Anaerobic mineralization of toluene by enriched sediments with quinones and humus as terminal electron acceptors

The anaerobic microbial oxidation of toluene to CO2 coupled to humus respir ation was demonstrated by use of enriched anaerobic sediments from the Amst erdam petroleum harbor (APH) and the Rhine River. Both highly purified soil humic acids (HPSHA) and the humic quinone moiety model compound anthraquin one-2,6-disulfonate (AQDS) were utilized as terminal electron acceptors. Af ter 2 weeks of incubation, 50 and 85% of added uniformly labeled [C-13] tol uene were recovered as (CO2)-C-13 in HPSHA- and AQDS-supplemented APH sedim ent enrichment cultures, respectively; negligible recovery occurred in unsu pplemented cultures. The conversion of [C-13] toluene agreed with the high level of recovery of electrons as reduced humus or as anthrahydroquinone-2, 6-disulfonate. APH sediment was also able to use nitrate and amorphous mang anese dioxide as terminal electron acceptors to support the anaerobic biode gradation of toluene. The addition of substoichiometric amounts of humic ac ids to bioassay reaction mixtures containing amorphous ferric oxyhydroxide as a terminal electron acceptor led to more than 65% conversion of toluene (1 mM) after 11 weeks of incubation, a result which paralleled the partial recovery of electron equivalents as acid-extractable Fe(II). Negligible con version of toluene and reduction of Fe(III) occurred in these bioassay reac tion mixtures when humic acids were omitted. The present study provides cle ar quantitative evidence for the mineralization of an aromatic hydrocarbon by humus-respiring microorganisms. The results indicate that humic substanc es may significantly contribute to the intrinsic bioremediation of anaerobi c sites contaminated with priority pollutants by serving as terminal electr on acceptors.