Extracellular Polymeric Substances from Geobacter sulfurreducens Biofilms in Microbial Fuel Cells

Abstract

Bioelectrochemical systems (BESs) are hybrid systems using electroactive bacteria and solid electrodes, which serve as electron donor or acceptor for microorganisms. When forming a biofilm on the electrode, bacteria secrete extracellular polymeric substances (EPSs). However, EPS excretion of electroactive biofilms in BES has been rarely studied so far. Consequently, the aim of this study is to develop a routine including the electrochemical cultivation, biofilm harvesting, fractionation, and biochemical analysis of the EPS secreted by Geobacter sulfurreducens under electroactive conditions. G. sulfurreducens was cultivated in microbial fuel cell mode on graphite-based electrodes polarized to +400 mV versus Ag/AgCl for 8 d. A maximum current density of 172 ± 29 μA cm–2 was reached after 7 d. The EPS secreted from the biofilms were harvested and fractioned into soluble, loosely bound, and tightly bound EPS and biochemically analyzed. Electroactive cultures secreted significantly more EPSs compared to cells grown under standard heterotrophic conditions (fumarate respiration). With 116 pg per cell, the highest amount of EPSs was measured for the soluble EPS fraction of G. sulfurreducens using anodic respiration, followed by the tightly bound (18 pg cell–1) and loosely bound (11 pg cell–1) fractions of the EPS. Proteins were found to dominate all EPS fractions of the biofilms grown under electrochemical conditions. To the best of the authors’ knowledge, these experiments are the first approach toward a complete analysis of the main EPS components of G. sulfurreducens under anode-respiring conditions.

Characterization and significance of extracellular polymeric substances, reactive oxygen species, and extracellular electron transfer in methanogenic biocathode

“…The EPS composition of the anode biofilms in both reactors was quite similar and was not affected by the differences in the cathode electrode between the two configurations. Protein was found as the major EPS component in anode biofilms in this study, which is consistent with recent reports on EPS composition in pure culture Geobacter biofilms (Stöckl et al, 2019; Tan et al, 2019). Geobacter species were also abundant in anode biofilms of both reactors in this study (discussed later).…”

“…Notably, EPS-associated proteins with redox-active features are a significant contributor to the EET from EAB to the anode (Stöckl et al, 2019;Tan et al, 2019;Xiao et al, 2017). Moreover, higher levels of redox-active proteins in anode biofilms were correlated with higher EET efficiency.…”

Redox‐Active Polymers Connecting Living Microbial Cells to an Extracellular Electrical Circuit

“…It is known that EPS constitutes 50-90% of the total organic matter in the biofilm. [68] EPS of some EET-active microbes contains redox-active molecules, such as heme-binding proteins, humic substances, and flavins, [69][70][71][72] which endows the biofilms with redox-activity. Xiao et al investigated the role of EPS in the EET of Shewanella oneidensis (S. oneidensis) MR-1 by comparing the electron transfer rate constants of c-type cytochromes in the intact cells and EPS-depleted cells.…”

“…It is known that EPS constitutes 50–90% of the total organic matter in the biofilm. [ 68 ] EPS of some EET‐active microbes contains redox‐active molecules, such as heme‐binding proteins, humic substances, and flavins, [ 69–72 ] which endows the biofilms with redox‐activity. Xiao et al.…”

Recovery of Metals from Acid Mine Drainage by Bioelectrochemical System Inoculated with a Novel Exoelectrogen, Pseudomonas sp. E8

“…Therefore, it is necessary to identify the important genes involved in the electron transfer for electricity generation of this novel exoelectrogen by comparative transcriptomic analyses of the attached cells on the surface of the anode and the planktonic cells in the MFC in the near future. The extracellular polymeric substances (EPS) are important for the function roles of single microorganism and consortium [39,40]. Characterization the compositions and redox properties of the EPS of Pseudomonas sp.…”