Anode Biofilms of Geoalkalibacter ferrihydriticus Exhibit Electrochemical Signatures of Multiple Electron Transport Pathways

Abstract

Thriving under alkaliphilic conditions, Geoalkalibacter ferrihydriticus (Glk. ferrihydriticus) provides new applications in treating alkaline waste streams as well as a possible new model organism for microbial electrochemistry. We investigated the electrochemical response of biofilms of the alkaliphilic anode-respiring bacterium (ARB) Glk. ferrihydriticus voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chronoamperometry. We observed there to be at least four dominant electron transfer pathways, with their contribution to the overall current produced dependent on the set anode potential. These pathways appear to be manifested at midpoint potentials of approximately -0.14 V, -0.2 V, -0.24 V, and -0.27 V vs standard hydrogen electrode. The individual contributions of the pathways change upon equilibration from a set anode potential to another anode potential. Additionally, the contribution of each pathway to the overall current produced is reversible when the anode potential is changed back to the original set potential. The pathways involved in anode respiration in Glk. ferrihydriticus biofilms follow a similar, but more complicated, pattern as compared to those in the model ARB, Geobacter sulfurreducens. This greater diversity of electron transport pathways in Glk. ferrihydriticus could be related to its wider metabolic capability (e.g., higher pH and larger set of possible substrates, among others).

Thermophiles; or, the Modern Prometheus: The Importance of Extreme Microorganisms for Understanding and Applying Extracellular Electron Transfer

“…Obtaining data in order to construct models for EET often proceeds by cultivating dissimilatory metal reducing microbes on electrodes attached to voltammeters or potentiostats in bioelectrochemical systems (BES) or microbial electrochemical cells (MxCs) (Marcus et al, 2007, 2011; Hamelers et al, 2011; Peng et al, 2013; Renslow et al, 2013). Once grown on electrodes, electrochemical techniques including CV, CA, and EIS can be used to monitor and characterize the influence of a Gram-positive thermophilic cellular physiology on the conductivity of the extracellular matrix (K bio ) and changes in the redox potential (E KA ) of electron channeling proteins during EET (Srikanth et al, 2008; Marsili et al, 2010; Strycharz et al, 2011; Yang et al, 2012; Badalamenti et al, 2013; Parameswaran et al, 2013; Yoho et al, 2014, 2015; Tan et al, 2017; Lusk et al, 2018b). For example, research using these electrochemical techniques has already shown the presence of multiple redox pathways and proton coupled EET in exoelectrogens, including Gram-positive thermophiles (Badalamenti et al, 2013; Fu et al, 2013; Yoho et al, 2014, 2015; Zacharoff et al, 2016; Lusk et al, 2018b).…”

Spatially Resolved Electron Transport through Anode‐Respiring Geobacter sulfurreducens Biofilms: Controls and Constraints

“…Korth and Harnisch (2019) [52] recently presented a model of direct EET that demonstrates that energy harvest not only depends on anode potential but can be determined by intracellular conditions. Thus, earlier studies showed that both pH [53][54][55] and electric potential dependencies [42][43][44][45][46]56,57] shape cell activity within biofilms. However, until recently, no robust, direct, and spatially resolved cell activity data was available.…”

Repeatability of low scan rate cyclic voltammetry in bioelectrochemical systems and effects on their performance

“…If there was a single working potential causing the oxidation peak in both forward and reverse scans, the intensity in the CA corresponding to the peak potential in the LSV should remain constant at its initial value for a given time. The results of the forward peak also should apply to the peak in the reverse scan because, according to Yoho et al ., the pathways used for electron transport during anode respiration appear to be reversible.…”