In this study, we examined the stratification of microbial processes and the associated redox potential changes in biofilms using microelectrode techniques. Two types of biofilms, each with a different combination of microbial processes, were examined. The first type carried aerobic oxidation and sulfate reduction, while the second one provided aerobic oxidation and nitrification. The microelectrodes used were oxygen, sulfide, ammonium, pH and redox potential microelectrodes. The results of this study provide the following new experimental evidence: (1) The aerobic/sulfate-reducing biofilm had a clearly stratified structure with depth. In this biofilm, aerobic oxidation took place only in a shallow layer near the surface and sulfate reduction occurred in the deeper anoxic zone. The boundary between these two processes was well defined. (2) The aerobic/nitrifying biofilm also had a stratified structure with depth. In this biofilm, though aerobic oxidation took place throughout the biofilm depth, more nitrification occurred in the deeper section of the biofilm. The boundary between these two processes, however, was less well defined. (3) Redox potential could be an indicator for the existence of certain microbial processes in biofilms. The redox potential profile changes were correlated to shifts of microbial processes in both types of biofilms. The redox potential profiles in these biofilms can be used to elucidate the stratification of microbial processes in the biofilms.

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