Degradation of phenol under meso- and thermophilic, anaerobic conditions

Based on the results of preliminary studies on phenol degradation under mes ophilic conditions with a mixed methanogenic culture, we proposed a degrada tion pathway in which phenol is fermented to acetate: Part of the phenol is reductively transformed to benzoate while the rest is oxidised, forming ac etate as end product. According to our calculations, this should result in three moles of phenol being converted to two moles of benzoate and three mo les df acetate (3pheno1+2CO(2)+3H(2)O --> 3acetate+2benzoate) : To assess t he validity of our hypothesis concerning the metabolic pathway, we studied the transformation of phenol under mesophilic and thermophilic conditions i n relation to the availability of hydrogen. Hence, methanogenic meso- and t hermophilic cultures amended with phenol were run with or without an added over-pressure of hydrogen under methanogenic and non-methanogenic condition s. Bromoethanesulfonic acid (BES) was used to inhibit methanogenic activity . In the mesophilic treatments amended with only BES, about 70% of the carb on in the products found was benzoate. During the course of phenol transfor mation in these BES-amended cultures, the formation pattern of the degradat ion products changed: Initially nearly 90% of the carbon from phenol degrad ation was recovered as benzoate, whereas later in the incubation, in additi on to benzoate formation, the aromatic nucleus degraded completely to aceta te. Thus, the initial reduction of phenol to benzoate resulted in a lowerin g of H-2 levels, giving rise to conditions allowing the degradation of phen ol to acetate as the end product. Product formation in bottles amended with BES and phenol occurred in accordance with the hypothesised pathway; howev er, the overall results indicate that the degradation of phenol in this sys tem is more complex. During phenol transformation under thermophilic condit ions, no benzoate was observed and no phenol was transformed in the BES-ame nded cultures. This suggests that the sensitivity of phenol transformation to an elevated partial pressure of H-2 is higher under thermophilic conditi ons than under mesophilic ones. The lack of benzoate formation could have b een due to a high turnover of benzoate or to a difference in the phenol deg radation pathway between the thermophilic and mesophilic cultures. (C) 1999 Academic Press.