Potential direct interspecies electron transfer (DIET) from Clostridium spp. to Methanosarcina barkeri in methanogenic isolates

Abstract:

[Objective] Geobacter metallireducens, a Gram-negative bacterium, can directly transfer electron to acetoclastic methanogens such as Methanosaeta harundinacea and Methanosarcina barkeri for reducing carbon dioxide to methane. Our previous results showed that Methanosarcina mazei and Geobacteraceae formed aggregates in an iron(III)-reducing enrichment culture indicating direct interspecies electron transfer. However, the capability of direct electron transfer with methanogens for Gram-positive iron(III)-reducing bacteria such as Clostridium spp. is still unknown. [Methods] In this further study, methanogenic isolates (S6) was achieved from the iron (III)-reducing enrichment by roll-tube (Hungate) method with ethanol as the sole electron donor. We used terminal restriction fragment length polymorphism (T-RFLP) and clone library analysis to investigate the community of S6 and used electrochemical method such as cyclic voltammetry (CV) to confirm the electroactivity of isolates. [Results] Clone library analysis of 16S rRNA gene showed that Clostridium spp. (close to C. tunisiense) and Methanosarcina barkeri was predominant in the bacterial and archaeal community, respectively. Interestingly, addition of G. metallireducens into S6 did not increase the ability of both iron(III) reduction and methanogenesis, indicating that Clostridium spp. may play a similar role in direct interspecies electron transfer from G. metallireducens to M. barkeri. Furthermore, current generation of the S6 suspension dramatically decreased when contact between the organisms and the electrodes was prevented by dialysis bag, and CV showed no obvious redox peaks. [Conclusion] These results suggested that there was direct electron transfer in the methanogenic isolates. This work demonstrate that the dominant Gram-positive Clostridium spp. can potentially directly transfer electron to M. barkeri in methanogenic isolates.