Periodic Reporting for period 1 - EMES (Enhanced Microbial Electrosynthesis and Visualization of Microbial Metabolism)
Reporting period: 2018-01-01 to 2019-12-31
Electroactive bacteria are present in a whole host of environments and have recently garnered attention for both, fundamental studies, and emerging applications such as microbial fuel cells. G. sulfurreducens is amongst the most common and promising of these and devices featuring these species have attained some of the highest current densities to date. While the mechanistic details of the EET mechanism of G. sulfurreducens – loaded electrodes in anodic mode are beginning to emerge, the recently established cathodic mode remains rather ambiguous. To shed light on the mechanism of G. sulfurreducens EET, we carried out an extensive study on their biofilms as they grow on electrodes in both anodic and cathodic reaction modes, utilizing electrochemistry, Raman spectroscopy, quartz crystal microbalance measurements, and electron microscopy, with a focus on the role of cytochromes under these two conditions.
Overall objectives of the project:
-Development of a high-performing electrode for microbial electrogenesis and microbial electrosynthesis
-Development of photoanodes for microbial electrosynthesis
-Investigation of bacterial extracellular electron uptake (cathodic) mechanism
We carried out a multifaced study on the growth and electrogenesis of G. sulfurreducens in systematically switching between anodic and cathodic modes on inverse-opal indium-doped tin oxide (IO-ITO) electrodes. In addition to the conventional electrochemical experiments, we performed complementary studies using in situ resonance Raman spectroscopy, UV-Vis absorbance spectroscopy, quartz-crystal microbalance with dissipation (QCM-D), and ex-situ scanning and transmission electron microscopy (SEM and TEM) to piece together clues behind the mechanism of their anodic and cathodic electron transfer. Using this comprehensive set of measurements, we found that anodic mode function is mainly linked to the biofilm’s cytochrome expression, but the cathodic mode likely operates through an alternate channel. We propose that a Fe-containing soluble species that can either come from Fe ions in the medium or alternatively be scavenged from cytochromes is contributing to cathodic mode charge transfer under our set of reaction conditions. The presented findings add insight to G. sulfurreducens' function in its natural environments and in emerging biotechnologies, as well as press for a closer look at the multitude of EET pathways present in biological systems.
In addition, the project provided a better clue for the mechanism involved in bacterial extracellular electron uptake from a cathode. The insights obtained from the project is anticipated to greatly bolster the community’s understanding of bacterial electron uptake processes and applicability to other reactions.