Evidence for aromatic ring reduction in the biodegradation pathway of carboxylated naphthalene by a sulfate reducing consortium

Naphthalene was used as a model compound in order to study the anaerobic pa thway of polycyclic aromatic hydrocarbon degradation. Previously we had det ermined that carboxylation is an initial step for anaerobic metabolism of n aphthalene, but no other intermediate metabolites were identified (Zhang & Young 1997). In the present study we further elucidate the pathway with the identification of six novel naphthalene metabolites detected when cultures were fed naphthalene in the presence of its analog 1-fluoronaphthalene. Re sults from cultures supplemented with either deuterated naphthalene or non- deuterated naphthalene plus [C-13]bicarbonate confirm that the metabolites originated from naphthalene. Three of these metabolites were identified by comparison with the following standards: 2-naphthoic acid (2-NA), 5,6,7,8-t etrahydro-2-naphthoic acid, and decahydro-2-naphthoic acid. The presence of 5,6,7,8-tetrahydro-2-NA as a metabolite of naphthalene degradation indicat es that the first reduction reaction occurs at the unsubstituted ring, rath er than the carboxylated ring. The overall results suggest that after the i nitial carboxylation of naphthalene, 2-NA is sequentially reduced to decahy dro-2-naphthoic acid through 5 hydrogenation reactions, each of which elimi nated one double bond. Incorporation of deuterium atoms from D2O into 5,6,7 ,8-tetrahydro-2-naphthoic acid suggests that water is the proton source for hydrogenation.