Microbial long distance electron transfer in contaminated aquifers.

Abstract

In marine sediments, filamentous Desulfobulbaceae have been shown to bridge sulfide oxidation and oxygen reduction over centimeter distances. The spatially separated redox half reactions lead to an alkaline pH where protons are consumed by oxygen reduction and to an acidic pH where sulfide is oxidized to sulfate. To analyze if long distance electron transfer occurs in contaminated groundwater we set up batch experiments with natural sediments amended with FeS as electron donor. After 2-3 months, the pore water showed pH, oxygen, and sulfide gradients characteristic of long distance electron transfer of 4-20 mm. Spatially resolved analysis of the microbial communities by tRFLP and 454 pyrosequencing indicated a relative abundance of Desulfobulbaceae of up to 45% within the suboxic zone. Fluorescence in situ hybridization revealed that the sequences belonged to filamentous Desulfobulbaceae. We then took intact sediment cores from the capillary fringe of a tar oil contaminated aquifer at 7 meters depth and analyzed the microbial communities at cm resolution. The filamentous Desulfobulbaceae were not only present in situ but accounted for up to 27% Desulfobulbaceae of the total bacterial numbers. An estimate of the total abundance showed about 3-4 m filamentous Desulfobulbaceae per ml sediment. These results suggest long distance electron transfer naturally occurs in contaminated aquifers and might be an important process in the recycling of the electron acceptor sulfate. Thus, the newly discovered process might be important in increasing degradation rates close to the capillary fringe