Methane-linked mechanisms of electron uptake from cathodes byMethanosarcina barkeri

Abstract

20The Methanosarcinales, a lineage of cytochrome-containing methanogens, have recently been proposed to participate in direct extracellular electron transfer interactions within syntrophic communities. To shed light on this phenomenon, we applied electrochemical techniques to measure electron uptake from cathodes by Methanosarcina barkeri, which is an important model organism that is genetically 25 tractable and utilizes a wide range of substrates for methanogenesis. Here we confirm the ability of M. barkeri to perform electron uptake from cathodes and show that this cathodic current is linked to quantitative increases in methane production. The underlying mechanisms we identified include, but are not limited to, a recently proposed association between cathodes and methanogen-derived extracellular enzymes (e.g. 30 hydrogenases) that can facilitate current generation through the formation of reduced and diffusible methanogenic substrates (e.g. hydrogen). However, after minimizing the contributions of such extracellular enzymes and using a mutant lacking hydrogenases, we observe a lower potential hydrogen-independent pathway that facilitates cathodic activity coupled to methane production in M. barkeri. Our electrochemical 35 measurements of wild-type and mutant strains point to a novel and extracellularenzyme-free mode of electron uptake able to take up electrons at potentials lower than -498 mV vs. SHE (over 100 mV more reduced than the observed hydrogenase midpoint potential under these conditions). These results suggest that M. barkeri can perform multiple modes (hydrogenase-mediated and free extracellular enzyme-independent) of 40 electrode interactions on cathodes including a mechanism pointing to a direct interaction, which has significant applied and ecological implications. ImportanceMethanogenic Archaea are of fundamental applied and environmental relevance. This is largely due to their activities in a wide range of anaerobic environments, generating 45 gaseous reduced carbon that can be utilized as a fuel source. While the bioenergetics of a wide variety of methanogens has been well studied with respect to soluble substrates, mechanistic understanding of their interaction with solid phase redox active compounds is limited. This work provides insight into solid phase redox interactions in Methanosarcina using electrochemical methods. We highlight a previously undescribed 50 mode of electron uptake from cathodes, that is potentially informative of direct interspecies electron transfer interactions in the Methanosarcinales.

Putative Extracellular Electron Transfer in Methanogenic Archaea

“…Recently, it was shown that hydrogenases in combination with ferredoxin (Fd) and F 420 from M. barkeri could attach to the electrode surface and catalyze the formation of H 2 . Then, the produced H 2 could be consumed rapidly by M. barkeri , resulting in a low or undetectable level of H 2 accumulation ( Rowe et al, 2019 ).…”

“…However, a hydrogenase deletion mutant of M. barkeri still exhibited the ability of electromethanogenesis with the cathode potential poised at –0.484 V vs. SHE, indicating a hydrogenase-independent mechanism to facilitate the cathodic activity ( Rowe et al, 2019 ). Furthermore, M. barkeri is capable of conducting DIET to accept electrons from syntrophic growth with G. metallireducens on ethanol ( Rotaru et al, 2014a ; Holmes et al, 2018 ).…”

“…It is possible that hydrogenases are coupled with ferredoxin (Fd) and F 420 from M. mazei to capture electrons from cathodes to form H 2 , which is then consumed for the CH 4 production. However, the experimental evidence for this speculation has yet to be revealed ( Welte and Deppenmeier, 2014 ; Rowe et al, 2019 ). M. soehngenii is a strict non-hydrogenotrophic methanogen.…”

Suppressing peatland methane production by electron snorkeling through pyrogenic carbon

“…Dependency of electron snorkeling kinetics on environmental terminal electron acceptors.The solid black line is the trend line of measured data and the dashed black line is the extrapolated trend. The reduction potentials and the corresponding terminal electron acceptors listed on the y axes were adopted from ref11,39,48 . The reduction potential (green line) of pyrogenic carbon functional groups was estimated based on direct electrochemical measurement 16 and mediated iron49 and nitrate 50 reduction.…”