Page 92 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 4. Robustness of coal microbial community after nutrient amendment
propanol and aromatic hydrocarbons like toluene and phenol (Lovley and Phillips 1988; Lovley, Holmes and Nevin 2004; Aklujkar et al. 2009; Kuntze et al. 2011; Zhang et al. 2013). In G. metallireducens, carbon fluxes can be balanced via the acetate kinase/phosphotransacetylase (ACK/PTA) pathway that converts acetyl-CoA to acetate with the generation of ATP, as shown for Escherichia coli (el-Mansi and Holms 1989). This mechanism is present in G. metallireducens and is hypothesized to be used when excess organic compounds are present (Lovley and Chapelle 1995; Aklujkar et al. 2009; Speers and Reguera 2012). Whether this pathway can provide acetate for methanogenesis in coal wells should be further investigated. Sequence analyses of metal reduction and complex organic compound degradation genes supported the potential role of G. metallireducens in these processes (all with 100% protein sequence identity, Table S2). Fe(III) reduction potential was analyzed based on cytochromes involved in iron reduction and electron transfer mechanisms across the membrane. We found the diheme cytochrome c peroxidase MacA in the G. metallireducens draft genome (100% aa identity). In G. sulfurreducens, MacA is involved in the electron transfer pathway to Fe(III) (Butler et al. 2004; Seidel et al. 2012). We also detected the triheme cytochrome c7 Gmet_2902 (homologous to G. sulfurreducens PpcA). Previous studies in both Geobacter species established that these cytochromes are essential for electron transfer reactions but do not directly reduce Fe(III) (Afkar and Fukumori 1999; Lloyd et al. 2003). As a candidate for Fe(III) reduction we identified a gene encoding Gmetc6, a homologue of a putative c-type outer membrane cytochrome (OmcF), that showed 100% protein sequence identity to the 74 aa conserved domain, indicating presence of an outer membrane electron transfer mechanism (Kim et al. 2005; Mehta et al. 2005). Genes encoding geopilin (PilA) and flagellar proteins for motility (FliCDS) were found and indicated the capacity for chemotaxis, direct substrate contact and importantly also electron transfer (Childers, Ciufo and Lovley 2002; Tremblay et al. 2012; Holmes et al. 2016). These factors can provide competitive advantages in the presence of difficult to dissolve Fe(III) and Mn(IV) minerals at high pH. Together with the capacity to oxidize aromatic hydrocarbons, this supports the potential role of Geobacter in
metal reduction in the amended coal well.
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