Methane production byMethanothrix thermoacetophilavia direct interspecies electron transfer withGeobacter metallireducens

Abstract

Methanothrixis widely distributed in natural and artificial anoxic environments and plays a major role in global methane emissions. It is one of only two genera that can form methane from acetate dismutation and through participation in direct interspecies electron transfer (DIET) with exoelectrogens. AlthoughMethanothrixis a significant member of many methanogenic communities, little is known about its physiology. In this study, transcriptomics helped to identify potential routes of electron transfer during DIET betweenGeobacter metallireducensandMethanothrix thermoacetophila. Additions of magnetite to cultures significantly enhanced growth by acetoclastic methanogenesis and by DIET, while granular activated carbon (GAC) amendments impaired growth. Transcriptomics suggested that the OmaF-OmbF-OmcF porin complex and the octaheme outer membranec-type cytochrome, Gmet_0930, were important for electron transport across the outer membrane ofG. metallireducensduring DIET withMx. thermoacetophila. Clear differences in the metabolism ofMx. thermoacetophilawhen grown via DIET or acetate dismutation were not apparent. However, genes coding for proteins involved in carbon fixation and a surface associated quinoprotein, SqpA, were highly expressed in all conditions. Expression of gas vesicle genes was significantly lower in DIET- than acetate-grown cells, possibly to facilitate better contact between membrane associated redox proteins during DIET. These studies reveal potential electron transfer mechanisms utilized by bothGeobacterandMethanothrixduring DIET and provide important insights into the physiology ofMethanothrixin anoxic environments.

Electrobiocorrosion by microbes without outer‐surface cytochromes

“…Thus, the current evidence suggests that Type I Methanosarcina and Methanothrix species, both of which lack outer‐surface cytochromes, are capable of electrobiocorrosion. This is consistent with DIET studies in which Type I Methanosarcina and Methanothrix species directly accepted electrons from G. metallireducens without a H 2 intermediary 31,35,37,38,46–48 . The electrical contacts on the outer surface of Type I Methanosarcina and Methanothrix remain to be determined.…”

“…Potential candidates include putative outer‐surface proteins whose genes were highly expressed in M. barkeri and Mx. thermoacetophila during growth via DIET, including surface‐associated quinoproteins 38,46,49 .…”

“…However, Fe 0 oxidation under conditions that rule out the possibility of H 2 serving as an intermediary electron carrier is required to more fully evaluate this possibility. Methanothrix harundinaceae 37 and Methanothrix thermoacetophila 38 are also capable of accepting electrons via DIET despite a lack of outer‐surface cytochromes. Their potential for electrobiocorrosion has also not been previously reported.…”

Detrimental Impact of a Type VI Secretion System on Direct Interspecies Electron Transfer

“…These included Gmet_0930, which codes for an octaheme outer membrane c -type cytochrome and Gmet_0910, the gene for the outer membrane c -type cytochrome, OmcF, from the PccF porin-cytochrome complex (59). Both Gmet_0930 and Gmet_0910 are important for Fe(III) oxide reduction and DIET-based growth (17, 59, 80). Gmet_2029, which codes for a lipopolysaccharide protein likely to be involved in biofilm formation and required for Fe(III) oxide reduction (80), was expressed more than 3 fold higher in DIET-grown cells, but pilA was not more highly expressed by DIET-grown G. metallireducens cells (Table S2a).…”

“…Most of the initial research following the discovery of DIET (9) focused on identifying which microbes have the potential to participate in DIET and the organic substrates that can support DIET (5,8,(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21). Study of the expression of genes and proteins that enhance electron exchange between species has also been emphasized (5,9,10,13,(22)(23)(24)(25).…”