Abstract
Nitrate is considered as a contamination since it’s over discharging to water incurs environmental problems. However, nitrate is an ideal electron sink for anaerobic pollutant degraders desiring electron acceptors due to the high redox potential. Unfortunately, not all degraders can directly reduce nitrate, and the anaerobic direct interspecies electron transfer (DIET) between degraders and denitrifiers has not been confirmed yet. Here we demonstrated that syntrophic growth of Geobacter sulfurreducens PCA with denitrifying microbial community at anaerobic condition eliminated the lag phase of 15 h and improved the denitrification rate by 13∼51% over a broad C/N ratio of 0.5 to 9. Quantitative PCR revealed that G. sulfurreducens selectively enhanced the expression of nirS coding for a cytochrome cd1-nitrite reductase, resulting in a fast and more complete denitrification. Geobacter also selectively enriched its potential denitrifying partners – Diaphorobacter, Delftia, and Shinella – to form spherical aggregates. More studies of the binary culture system need to be carried out to confirm the syntrophic mechanism of Geobacter and denitrifiers in the future. These findings extend our knowledge on understanding the anaerobic bacterial interspecies electron transfer in the denitrification process, which has broader implications in fast selection and stabilization of denitrifiers in wastewater treatment plant, and general understanding of ecology for nitrogen and metal cycling.
Highlights
Interspecies electron transfer is critical in anaerobic digester due to the lack of electron acceptors
Very interestingly, when G. sulfurreducens PCA was mixed with the denitrifying microbial community (Group G-D), nitrate reduction started immediately without any lag period
It is hypothesized that Geobacter and several denitrifiers fast united as a strategy to survive through interspecies electron transfer, so that the lag phase seems to be eliminated
Summary
Interspecies electron transfer is critical in anaerobic digester due to the lack of electron acceptors. It had been demonstrated that the syntrophic growth of G. sulfurreducens and Wolinella succinogenes (not use acetate) can oxidize acetate and reduce nitrate to ammonium efficiently. The electrons from acetate oxidation was transferred probably through interspecies hydrogen transfer (0.02 to 0.04 Pa) to W. succinogenes for nitrate reduction (Cord-Ruwish et al, 1998) This process is called “dissimilatory nitrate reduction to ammonium (DNRA),” which is different from the denitrification with N2 or N2O as the end product. Interspecies electron transfer between Geobacter and denitrifier was only found between G. sulfurreducens and an autotrophic Thiobacillus denitrificans when conductive nanoFe3O4 was added (Kato et al, 2012) This coculture seems to experience a DNRA process with ammonium as the end product. Denitrification at different C/N ratio was investigated using this syntrophic community
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