Development and application of microelectrodes in biofilms

The objective of this dissertation was to develop novel microelectrode techniques and to use microelectrodes for microscale chemistry characterization in biofilms. The first part of the dissertation focused on the development of microelectrodes operated in biofilms respiring on electrodes. The novel improvement required that microelectrodes be compatible within a three-electrode electrochemical cell. Redox potential, pH and hydrogen peroxide microelectrodes were successfully constructed with built-in silver/silver chloride reference electrodes that allowed for such measurements. By using these novel redox potential and pH microelectrodes we found that the decreasing redox potential in Shewanella oneidensis MR-1 biofilms from the bulk phase was not due to pH changes. Shewanella oneidensis MR-1 biofilms were not redox-controlled when they respired on electrodes. The second part of the dissertation focused on the development of a flavin microelectrode to detect flavins, known chemicals playing a role as electron mediators in Shewanella oneidensis MR-1. We successfully developed the novel microelectrode and operated it using a unique voltammetric technique, square wave voltammetry. It was found that the flavin concentration reached 0.7 microM, increasing near the bottom of the Shewanella oneidensis MR-1 biofilm, where no oxygen was present. The third part of the dissertation focused on microelectrode mapping techniques to quantify microscale geochemical gradients in Hanford 300A subsurface sediment biofilms. We found that there were significant differences in geochemical parameters across the sediment biofilm/water interface in the presence and absence of U(VI) under oxic and anoxic conditions. The results also revealed the presence of geochemical hotspots, i.e., microenvironments with relatively high or low biogeochemical activities in sediment biofilms, which had important practical implications for bioremediation processes. Overall, it is concluded that diffusion limitation in biofilms and reactions result in significant differences in chemistries between bulk and biofilm. This means that research conclusions drawn only from bulk data can be misleading. Thus, development and application of novel microelectrodes and microelectrode techniques are critical to identify the microscale chemical processes occurring inside biofilms.