Selecting Anode-Respiring Bacteria Based on Anode Potential: Phylogenetic, Electrochemical, and Microscopic Characterization

Abstract

Anode-respiring bacteria (ARB) are able to transfer electrons contained in organic substrates to a solid electrode. The selection of ARB should depend on the anode potential, which determines the amount of energy available for bacterial growth and maintenance. In our study, we investigated how anode potential affected the microbial diversity of the biofilm community. We used a microbial electrolysis cell (MEC) containing four graphite electrodes, each at a different anode potential (E(anode) = -0.15, -0.09, +0.02, and +0.37 V vs SHE). We used wastewater-activated sludge as inoculum, acetate as substrate, and continuous-flow operation. The two electrodes at the lowest potentials showed a faster biofilm growth and produced the highest current densities, reaching up to 10.3 A/m(2) at the saturation of an amperometric curve; the electrode at the highest potential produced a maximum of 0.6 A/m(2). At low anode potentials, clone libraries showed a strong selection (92-99% of total clones) of an ARB that is 97% similar to G. sulfurreducens. At the highest anode potential, the ARB community was diverse. Cyclic voltammograms performed on each electrode suggest that the ARB grown at the lowest potentials carried out extracellular electron transport exclusively by conducting electrons through the extracellular biofilm matrix. This is supported by scanning electron micrographs showing putative bacterial nanowires and copious EPS at the lowest potentials. Non-ARB and ARB using electron shuttles in the diverse community for the highest anode potential may have insulated the ARB using a solid conductive matrix from the anode. Continuous-flow operation and the selective pressure due to low anode potentials selected for G. sulfurreducens, which are known to consume acetate efficiently and use a solid conductive matrix for electron transport.

Temperature, inocula and substrate: Contrasting electroactive consortia, diversity and performance in microbial fuel cells

“…It is hypothesised that higher anode potentials provide the bacteria with more energy and therefore lead to a greater diversity, while low potentials allow only the most capable electrogen, i.e. Geobacter [28], to colonise the anode. This would, somewhat counterintuitively, result in lower diversity where there is higher current densities at low electrode potentials.…”

“…This would, somewhat counterintuitively, result in lower diversity where there is higher current densities at low electrode potentials. Using 16S rDNA clone library construction and sequencing Torres et al, [28] have observed higher diversity when higher anode potentials were imposed. Conversely Zhu et al [29] have observed that diversity was not related to imposed anode potentials when measured using 454 pyrosequencing.…”

Microbial population and functional dynamics associated with surface potential and carbon metabolism

“…These studies revealed that different EETactive communities were established with various carbon substrates and different electrode potentials, and both parameters clearly affected the final community composition (Torres et al, 2009;White et al, 2009;Freguia et al, 2010;Kiely et al, 2011). However, it is poorly understood how the substrates and electrode potentials affect the developmental processes of EET-active microbial communities, and how metabolic networks are established and maintained within the community.…”

Reference and counter electrode positions affect electrochemical characterization of bioanodes in different bioelectrochemical systems

“…This IR drop may not only introduce errors in the measurements electrochemically, but also can result in biological changes in the bioanodes. When anodes in MFCs were acclimated to different set anode potentials they showed differences in startup time, power production, and electrochemical performance (Sun et al, 2012;Torres et al, 2009;Wang et al, 2009;Zhang et al, 2013;Zhu et al, 2013), likely due to the selection of different bacteria species with different anode potentials during the acclimation. Even with pure cultures the set potential can alter expression of different electron transport systems, thus affecting the actual reactions (Dumas et al, 2008;Wei et al, 2010).…”