Carbon nanotubes accelerate methane production in pure cultures of methanogens and in a syntrophic coculture


Carbon materials have been reported to facilitate direct interspecies electron transfer (DIET) between bacteria and methanogens improving methane production in anaerobic processes. In this work, the effect of increasing concentrations of carbon nanotubes (CNT) on the activity of pure cultures of methanogens and on typical fatty acid-degrading syntrophic methanogenic coculture was evaluated. CNT affected methane production by methanogenic cultures, although acceleration was higher for hydrogenotrophic methanogens than for acetoclastic methanogens or syntrophic coculture. Interestingly, the initial methane production rate (IMPR) by Methanobacterium formicicum cultures increased 17 times with 5 g. Butyrate conversion to methane by Syntrophomonas wolfei and Methanospirillum hungatei was enhanced (approximate to 1.5 times) in the presence of CNT (5 gL(-1)), but indications of DIET were not obtained. Increasing CNT concentrations resulted in more negative redox potentials in the anaerobic microcosms. Remarkably, without a reducing agent but in the presence of CNT, the IMPR was higher than in incubations with reducing agent. No growth was observed without reducing agent and without CNT. This finding is important to re-frame discussions and re-interpret data on the role of conductive materials as mediators of DIET in anaerobic communities. It also opens new challenges to improve methane production in engineered methanogenic processes.



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Salvador, AF; Martins, G; Melle-Franco, M; Serpa, R; Stams, AJM; Cavaleiro, AJ; Pereira, MA; Alves, MM

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This study was supported by the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. 323009 and by the Portuguese Foundation for Science and Technology (FCT) under the scope of the following programs: strategic funding of UID/BIO/04469/2013 unit, funded by COMPETE 2020 (POCI-01-0145-FEDER-006684), Project RECI/BBB-EBI/0179/2012 (FCOMP-01-0124-FEDER-027462), Project UID/CTM/50011/2013 (POCI-01-0145-FEDER-007679), fellowship awarded to Gilberto Martins (SFRH/BPD/80528/2011) under the scope of the program POPH/ESF and the sabbatical FCT fellowship reference SFRH/BSAB/113660/2015 attributed to Madalena Alves. The authors thank also the European Regional Development Fund under the scope of Norte2020 - Programa Operacional Regional do Norte, BioTecNorte operation (NORTE-01-0145-FEDER-000004). Finally, the authors thank to Luciana Pereira (Centre of Biological Engineering, University of Minho) and Fernando Pereira (Laboratory of Catalysis and Materials, Universidade do Porto) for providing the CNT for this study. Also a special acknowledgement is due to Orianna Bretschger from J Craig Venter Institute, for the interesting scientific discussions. All authors disclose any potential sources of conflict of interest.

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