Distribution of sulfate-reducing and methanogenic bacteria in anaerobic aggregates determined by microsensor and molecular analyses

Using molecular techniques and microsensors for H2S and CH4, we studied the population structure of and the activity distribution in anaerobic aggrega tes. The aggregates originated from three different types of reactors: a me thanogenic 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 surfa ce layer of 50 to 100 mu m thick The sulfidogenic aggregates contained a wi der sulfate-reducing zone (the first 200 to 300 mu m from the aggregate sur face) 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 signific ant sulfate-reducing activity. Methanogenic activity in the methanogenic-su lfidogenic 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, starti ng at ca. 100 mu m from the aggregate surface. The methanogenic activity wa s 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 metabolize d in different zones. The localization of the populations corresponded to t he microsensor data. A distinct layered structure was found in the methanog enic-sulfidogenic aggregates, with sulfate-reducing bacteria in the outer 5 0 to 100 mu m, methanogens in the inner part, and Eubacteria spp. (partly s yntrophic bacteria) filling the gap between sulfate-reducing and methanogen ic bacteria. In methanogenic aggregates, few sulfate-reducing bacteria were detected, while methanogens were found in the core. In the sulfidogenic ag gregates, sulfate-reducing bacteria were present in the outer 300 mu m, and methanogens were distributed over the inner part in clusters with syntroph ic bacteria.