Journal
PEERJ
Volume 6, Issue -, Pages -Publisher
PEERJ INC
DOI: 10.7717/peerj.4541
Keywords
Geobacter metallireducens; Methanosarcina barkeri; Electrically conductive pili (e-pili); Direct interspecies electron transfer (DIET); Magnetite; Ferrous iron; Ethanol metabolism; Stimulation; Co-cultures; Methane
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Funding
- Major Research Plan of the National Natural Science Foundation of China [91751112]
- General Programme of the National Natural Science Foundation of China [41573071, 41371257]
- Key Research Project of Frontier Science of the Chinese Academy of Sciences [QYZDJ-SSW-DQC015]
- Natural Science Foundation of Shandong Province [ZR2016DQ12]
- Young Taishan Scholars Programme [tsqn20161054]
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Background: Magnetite-mediated direct interspecies electron transfer (DIET) between Geobacter and Methanosarcina species is increasingly being invoked to explain magnetite stimulation of methane production in anaerobic soils and sediments. Although magnetite-mediated DIET has been documented in defined co-cultures reducing fumarate or nitrate as the electron acceptor, the effects of magnetite have only been inferred in methanogenic systems. Methods: Concentrations of methane and organic acid were analysed with a gas chromatograph and high-performance liquid chromatography, respectively. The concentration of HCl-extractable Fe(II) was determined by the ferrozine method. The association of the defined co-cultures of G. metallireducens and M. barkeri with magnetite was observed with transmission electron micrographs. Results: Magnetite stimulated ethanol metabolism and methane production in defined co-cultures of G. metallireducens and M. barkeri; however, magnetite did not promote methane production in co-cultures initiated with a culture of G. metallireducens that could not produce electrically conductive pili (e-pili), unlike the conductive carbon materials that facilitate DIET in the absence of e-pili. Transmission electron microscopy revealed that G. metallireducens and M. barkeri were closely associated when magnetite was present, as previously observed in G. metallireducens/G. sulfurreducens co-cultures. These results show that magnetite can promote DIET between Geobacter and Methanosarcina species, but not as a substitute for e-pili, and probably functions to facilitate electron transfer from the e-pili to Methanosarcina. Conclusion: In summary, the e-pili are necessary for the stimulation of not only G. metallireducens/G. sulfurreducens, but also methanogenic G. metallireducens/M. barkeri co-cultures with magnetite.
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