Stratified Microbial Structure and Activity in Sulfide- and Methane-Producing Anaerobic Sewer Biofilms

Abstract

bSimultaneous production of sulfide and methane by anaerobic sewer biofilms has recently been observed, suggesting that sulfate-reducing bacteria (SRB) and methanogenic archaea (MA), microorganisms known to compete for the same substrates, can coexist in this environment. This study investigated the community structures and activities of SRB and MA in anaerobic sewer biofilms (average thickness of 800 m) using a combination of microelectrode measurements, molecular techniques, and mathematical modeling. It was seen that sulfide was mainly produced in the outer layer of the biofilm, between the depths of 0 and 300 m, which is in good agreement with the distribution of SRB population as revealed by cryosection-fluorescence in situ hybridization (FISH). SRB had a higher relative abundance of 20% on the surface layer, which decreased gradually to below 3% at a depth of 400 m. In contrast, MA mainly inhabited the inner layer of the biofilm. Their relative abundances increased from 10% to 75% at depths of 200 m and 700 m, respectively, from the biofilm surface layer. High-throughput pyrosequencing of 16S rRNA amplicons showed that SRB in the biofilm were mainly affiliated with five genera, Desulfobulbus, Desulfomicrobium, Desulfovibrio, Desulfatiferula, and Desulforegula, while about 90% of the MA population belonged to the genus Methanosaeta. The spatial organizations of SRB and MA revealed by pyrosequencing were consistent with the FISH results. A biofilm model was constructed to simulate the SRB and MA distributions in the anaerobic sewer biofilm. The good fit between model predictions and the experimental data indicate that the coexistence and spatial structure of SRB and MA in the biofilm resulted from the microbial types and their metabolic transformations and interactions with substrates. Sewer biofilms comprise complex multispecies microflora with a typical thickness of only about 1 mm (1). Depending on the electron donors and electron acceptors present in the wastewater, different carbon transformation processes can occur in close proximity in the sewer biofilms. Domestic wastewater normally contains a significant concentration (ca. 100 to 1,000 M) of sulfate but negligible nitrite and nitrate concentrations (2, 3). Therefore, under anaerobic conditions (which normally occur in pressure sewers fully filled with wastewater), sulfate reduction carried out by sulfate-reducing bacteria (SRB) could be an important terminal electron-accepting process in the sewer biofilms. The sulfate reduction activity in anaerobic sewers is important, as the production of sulfide can be transferred to the gas phase of partially filled gravity sewers and cause extensive corrosion of concrete sewer pipes (4, 5). Also, the emission of sulfide from sewers can cause odor problems for the surrounding area and pose health risks to sewer workers (6, 7). Apart from sulfate reduction, methanogenesis caused by the respiration of methanogenic archaea (MA) could also be a key terminal process in anaerobic sewer biofilms (8, 9). Guisasola and ...

Measurement and understanding of methane emission from sewers

“…200-500 mg COD/L) for both SRB and MA to co-exist in sewer biofilms, thereby allowing simultaneous methane and sulfide production despite their competition for substrates (Conrad et al, 1987, Raskin et al, 1996, Robinson and Tiedje, 1984. This co-existence is due to stratification of these anaerobic microorganisms in the biofilm (Guisasola et al, 2008, Sun et al, 2014. SRB are found to dominate the microbial community in the top layers of the biofilms where the sulfate concentration is relatively high ( Fig.…”

“…MA of the Methanosataceae, Methanomicrobiales, Methanosarcinaceae, Methanococcales and Methanocaldococcaceae are detected in rising main sewer biofilms while Methanobacteriales is absent (Mohanakrishnan et al, 2009b). In a recent study of a sewer rising main biofilm it was found that 90% of the MA population belonged to the genus Methanosaeta, and these are obligate acetoclastic methanogens (Sun et al, 2014). MA that use other substrates such as hydrogen only accounted for less than 10% of the total MA population in the sewer biofilm.…”

“…2.2). However, MA require low-sulfate conditions, and absence of sulfate in the deeper layers of biofilms due to the diffusional limitation of sulfate combined with an adequate supply of methanogenesis precursors promotes its growth (Guisasola et al, 2008, Sun et al, 2014. …”

“…Over the years, most of the studies on sulfide and methane production in sewers have focused on sewer biofilms, particularly those in rising mains , Sun et al, 2014. These studies demonstrated that both sulfide and methane are generated in significant amounts from biofilms in anaerobic rising main sewers due to the biological sulfate reduction process mediated by sulfate reducing bacteria (SRB) and methanogenesis mediated by methanogenic archaea (MA) within the sewer biofilms that are attached to the walls of rising mains.…”

“…Previous studies have identified rising main sewers as a source of hydrogen sulfide and methane (Boon, 1995, Guisasola et al, 2008. Sulfate-reducing bacteria (SRB) and methanogenic archaea (MA) that grow in the biofilm attached to the inner surfaces of sewer walls are primarily responsible for sulfide and methane production in rising main sewers (Foley et al, 2009, Guisasola et al, 2008, Sun et al, 2014. Sediment in gravity sewers is also believed to be biologically active (Schmitt and Seyfried, 1992), and thus could contribute to sulfide and methane production.…”