Extracellular Electron Uptake by TwoMethanosarcinaSpecies

Abstract

9Direct electron uptake by prokaryotes is a recently described mechanism with a potential application for 10 energy and CO 2 storage into value added chemicals. Members of Methanosarcinales, an 11 environmentally and biotechnologically relevant group of methanogens, were previously shown to 12 retrieve electrons from an extracellular electrogenic partner performing Direct Interspecies Electron 13 45 (Holmes et al., 2018b; Shrestha et al., 2013). Geobacter's requirement for outer-surface proteins during 46 DIET was confirmed earlier with gene-deletion studies (Rotaru et al., 2014a(Rotaru et al., , 2014b. Thus, if 47 Geobacter lacked the ability to produce e.g. pili it was incapable of DIET. 48Although we understand how Geobacter releases electrons outside their cells during DIET, the way 49 Methanosarcinales retrieve DIET-electrons is poorly understood. A glimpse at this mechanism was 50 provided in a recent comparative transcriptomic study (Holmes et al., 2018b). In this study, the 51 transcriptomes of DIET co-cultures (G. metallireducens -Methanosarcina barkeri) were compared to 52 those of co-cultures performing interspecies H 2 -transfer (Pelobacter carbinolicus -M. barkeri). During 53 DIET, M. barkeri had higher expression of membrane-bound redox-active proteins like cupredoxins, 54 thioredoxins, pyrroloquinoline, and quinone-, cytochrome-or Fe-S containing proteins (Holmes et al., 55 2018b). Still, the exact mechanism of electron uptake by Methanosarcina has not been validated and 56 warrants further investigation. 57 Moreover, Methanosarcina can also retrieve electrons from electrically conductive particles charged by 58 Geobacter oxidizing organics (Shrestha and Rotaru, 2014). In effect, DIET is accelerated by electrically 59 conductive particles/minerals perhaps because they replace conductive and redox active surface proteins 60 diminishing cellular energy expenditure required to overexpress such surface constituents (Liu et al., 61 2012). For instance, co-cultures of G. metallireducens and M. barkeri were stimulated at the addition of 62 conductive particles, such as GAC (Liu et al., 2012), carbon cloth (Chen et al., 2014a), biochar (Chen et 63 al., 2014b), or magnetite (Wang et al., 2018). On the other hand, the addition of non-conductive 64 materials did not stimulate DIET (Chen et al., 2014a; Rotaru et al., 2018a). In addition, conductive 65 particles appear to play a significant role in interspecies interactions from natural and artificial 66 ecosystems such as sediments, soils, rice paddies or anaerobic digesters (Holmes et al., 2017a; Rotaru et 67 al., 2018b; Ye et al., 2018; Zhang et al., 2018). In these cases, the addition of conductive particles 68 enriched for DIET-related Methanosarcinales (Dang et al., 2016; Holmes et al., 2017b; Rotaru et al., 69 2018a; Zheng et al., 2015). However, exceptions were observed since occasionally conductive particles 70 enriched for H 2 -utilizing methanogens of the genus Methanospirillum (Lee et al., 2016) or 71Methanobacterium (Lin et al., 2017; Zhua...

Extracellular electron uptake in Methanosarcinales is independent of multiheme c-type cytochromes

“…G. metallireducens, G. hydrogenophilus and R. ferrireducens were maintained on the typical co-culture freshwater media 10 with 55 mM ferric citrate as electron acceptor. The two Geobacter species were maintained with 10-20 mM ethanol as electron donor 11,12 whereas Rhodoferax was maintained with 5 mM glucose. P. carbinolicus which was used as H 2 -donating strain (DIET-negative control), was pre-cultivated under fermentative conditions on 10 mM acetoin as previously described 2 .…”

“…Methanothrix soehengii, Methanosaeta harundinacea, Methanospirilum hungatei and Methanoculleus marisnigri were pre-grown on the typical co-culture freshwater media however without ethanol 10 Prior to co-cultivation, all Methanosarcina cultures were pre-grown on their respective substrates (20-30 mM acetate and/or 20 mM methanol) in a modified DSMZ 120c media in which tryptone was omitted and NaCl was reduced down to 1 g/L 11,12 . M. mazei (wild type and 633 k.o.)…”

“…Co-cultures were set up without a methanogenic electron donor. As such, Methanosarcina co-cultures were prepared in modified DSMZ 120c media [10][11][12] , while co-cultures with Methanothrix/Methanosaeta or strict H 2 -utilizing methanogens (Methanospirillum and Methanoculleus) in modified freshwater media 10 .…”

“…Determination of methane (CH 4 ), hydrogen gas (H 2 ), ethanol and volatile fatty acids (e.g. acetate, formate) was carried out as described before 11,12 . Briefly, for experiments done at the University of Southern Denmark, gases were measured with a Trace 1300 gas chromatograph (GC) (Thermo-Scientific) equiped with a TracePLOT ™ TG-BOND Msieve 5A column and a thermal conductivity detector (TCD).…”

BalticMethanosarcinaandClostridiumcompete for electrons from metallic iron

“…By inference we propose that Methanosarcina might be able to retrieve electrons directly from Fe 0 . This is because we recently reported that Methanosarcina barkeri could retrieve electrons directly from a poised cathode at −400 mV under conditions unfavorable for abiotic H 2 -evolution [23]. The same Methanosarcina was previously shown to grow on Fe 0 (although assumedly using H 2 ) [24] retrieve electrons from electrogenic syntrophic partners [25,26] or via electrically conductive particles [27–29].…”