Abstract
Aromatic hydrocarbons belong to the most important ground water contaminants. Especially in the absence of molecular oxygen their degradation proceeds very slowly, leading to the formation of long contaminant plumes and pollutant transport over long distances. Such processes endanger the drinking water supply in many areas. The objective of this work was the examination of microbial degradation pathways and enrichment of new cultures. Based on laboratory results, conclusions should be drawn about potential natural attenuation in contaminated ground water. The sulphate-reducing culture N47 can utilise naphthalene or 2-mnethylnaphthalene as the sole carbon source and electron donor. Anaerobic degradation of 2-methylnaphthalene is initiated by an addition of fumarate to the methyl group producing the first intermediate, naphthyl-2-methyl-succinate. In a subsequent b-oxidation of the original methyl atom, the central metabolite 2-naphthoic acid is generated. In the following pathway, the aromatic ring system is reduced, cleaved, and finally oxidized to CO2. The activities of two enzymes involved in the upper degradation pathway of 2-methylnaphthalene, succinyl-CoA:naphthyl-2-methyl-succinate CoA-transferase and naphthyl-2-methyl-succinyl-CoA dehydrogenase, were measured in crude cell extracts. Succinyl-CoA:naphthyl-2-methyl-succinate CoA-transferase was not inhibited by sodium borohydride or hydroxylamine indicating that this enzyme belongs to the family III of CoA-transferases similar to the corresponding enzyme in the anaerobic toluene degradation pathway. The enzymatic activity of naphthyl-2-methyl-succinyl-CoA dehydrogenase could only be detected using phenazine methosulphate (PMS) as electron acceptor. No activity was observed with natural electron acceptors such as nicotinamide adenine dinucleotide (NAD+) or flavin adenine dinucleotide (FAD). Also the activation mechanism of naphthalene was investigated. Contrary to previous reports postulating a carboxylation, it could be demonstrated that the initial reaction in the anaerobic degradation of naphthalene is methylation to 2-methylnaphthalene. Naphthyl-2-methyl-succinate and naphthyl-2-methylene-succinate, specific metabolites occurring exclusively during anaerobic degradation of 2-methylnaphthalene were detected during growth on naphthalene. Additionally, enzymes of the anaerobic 2-methylnaphthalene degradation pathway could be detected in naphthalene-grown cells with similar activities. When the cells were transferred from 2-methylnaphthalene to naphthalene, the lag-phase lasted for almost 100 days indicating that additional catabolic enzymes have to be activated in this case. In the opposite case, after the transfer from naphthalene to 2-methylnaphthalene, the cells grew immediately demonstrating that no new enzymes had to be induced. As contaminant plumes in ground water contain normally mixtures of numerous substances, the influence of diverse polycyclic and heterocyclic compounds on growth on naphthalene or on 2-methylnaphthalene was investigated. Cometabolic transformation of the majority of the offered cosubstrates was observed, whereas they were converted to the corresponding carboxylic acids, frequently to several isomers. Some compounds were methylated and transformed to the corresponding methyl-succinic acids. In few cases, a partial or total inhibition of growth was detected. As one of the principal obstacles in the investigation of anaerobic degradation of aromatic compounds is the lack of appropriate cultures, new enrichments with different substrates and electron donors were prepared. Amongst others, a novel sulphate-reducing biphenyl degrading enrichment culture was obtained. 4-biphenylic acid was detected in culture supernatants as a putative metabolite. The culture was also able to convert cometabolically 4-fluorobiphenyl to 4-flourobiphenyl-carboxylic acid. The data obtained during this work contribute significantly to a better understanding of anaerobic degradation of polycyclic aromatic compounds.
Author and committee
dc:creator, dc:contributor.*- Author
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- Safinowski, Michael
Identifiers
dc:identifier.*- Identifier
- hdl:10900/48811