RECOVERY OF HUMIC-REDUCING BACTERIA FROM A DIVERSITY OF ENVIRONMENTS

To evaluate which microorganisms might be responsible for microbial re duction of humic substances in sedimentary environments, humic-reducin g bacteria were isolated from a variety of sediment types. These inclu ded lake sediments, pristine and contaminated wetland sediments, and m arine sediments. In each of the sediment types, all of the humic reduc ers recovered with acetate as the electron donor and the humic substan ce analog, 2,6-anthraquinone disulfonate (AQDS), as the electron accep tor were members of the family Geobacteraceae. This was true whether t he AQDS-reducing bacteria were enriched prior to isolation on solid me dia or were recovered from the highest positive dilutions of sediments in liquid media. All of the isolates tested not only conserved energy to support growth from acetate oxidation coupled to AQDS reduction bu t also could oxidize acetate with highly purified soil humic acids as the sole electron acceptor. All of the isolates tested were also able to grow with Fe(III) reduction is a common feature of all members of t he Geobacteraceae. These studies demonstrate that the potential for mi crobial humic substance reduction can be found in a wide variety of se diment types and suggest that Geobacteraceae species might be importan t humic-reducing organisms in sediments.