Recovery of humic-reducing bacteria from a diversity of environments

Abstract

To evaluate which microorganisms might be responsible for microbial reduction of humic substances in sedimentary environments, humic-reducing bacteria were isolated from a variety of sediment types. These included lake sediments, pristine and contaminated wetland sediments, and marine sediments. In each of the sediment types, all of the humic reducers recovered with acetate as the electron donor and the humic substance analog, 2,6-anthraquinone disulfonate (AQDS), as the electron acceptor were members of the family Geobacteraceae. This was true whether the AQDS-reducing bacteria were enriched prior to isolation on solid media 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 but 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) serving as the sole electron acceptor. This is consistent with previous studies that have suggested that the capacity for Fe(III) reduction is a common feature of all members of the Geobacteraceae. These studies demonstrate that the potential for microbial humic substance reduction can be found in a wide variety of sediment types and suggest that Geobacteraceae species might be important humic-reducing organisms in sediments.

FIG. 1

Growth of strain JW-3 (squares) and AQDS reduction (circles) with acetate (2 mM) as the electron donor. The results of one representative experiment of triplicate determinations are depicted.

FIG. 2

Humic substance-dependent oxidation of [¹⁴C]acetate to ¹⁴CO₂ by strains JW-3 and TC-4.

FIG. 3

Phylogenetic tree showing placement of the humic-reducing isolates within the family Geobacteraceae. A total of 812 positions of the 16S rRNA sequences were used to infer the phylogenetic relationships. The sequence of E. coli was included as an outgroup. Bar, one evolutionary distance unit.

FIG. 4

Fe(II) and Fe(III) concentration profile and MPN counts of humic reducers with depth at the Talladega Wetland.

FIG. 5

Phylogenetic placement of Geobacteraceae partial 16S rDNA sequences obtained from acetate-oxidizing, humic-reducing enrichments. A total of 214 positions were considered. The sequence of D. desulfuricans was included as an outgroup. Bar, one evolutionary distance.