Identical Ring Cleavage Products during Anaerobic Degradation of Naphthalene, 2-Methylnaphthalene, and Tetralin Indicate a New Metabolic Pathway

Abstract

Anaerobic degradation of naphthalene, 2-methylnaphthalene, and tetralin (1,2,3,4-tetrahydronaphthalene) was investigated with a sulfate-reducing enrichment culture obtained from a contaminated aquifer. Degradation studies with tetralin revealed 5,6,7,8-tetrahydro-2-naphthoic acid as a major metabolite indicating activation by addition of a C 1 unit to tetralin, comparable to the formation of 2-naphthoic acid in anaerobic naphthalene degradation. The activation reaction was specific for the aromatic ring of tetralin; 1,2,3,4-tetrahydro-2-naphthoic acid was not detected. The reduced 2-naphthoic acid derivatives tetrahydro-, octahydro-, and decahydro-2-naphthoic acid were identified consistently in supernatants of cultures grown with either naphthalene, 2-methylnaphthalene, or tetralin. In addition, two common ring cleavage products were identified. Gas chromatography-mass spectrometry (GC-MS) and high-resolution GC-MS analyses revealed a compound with a cyclohexane ring and two carboxylic acid side chains as one of the first ring cleavage products. The elemental composition was C 11 H 16 O 4 (C 11 H 16 O 4 -diacid), indicating that all carbon atoms of the precursor 2-naphthoic acid structure were preserved in this ring cleavage product. According to the mass spectrum, the side chains could be either an acetic acid and a propenic acid, or a carboxy group and a butenic acid side chain. A further ring cleavage product was identified as 2-carboxycyclohexylacetic acid and was assumed to be formed by ␤-oxidation of one of the side chains of the C 11 H 16 O 4 -diacid. Stable isotope-labeling growth experiments with either 13 C-labeled naphthalene, per-deuterated naphthalene-d 8 , or a 13 C-bicarbonatebuffered medium showed that the ring cleavage products derived from the introduced carbon source naphthalene. The series of identified metabolites suggests that anaerobic degradation of naphthalenes proceeds via reduction of the aromatic ring system of 2-naphthoic acid to initiate ring cleavage in analogy to the benzoylcoenzyme A pathway for monoaromatic hydrocarbons. Our findings provide strong indications that further degradation goes through saturated compounds with a cyclohexane ring structure and not through monoaromatic compounds. A metabolic pathway for anaerobic degradation of bicyclic aromatic hydrocarbons with 2-naphthoic acid as the central intermediate is proposed.

Anaerobic phenanthrene mineralization by a carboxylating sulfate-reducing bacterial enrichment

“…To our knowledge, this is the first determination of the 16S rRNA sequence phylogeny of a sulfate-reducing phenanthrene-degrading culture. To date, two possible mechanisms for anaerobic PAH activation have been reported, with most work focused on the metabolism of two-ring compounds such as naphthalene and 2-methylnaphthalene (Sullivan et al, 2001;Annweiler et al, 2000Annweiler et al, , 2002. Only one study has helped clarify the metabolism of phenanthrene (Zhang and Young, 1997).…”

Anaerobic Catabolism of Aromatic Compounds: a Genetic and Genomic View

“…In contrast, due to the fully aromatic character of the benzene ring in benzoyl-CoA (and its methyl, hydroxyl, and amino derivatives), its reduction is mechanistically difficult to achieve, and it requires not only a lowpotential electron donor, ferredoxin, but also a complex reductase system coupled to an input of energy (30,31,33,116). A similar reductive strategy is thought to occur during the anaerobic dearomatization of the polycyclic intermediates, e.g., naphthoyl-CoA, generated during the anaerobic catabolism of polycyclic aromatic compounds (8,374). Although the anaerobic degradation of heterocyclic aromatic compounds has been less studied than that of the homocyclic compounds, and it usually involves central intermediates of low aromaticity that are subject to a hydrolase-type ring cleavage reaction (21,74,111,176,178,366), some intermediates, e.g., hydroxynicotinate ( Fig.…”

“…The metabolites are as follows: cyclohex-1,5-diene-1-carbonyl-CoA (1), cyclohex-1-ene-1-carbonyl-CoA (2), 2-hydroxycyclohexane-1-carbonyl-CoA (3), 6-hydroxycyclohex-1-ene-1-carbonyl-CoA (4), 2-ketocyclohexane-1-carbonyl-CoA (5), 6-ketocyclohex-1-ene-1-carbonyl-CoA (6), pimelyl-CoA (7), and 3-hydroxypimelyl-CoA (8). The enzymes are also indicated, and the names correspond to those of the corresponding genes.…”

Intrinsic Aromatic Hydrocarbon Biodegradation for Groundwater Remediation

“…Degradation of PAH in the absence of oxygen has been documented only in a few reports. Carboxy derivates have been observed as the first intermediates (Zhang and Young, 1997;Meckenstock et al, 2000), and recently a reductive 2-naphthoic acid pathway was proposed for naphthalene degradation under sulfate reducing conditions (Annweiler et al, 2002). Pollutant degradability is most significantly affected by the availability of the different TEA.…”