Peripheral and central pathways involved in the biodegradation of monoaromatic compounds under anaerobic conditions.

Abstract

Overall, the work in this dissertation contributes to our understanding of anaerobic biodegradation pathways, the role of syntrophic interactions in substrates degradation, and demonstrates a novel benzoate fermentation ability in S. aciditrophicus. (Abstract shortened by UMI.)

The metabolism of benzoate, cyclohex-1-ene carboxylate and cyclohexane carboxylate by Syntrophus aciditrophicus in coculture with hydrogen-using microorganisms was studied. Cocultures of S. aciditrophicus and Methanospirillum hungatei readily metabolized cyclohexane carboxylate and cyclohex-1-ene carboxylate at a rate slightly faster than that for benzoate. These results suggest that a unique ring-reduction mechanism operates in S. aciditrophicus which is probably imposed by energetic constraints encountered during syntrophic benzoate metabolism.

The anaerobic degradation of monoaromatic compounds was studied under a variety of terminal electron-accepting conditions to test the susceptibility to and possible metabolic pathways involved in toluic acids degradation under methanogenic and sulfate-reducing conditions, to determine the pathway for benzoate degradation in the syntrophic bacterium Syntrophus aciditrophicus , as well as to determine whether toluene degradation under sulfate-reducing conditions is dependent on interspecies hydrogen transfer. All toluic acid isomers were degraded under sulfate-reducing conditions, with m-toluate degradation occurring at the fastest rate. Benzoate, isophthalate, and m-carboxybenzylsuccinate were detected as transient intermediates in m-toluate-degrading, sulfate-reducing enrichments. All three toluate isomers were also degraded under methanogenic conditions with phthalic, isophthalic and terephthalic acids transiently accumulating in the culture supernatants of o-, m- and p-toluate methanogenic cultures, respectively. o-Carboxybenzaldehyde was detected in the culture supernatant of o-toluate methanogenic enrichments. These results show that the biodegradation of toluic acids is initiated at the methyl group under both electron-accepting conditions and that the formation of m-carboxybenzylsuccinic acid is an early step in m-toluate degradation under sulfate-reducing conditions.

S. aciditrophicus also metabolized benzoate in pure culture in the absence of hydrogen-utilizing partners or terminal-electron acceptors. S. aciditrophicus produced approximately 0.5 mol of cyclohexane carboxylate and 1.5 mol of acetate per mol of benzoate when grown in pure cultures compared to 3 mol of acetate and 0.75 mol of methane per mol of benzoate in coculture with M. hungatei.