Physiologically distinct anaerobic microorganisms were explored for their ability to oxidize different substrates with humic acids or the humic analogue, anthraquinone-2,6-disulphonate (AQDS), as a terminal electron acceptor. Most of the microorganisms evaluated including, for example, the halorespiring bacterium, Desulfitobacterium PCE1, the sulphate-reducing bacterium, Desulfovibrio G11 and the methanogenic archaeon, Methanospirillum hungatei JF1, could oxidize hydrogen linked to the reduction of humic acids or AQDS. Desulfitobacterium dehalogenans and Desulfitobacterium PCE1 could also convert lactate to acetate linked to the reduction of humic substances. Humus served as a terminal electron acceptor supporting growth of Desulfitobacterium species, which may explain the recovery of these microorganisms from organic rich environments in which the presence of chlorinated pollutants or sulphite is not expected. The results suggest that the ubiquity of humus reduction found in many different environments may be as a result of the increasing number of anaerobic microorganisms, which are known to be able to reduce humic substances.

The anaerobic microbial oxidation of toluene to CO 2 coupled to humus respiration was demonstrated by use of enriched anaerobic sediments from the Amsterdam petroleum harbor (APH) and the Rhine River. Both highly purified soil humic acids (HPSHA) and the humic quinone moiety model compound anthraquinone-2,6-disulfonate (AQDS) were utilized as terminal electron acceptors. After 2 weeks of incubation, 50 and 85% of added uniformly labeled

The capacity of an anaerobic granular sludge for serving as an immobilizing mechanism for quinone-respiring bacteria was evaluated. The inoculum was continuously fed with a basal medium containing the humic model compound, anthraquinone-2,6-disulfonate (AQDS), as a terminal electron acceptor. Complete reduction of AQDS was achieved by the granular sludge for a prolonged period in an anaerobic bioreactor provided with a mixture of volatile fatty acids as a substrate. Phylogenetic analysis revealed the enrichment and immobilization of AQDS-respiring bacteria appearing as dominant organisms in the microbial population of the AQDS-supplemented reactor, compared to a reactor control operated under methanogenic conditions. The consistent quinone-reducing capacity observed in the consortium indicates that it is feasible to apply quinone-reducing microorganisms in continuous bioreactors and this ability can potentially be important in wastewaters rich in humic substances. The quinone reducing activity could also be applied to accelerate the conversion of xenobiotics susceptible to reductive biotransformations such as azo dyes and polychlorinated compounds in continuous bioreactors.

A quinone-respiring, enrichment culture derived from methanogenic granular sludge was phylogenetically characterized by using a combined cloning-denaturing gradient gel electrophoresis (DGGE) method, which revealed that the consortium developed was dominated by a single microorganism: 97% related, in a sequence of 1520 base pairs, to Geobacter sulfurreducens. The enrichment culture could grow with acetate, formate or H2 when humic acids, the humic model compound, anthraquinone-2,6-disulfonate (AQDS), or chelated Fe(III) was provided as a terminal electron acceptor. The occurrence of a humic acid- or quinone-respiring microorganism in the microbial community of a wastewater treatment system suggests that this type of microorganisms may play a potential role in anaerobic bioreactors treating humus-containing wastewaters.