Metagenomic and Metatranscriptomic Characterization of a Microbial Community That Catalyzes Both Energy-Generating and Energy-Storing Electrode Reactions

Abstract

Electroactive bacteria are living catalysts, mediating energy-generating reactions at anodes or energy storage reactions at cathodes via extracellular electron transfer (EET). The Cathode-ANode (CANode) biofilm community was recently shown to facilitate both reactions, however, the identity of the primary constituents and underlying molecular mechanisms remain unknown. Here, we used metagenomics and metatranscriptomics to characterize the CANode biofilm. We show that a previously uncharacterized member of the family Desulfobulbaceae, Desulfobulbaceae -2, which had <1% relative abundance, had the highest relative gene expression and accounted for over 60% of all differentially expressed genes. At the anode potential, differential expression of genes for a conserved flavin oxidoreductase (Flx) and heterodisulfide reductase (Hdr) known to be involved in ethanol oxidation suggests a source of electrons for the energy-generating reaction. Genes for sulfate and carbon dioxide reduction pathways were expressed by Desulfobulbaceae -2 at both potentials and are the proposed energy storage reactions. Reduction reactions may be mediated by direct electron uptake from the electrode, or from hydrogen generated at the cathode potential. The Desulfobulbaceae -2 genome is predicted to encode at least 85 multi-heme (≥3 hemes) c -type cytochromes, some with as many as 26 heme-binding domains, that could facilitate reversible electron transfer with the electrode. Gene expression in other CANode biofilm species was also affected by the electrode potential, although to a lesser extent, and we cannot rule out their contribution to observed current. Results provide evidence of gene expression linked to energy storage and energy-generating reactions and will enable development of the CANode biofilm as a microbially-driven rechargeable battery. IMPORTANCE Microbial electrochemical technologies (METs) rely on electroactive bacteria to catalyze energy-generating and energy storage reactions at electrodes. Known electroactive bacteria are not equally capable of both reactions and METs are typically configured to be unidirectional. Here we report on genomic and transcriptomic characterization of a recently described microbial electrode community called the Cathode-ANode (CANode). The CANode community is able to generate or store electrical current based on the electrode potential. During periods where energy is not needed, electrons generated from a renewable source, such as solar power, could be converted into energy storage compounds to later be reversibly oxidized by the same microbial catalyst. Thus, the CANode system can be thought of as a living “rechargeable battery”. Results show that a single organism may be responsible for both reactions demonstrating a new paradigm for electroactive bacteria.

Alternating Polarity as a Novel Strategy for Cultivating Electro-Methanogenic Microbial Communities Capable of Robust Biogas Production

“…Limited evidence suggests that high frequency does not favor the assembly of electro-methanogenic communities [58, 59]. For example, alternating the polarity every 10 minutes or 2 hours did not lead to the selection of methanogens or Geobacter , and the resulting communities did not produce biogas [60, 61]. It is possible that the alternation frequency should be longer than the doubling time of the populations to be selected so that they have sufficient time to adapt to the electrochemical stimulation [60, 61].…”

“…For example, alternating the polarity every 10 minutes or 2 hours did not lead to the selection of methanogens or Geobacter , and the resulting communities did not produce biogas [60, 61]. It is possible that the alternation frequency should be longer than the doubling time of the populations to be selected so that they have sufficient time to adapt to the electrochemical stimulation [60, 61]. Moreover, the alternation frequency can be asymmetric, and a longer cathodic cycle may be more favorable, considering that methanogens typically have a longer doubling time than bacteria.…”

Electricity-Driven Microbial Metabolism of Carbon and Nitrogen: A Waste-to-Resource Solution

“…154 Key enzymes of rTCA and genes for WLP were expressed differently, when reversing the polarity of mixed culture-based bioelectrodes. 155 In a heterogeneous catalyst− microbiome hybrid system, the required genes in both RBO and FAB pathways for MCFA production were identified in metagenomic profiles, whereas only the functional proteins involved in the RBO pathway could be detected by proteomic analysis. 156 In addition, proteomes and metabolomes were used to understand the energy-conservation process for light harvesting and chemical production in CdS-M. thermoacetica hybrid systems (Figure 7f).…”

“…Transcriptional analysis suggested that reductive citric acid cycle (rTCA) and WLP were responsible for CO 2 fixation, regardless of different cathode types . Key enzymes of rTCA and genes for WLP were expressed differently, when reversing the polarity of mixed culture-based bioelectrodes . In a heterogeneous catalyst–microbiome hybrid system, the required genes in both RBO and FAB pathways for MCFA production were identified in metagenomic profiles, whereas only the functional proteins involved in the RBO pathway could be detected by proteomic analysis .…”

Carbon source priority and availability limit bidirectional electron transfer in freshwater mixed culture electrochemically active bacterial biofilms

“…Some electrochemically active bacteria (EAB) are able to switch between electrogenic and electroautotrophic EET (Ross et al 2011 ; Okamoto et al 2014 ; Yu et al 2015 , 2018 ). This was later further investigated under the term bidirectional EET or canodic (from cathodic + anodic) (Yates et al 2017 ; Mickol et al 2021 ), which implies the ability of EAB to adapt and rapidly transition between electrogenic and -autotrophic EET (Pous et al 2016 ; Yates et al 2017 ; Izadi et al 2021 ; Xie et al 2021 ). This research paved the way for combined bioelectrochemical energy storage (electroautotrophy) and power generation (electrogenesis) in a single device.…”