Microsensor measurements of sulfate reduction and sulfide oxidation in compact microbial communities of aerobic biofilms.

The microzonation of O(2) respiration, H(2)S oxidation, and SO(4) reduction in aerobic trickling-filter biofilms was studied by measuring concentration profiles at high spatial resolution (25 to 100 mum) with microsensors for O(2), S, and pH. Specific reaction rates were calculated from measured concentration profiles by using a simple one-dimensional diffusion reaction model. The importance of electron acceptor and electron donor availability for the microzonation of respiratory processes and their reaction rates was investigated. Oxygen respiration was found in the upper 0.2 to 0.4 mm of the biofilm, whereas sulfate reduction occurred in deeper, anoxic parts of the biofilm. Sulfate reduction accounted for up to 50% of the total mineralization of organic carbon in the biofilms. All H(2)S produced from sulfate reduction was reoxidized by O(2) in a narrow reaction zone, and no H(2)S escaped to the overlying water. Turnover times of H(2)S and O(2) in the reaction zone were only a few seconds owing to rapid bacterial H(2)S oxidation. Anaerobic H(2)S oxidation with NO(3) could be induced by addition of nitrate to the medium. Total sulfate reduction rates increased when the availability of SO(4) or organic substrate increased as a result of deepening of the sulfate reduction zone or an increase in the sulfate reduction intensity, respectively.