Stratification of microbial metabolic processes and redox potential change in an aerobic biofilm studied using microelectrodes

Abstract

In this study we used oxygen, sulfide, redox potential and pH microelectrodes to examine the stratification of microbial metabolic processes and the change of redox potential within an aerobic biofilm used to treat azo dye containing wastewater. These microelectrodes have tip diameters of 3 to 20 μm and a high spatial resolution. They were used to measure the profiles of oxygen, total dissolved sulfide, redox potential and pH as a function of depth in the biofilm. These profiles demonstrated that oxygen was depleted at 550 μm from the surface and the deeper section of the biofilm was actually anaerobic. While aerobic oxidation took place only in a shallow layer near the surface, sulfate reduction occurred in the deeper anaerobic zone, even with a low concentration of sulfate (6.75 mg/1 as SO2-) in the bulk solution. We discovered a sharp decrease of redox potential (271 mV) from a positive potential to a negative potential within a very narrow band of 50 μm near the interface between the aerobic zone and the sulfate reduction zone. The new experimental findings support the concept of stratification of the microbial metabolic processes in biofilms.