Selecting anode-respiring bacteria based on anode potential: phylogenetic, electrochemical, and microscopic...

Area of Science:

• Microbiology
• Electrochemistry
• Environmental Science

Background:

• Anode-respiring bacteria (ARB) utilize organic substrates and transfer electrons to solid electrodes.
• Anode potential influences the energy available for ARB growth and maintenance.
• Understanding ARB community dynamics under varying anode potentials is crucial for optimizing bioelectrochemical systems.

Purpose of the Study:

• To investigate the effect of different anode potentials on the microbial diversity of anode-respiring bacteria biofilms.
• To determine the dominant ARB species and their electron transfer mechanisms at various anode potentials.
• To assess the impact of anode potential on biofilm growth and current generation.

Main Methods:

• Utilized a microbial electrolysis cell with four graphite electrodes at distinct anode potentials (-0.15, -0.09, +0.02, +0.37 V vs SHE).
• Inoculated with wastewater-activated sludge and used acetate as the substrate under continuous-flow operation.
• Employed clone libraries, cyclic voltammograms, and scanning electron microscopy to analyze microbial communities and electron transfer.

Main Results:

• Electrodes at lower potentials (-0.15, -0.09 V) exhibited faster biofilm growth and significantly higher current densities (up to 10.3 A/m²).
• Low anode potentials strongly selected for ARB closely related to Geobacter sulfurreducens (92-99% of clones).
• The ARB community was diverse at the highest anode potential (+0.37 V), with lower current generation (0.6 A/m²).

Conclusions:

• Low anode potentials act as a selective pressure favoring Geobacter sulfurreducens, which efficiently uses solid conductive matrices for electron transport.
• The mechanism of extracellular electron transfer at low potentials involves direct conduction through the biofilm matrix, supported by bacterial nanowires and EPS.
• Community diversity at higher anode potentials may involve electron shuttling, potentially hindering direct electron transfer by ARB to the anode.