid sequence shows some similarity to the uroporphyrinogen decarboxylases from Synechococcus and Saccharomyces. The enzyme catalyzed the reverse reactions, that is, the carboxylation of phenol to 4-hydroxybenzoate and of catechol to 3,4-dihydroxybenzoate. The carboxylation did not require ATP.

Spore-forming bacteria that carboxylate phenol to benzoic acid under anaerobic conditions

A methanogenic consortium transforming phenol to benzoic acid was submitted to different treatments to characterize the carboxylating microorganisms and eventually to facilitate their isolation. Under aerobic conditions, phenol was not transformed by the consortium and no growth was observed on solid medium. The consortium from an inoculum that was treated with heat, or heat and ethanol, retained the ability to carboxylate phenol under strictly anaerobic conditions. Electron microscopic observations of the consortium from an inoculum that was heated for 15 min at 80 °C revealed only Gram-positive bacilli. In this culture, methane production was not detected and benzoic acid accumulated. Five colonies with distinct morphologies were isolated from this culture on solid medium. Four of these strains were identified as Clostridium spp. In contrast to the untreated culture, none of the strains isolated were able to carboxylate phenol in pure culture or in coculture, nor could they decarboxylate or dehydroxylate 4-hydroxybenzoic acid, or oxidize 2-hydroxybenzyl alcohol, or O-demethylate anisole or 2-methoxyphenol. Also, the consortium from a treated inoculum retained its ability to decarboxylate and dehydroxylate 4-hydroxybenzoic acid forming phenol and benzoic acid, respectively, but could not accomplish the other reactions. These results suggest that spore-forming microorganisms are involved in the carboxylation of phenol and in the decarboxylation and dehydroxylation of 4-hydroxybenzoic acid.Key words: spore-forming bacteria, phenol, benzoic acid, methanogenic conditions, carboxylation.

The anaerobic degradation of 3-chloro-4-hydroxybenzoate in freshwater sediment proceeds via either chlorophenol or hydroxybenzoate to phenol and subsequently to benzoate

To study the anaerobic degradation of the chimera 3-chloro-4-hydroxybenzoate (3-Cl,4-OHB), anaerobic freshwater sediment samples from the vicinity of Athens, Ga., were adapted for the transformation of 4-hydroxybenzoate (4-OHB), 3-chlorobenzoate (3-CB), 2-chlorophenol (2-CP), and 2,4-dichlorophenol (2,4-DCP). In nonadapted samples, both 4-OHB (product of aryl dechlorination) and 2-CP (product of aryl decarboxylation) were observed as intermediates in the transformation of 3-Cl,4-OHB to phenol. The accumulated phenol was subsequently transformed to benzoate, an intermediate in the conversion to methane and CO2. In 4-OHB-adapted samples (i.e., samples adapted for aryl decarboxylation), 2-CP was the first intermediate which was subsequently dechlorinated to phenol. In 3-CB-adapted samples (i.e., samples adapted for meta-chlorobenzoate dehalogenation), 3-Cl,4-OHB was stoichiometrically dechlorinated to 4-OHB. In 2-CP-adapted samples (i.e., samples adapted for ortho-chlorophenol dehalogenation), 4-OHB was the first major intermediate. Furthermore, 3-CB was not dechlorinated in 2-CP-adapted sediment samples, suggesting the possibility that different 3-Cl,4-OHB dechlorinating systems were induced in the 2-CP- and 3-CB-adapted sediments. Adaptation of sediment samples for dechlorination of 2,4-DCP did not lead to adaptation for dechlorination of 3-Cl,4-OHB. However, 3-Cl,4-OHB was dechlorinated to 4-OHB in our stable, sediment-free 2,4-DCP-dechlorinating enrichment, isolated previously from the same environment.(ABSTRACT TRUNCATED AT 250 WORDS)