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Abstract

Using molecular techniques and microsensors for H2S and CH4, we studied the population structure of and the activity distribution in anaerobic aggregates. The aggregates originated from three different types of reactors: a methanogenic reactor, a methanogenic-sulfidogenic reactor, and a sulfidogenic reactor. Microsensor measurements in methanogenic-sulfidogenic aggregates revealed that the activity of sulfate-reducing bacteria (2 to 3 mmol of S2- m(-3) s(-1) or 2 x 10(-9) mmol s(-1) per aggregate) was Located in a surface layer of 50 to 100 mu m thick The sulfidogenic aggregates contained a wider sulfate-reducing zone (the first 200 to 300 mu m from the aggregate surface) with a higher activity (1 to 6 mmol of S2- m(-3) s(-1) or 7 x 10(-9) mol s(-1) per aggregate). The methanogenic aggregates did not show significant sulfate-reducing activity. Methanogenic activity in the methanogenic-sulfidogenic aggregates (1 to 2 mmol of CH4 m(-3) s(-1) or 10(-9) mmol s(-1) per aggregate) and the methanogenic aggregates (2 to 4 mmol of CH4 m(-3) s(-1) or 5 x 10(-9) mmol s(-1) per aggregate) was located more inward, starting at ca. 100 mu m from the aggregate surface. The methanogenic activity was not affected by 10 mM sulfate during a 1-day incubation. The sulfidogenic and methanogenic activities were independent of the type of electron donor (acetate, propionate, ethanol, or H-2), but the substrates were metabolized in different zones. The localization of the populations corresponded to the microsensor data. A distinct layered structure was found in the methanogenic-sulfidogenic aggregates, with sulfate-reducing bacteria in the outer 50 to 100 mu m, methanogens in the inner part, and Eubacteria spp. (partly syntrophic bacteria) filling the gap between sulfate-reducing and methanogenic bacteria. In methanogenic aggregates, few sulfate-reducing bacteria were detected, while methanogens were found in the core. In the sulfidogenic aggregates, sulfate-reducing bacteria were present in the outer 300 mu m, and methanogens were distributed over the inner part in clusters with syntrophic bacteria.