Microsensor Measurements of Sulfate Reduction and Sulfide Oxidation in Compact Microbial Communities of Aerobic Biofilms

Abstract

The microzonation of 02 respiration, H2S oxidation, and so42reduction in aerobic trickling-filter biofilms was studied by measuring concentration profiles at high spatial resolution (25 to 100 ,um) with microsensors for 02, S2, 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 H2S produced from sulfate reduction was reoxidized by 02 in a narrow reaction zone, and no H2S escaped to the overlying water. Turnover times of H2S and 02 in the reaction zone were only a few seconds owing to rapid bacterial H2S oxidation. Anaerobic H2S oxidation with N03could be induced by addition of nitrate to the medium. Total sulfate reduction rates increased when the availability of s042or organic substrate increased as a result of deepening of the sulfate reduction zone or an increase in the sulfate reduction intensity, respectively.

Perspectives and predictions on the microbial ecology of the hyporheic zone

“…(Paerl & Pinckney, 1996). It also has been documented in the biofilm of an aerobic trickling filter (Kuhl & Jorgensen, 1992), which is more similar to the biofilms occurring in hyporheic zones in that it was micrometres thick and exposed to a constant interstitial flow of oxygenated water; here oxidation of H2S accounted for 48% of total oxygen consumption. This suggests that the micrometres-thick epilithic biofilms within stream beds also may create suitable micro-environments, in which chemolithotrophic processes can play an important role in subsurface oxygen dynamics and might contribute significantly to hyporheic microbial production.…”

“…Because the matrix isolates microcolonies from the bulk water, biofilms allow metabolic activities which are quite different from those expected from the physicochemical parameters of the bulk water (Paerl & Pinckney, 1996). Thus, as mentioned earlier, anaerobic activities are able to occur in aerobic habitats, and aerobic and anaerobic processes, such as sulphide oxidation and sulphate reduction, can occur in different layers of the same biofilm (Kuhl & Jorgensen, 1992). Because the matrix maintains a close proximity between microcolonies of different bacterial types, sites of specific limited niches can be created.…”

“…Descriptions of hyporheic microbial ecology must incorporate these properties of biofilm dynamics. Kuhl & Jorgensen (1992) have shown that entire cycles of sulphur and nitrogen may occur within a biofilm, so that large-scale measures of nutrient transformations may not reveal the diversity of microbial processes occurring within the habitat; yet Paerl & Pinckney (1996) emphasize that measuring microscale processes is very important for obtaining realistic and representative macroscale fluxes.…”

“…These indicators of biofilm community structure are most powerful when combined with direct measurements of chemical concentrations and fluxes at known positions in the biofilm. Micro-electrodes are now available which measure concentrations of oxygen, pH, sulphide, ammonia, nitrate and nitrous oxide (Kuhl & Jorgensen, 1992;Schramm et al, 1996). With tips measuring only 2±50 mm, these micro-electrodes make it possible to study microzonations of bacterial substrates and products on scales of less than 100 mm (Kuhl & Jorgensen, 1992).…”

“…Micro-electrodes are now available which measure concentrations of oxygen, pH, sulphide, ammonia, nitrate and nitrous oxide (Kuhl & Jorgensen, 1992;Schramm et al, 1996). With tips measuring only 2±50 mm, these micro-electrodes make it possible to study microzonations of bacterial substrates and products on scales of less than 100 mm (Kuhl & Jorgensen, 1992). From concentration profiles of the various compounds, fluxes, and hence production rates, can be calculated, using Fick's first law of diffusion (Schramm et al, 1996).…”