Abstract
Syntrophic methanogenesis is an essential link in the global carbon cycle and a key bioprocess for the disposal of organic waste and production of biogas. Recent studies suggest direct interspecies electron transfer (DIET) is involved in electron exchange in methanogenesis occurring in paddy soils, anaerobic digesters, and specific co-cultures with Geobacter. In this study, we evaluate the possible involvement of DIET in the syntrophic oxidation of butyrate in the enrichments from two lake sediments (an urban lake and a natural lake). The results showed that the production of CH4 was significantly accelerated in the presence of conductive nanoscale Fe3O4 or carbon nanotubes in the sediment enrichments. Observations made with fluorescence in situ hybridization and scanning electron microscope indicated that microbial aggregates were formed in the enrichments. It appeared that the average cell-to-cell distance in aggregates in nanomaterial-amended enrichments was larger than that in aggregates in the non-amended control. These results suggested that DIET-mediated syntrophic methanogenesis could occur in the lake sediments in the presence of conductive materials. Microbial community analysis of the enrichments revealed that the genera of Syntrophomonas, Sulfurospirillum, Methanosarcina, and Methanoregula were responsible for syntrophic oxidation of butyrate in lake sediment samples. The mechanism for the conductive-material-facilitated DIET in butyrate syntrophy deserves further investigation.
Highlights
Methane (CH4) is an important greenhouse gas, and a well-known component of biogas, which is used as fuel
The stoichiometric conversion of butyrate in the first transfer of Erhai Lake (EHL) enrichment was obtained at 32 days in the presence of nanoFe3O4 compared with 42 days in the control (Figure 1E)
In all of the enrichments, we found that the addition of nanoFe3O4 substantially facilitated the syntrophic production of CH4 from butyrate oxidation
Summary
Methane (CH4) is an important greenhouse gas, and a well-known component of biogas, which is used as fuel. It was proposed that direct interspecies electron transfer (DIET) is an alternative to the interspecies H2/formate transfer in syntrophic methanogenesis (Morita et al, 2011; Rotaru et al, 2014b). The DIET-mediated syntrophic methanogenesis has been demonstrated in co-cultures of Geobacter metallireducens and Methanosaeta (Rotaru et al, 2014b) or Methanosarcina species (Rotaru et al, 2014a). The outer surface c-type cytochromes and electrically conductive pili are considered to play a role in mediating DIET when Geobacter species are involved (Lovley, 2012; Shrestha and Rotaru, 2014)
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