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

Abstract

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·L CNT. Butyrate conversion to methane by Syntrophomonas wolfei and Methanospirillum hungatei was enhanced (∼1.5 times) in the presence of CNT (5 g·L ), 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.

Extracellular Electron Uptake by TwoMethanosarcinaSpecies

“…Since H 2 could not be generated by G. metallireducens , a strict H 2 -utilizer like M. formicicum was rendered incapable of an interspecies association based on H 2 -transfer with this bacterium (Rotaru 2014b). However, more recent studies indicated that conductive carbon nanotubes stimulated methanogenesis by M. formicicum (Salvador et al, 2017). This implied that M. formicicum might be encouraged by the presence of conductive particle to interact syntrophically with Geobacter .…”

Methane Production and Conductive Materials: A Critical Review

“…In that work, higher concentrations of multiwalled CNT resulted in a growth medium with a more negative ORP, which benefited methanogenesis that ideally occurs at ORP ranging from −200 mV and −400 mV. 118 Nevertheless, Salvador and co-workers 18 also observed that in the absence of a reducing agent, the ORP increased (to values reaching approximately −200 mV), having the opposite effect verified in the assays performed with the reducing agent. Surprisingly, without reducing agent, the methanogenic activity of Methanobacterium formicicum still increased with increasing concentrations of multiwalled CNT.…”

“…In general, lag phases preceding methane production are reduced and methane production rates increase when CM are added to batch experiments. 8,16,[18][19][20]27,74,78,79 In continuous anaerobic digesters, CM also accelerate methane production and contribute for a more stable operation, allowing higher applied organic loading rates while maintaining high COD removal rates. 21,80−83 Only few studies used direct methanogenic substrates, such as acetate or H 2 /CO 2 , 18,24,84,85,98 while most studies used volatile fatty acids, such as butyrate and propionate, and alcohols as substrates to promote the syntrophic metabolism within microbial communities.…”

“…8,16,[18][19][20]27,74,78,79 In continuous anaerobic digesters, CM also accelerate methane production and contribute for a more stable operation, allowing higher applied organic loading rates while maintaining high COD removal rates. 21,80−83 Only few studies used direct methanogenic substrates, such as acetate or H 2 /CO 2 , 18,24,84,85,98 while most studies used volatile fatty acids, such as butyrate and propionate, and alcohols as substrates to promote the syntrophic metabolism within microbial communities. 8,[16][17][18]20,[22][23][24]78,79,82,85,96,100 More complex organic matter, such as dairy wastewater, 1,83,86 brewery wastewater, 20 beet sugar wastewater, 87 food waste, 13 municipal solid waste, 19 waste activated sludge 89,90,99,103,104 or leachate 81 have also been investigated.…”

Role and Potential of Direct Interspecies Electron Transfer in Anaerobic Digestion

“…However, recently, several studies revealed that carbon nanotubes successfully promoted methanogenesis in syntrophic co-cultures as well as in complex environmental matrices [79,80]. Li et al [79] explained how the cytotoxicity of carbon nanotubes could be reduced in experiments: in anaerobic condition, the oxidative stress by free radicals is less than in an aerobic environment, and the high-purity (>90% pure) carbon nanotubes used in this study might not induce any significant residual metal toxicity.…”