Abstract
The microbial electrolysis cell assisted anaerobic digestion holds great promises over conventional anaerobic digestion. This article reports an experimental investigation of extracellular polymeric substances (EPS), reactive oxygen species (ROS), and the expression of genes associated with extracellular electron transfer (EET) in methanogenic biocathodes. The MEC-AD systems were examined using two cathode materials: carbon fibers and stainless-steel mesh. A higher abundance of hydrogenotrophic Methanobacterium sp. and homoacetogenic Acetobacterium sp. appeared to play a major role in superior methanogenesis from stainless steel biocathode than carbon fibers. Moreover, the higher secretion of EPS accompanied by the lower ROS level in stainless steel biocathode indicated that higher EPS perhaps protected cells from harsh metabolic conditions (possibly unfavorable local pH) induced by faster catalysis of hydrogen evolution reaction. In contrast, EET-associated gene expression patterns were comparable in both biocathodes. Thus, these results indicated hydrogenotrophic methanogenesis is the key mechanism, while cathodic EET has a trivial role in distinguishing performances between two cathode electrodes. These results provide new insights into the efficient methanogenic biocathode development.
Highlights
The microbial electrolysis cell assisted anaerobic digestion holds great promises over conventional anaerobic digestion
In microbial electrolysis cell (MEC)-anaerobic digestion (AD) systems, methane can be produced via multiple pathways, such as (1) direct electron transfer from the cathode to electrotrophic methanogens coupled with C O2 reduction to methane, and (2) hydrogenotrophic methanogenesis of H2 produced via cathodic hydrogen evolution reaction (HER)[4,5,6,7]
Methane can be produced via direct interspecies electron transfer (DIET) between electroactive bacteria (EAB) and electrotrophic methanogens in cathode and anode electrodes[7,8,9]
Summary
The microbial electrolysis cell assisted anaerobic digestion holds great promises over conventional anaerobic digestion. EET-associated gene expression patterns were comparable in both biocathodes These results indicated hydrogenotrophic methanogenesis is the key mechanism, while cathodic EET has a trivial role in distinguishing performances between two cathode electrodes. These results provide new insights into the efficient methanogenic biocathode development. In MEC-AD systems, methane can be produced via multiple pathways, such as (1) direct electron transfer from the cathode to electrotrophic methanogens coupled with C O2 reduction to methane, and (2) hydrogenotrophic methanogenesis of H2 produced via cathodic hydrogen evolution reaction (HER)[4,5,6,7]. To the best of the authors’ knowledge, reports on biocathode EPS characteristics and expressions of EET genes in MEC-AD systems are still scarce
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