CARD-FISH analysis of a TCE-dechlorinating biocathode operated at different set potentials

Bioelectrochemical systems (BES) are increasingly being considered for bioremediation applications, such as the reductive transformation of chlorinated hydrocarbons in subsurface environments. These systems typically rely on a polarized solid-state electrode (i.e. a cathode) serving as electron donor for the microbially catalyzed reductive dechlorination of chlorinated contaminants. The microorganisms involved in dechlorinating biocathodes are not still identified. Particularly, it is not clear whether the same microorganisms responsible for the reductive dechlorination in 'conventional' bioremediation systems (i.e. those based on the supply of soluble substrates as electron donors) also play a role in BES. Here, we analyzed by CARD-FISH, the microbial composition of a dechlorinating biocathode operated at different set potential, in the range from -250 mV to -750 mV (vs. the standard hydrogen electrode, SHE). The rate and extent of TCE dechlorination, as well as of competing metabolisms (i.e. methanogenesis), were found to increase as the cathode potential decreased. The higher metabolic activities observed at the more reducing cathode potentials were mirrored by a higher total biomass concentration (as DAPI-stained cells) in the cathode effluent. CARD-FISH analysis revealed that Dehalococcoides was the dominant dechlorinating bacterial genus (from 65% to 100% of Bacteria) in the range from -550 mV to -750 mV, whereas it was abruptly outcompeted by other (yet unidentified) members of the Chloroflexi phylum, when the cathode was controlled in the range from -250 mV to -450 mV. Most probably, the observed changes in the microbial composition of the biocathode were driven by changes in the dominant mechanisms of electron transfer to TCE: mediated by the electrolytic production of H-2 gas (in the range from -550 mV to -750 mV), or direct (in the range of cathode potentials from -250 mV to -450 mV).