Microbial reductive dechlorination of trichloroethene to ethene with electrodes serving as electron donors without the external addition of redox mediators

In situ bioremediation of industrial chlorinated solvents, such as trichloroethene (TCE), is typically accomplished by providing an organic electron donor to naturally occurring dechlorinating populations. In the present study, we show that TCE dechlorinating bacteria can access the electrons required for TCE dechlorination directly from a negatively polarized (-450 mV vs. SHE) carbon paper electrode. In replicated batch experiments, a mixed dechlorinating culture, also containing Dehalococcoides spp., dechlorinated TCE to cis-dichloroethene (cis-DCE) and lower amounts of vinyl chloride (VC) and ethene using the polarized electrode as the sole electron donor. Conversely, neither VC nor ethene formation occurred when a pure culture of the electro-active microorganism Geobacter lovleyi was used, under identical experimental conditions. Cyclic voltammetry tests, carried out on the filter-sterilized supernatant of the mixed culture revealed the presence of a self-produced redox mediator, exhibiting a midpoint potential of around -400 mV (vs. SHE). This yet unidentified redox-active molecule appeared to be involved in the extracellular electron transfer from the electrode to the dechlorinating bacteria. The ability of dechlorinating bacteria to use electrodes as electron donors opens new perspectives for the development of clean, versatile, and efficient bioremediation systems based on a controlled subsurface delivery of electrons in support of biodegradative metabolisms and provides further evidence on the possibility of using conductive materials to manipulate and control a range of microbial bioprocesses.