Page 94 - 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
methanogenesis does not play a significant role in the amended coal well. BLASTX analysis of raw and de novo assembled reads that mapped to the methyl-coenzyme M reductase protein subunit A (McrA) sequence database indicated high identity to both Methanothrix concilii GP- 6 (1 contig with 94% protein sequence identity) and uncultured methanogens predominantly found in lake sediments (Denonfoux et al. 2013; Youngblut, Dell’Aringa and Whitaker 2014; Lin et al. 2015; Pump, Pratscher and Conrad 2015). More specifically, the one assembled contig was very similar to the McrA sequence of an uncultured Methanothrix sp. from 90 m deep Lake Pavin sediment (95% protein sequence identity) (Denonfoux et al. 2013), in line with high similarity of the 16S rRNA gene sequence to uncultured Methanothrix species from Lake Pavin sediment and water samples (Lehours et al. 2005, 2007; Borrel et al. 2012). It is interesting to note that Methanothrix co-occurred with Geobacter. A previous study by Jones et al. (2010) on bioaugmentation of sub-bituminous coal showed degradation of single-ring aromatics and long- chain alkanes and subsequent methane production by a Geobacter spp. and Methanothrix concilii dominated culture. Experiments on microbial aggregates and co-cultures showed that Methanothrix-Geobacter clusters are electrically conductive which suggested that direct interspecies electron transfer (DIET) plays an important role (Summers et al. 2010; Morita et al. 2011; Rotaru et al. 2014b; Holmes et al. 2017). These processes could also be relevant in the acetate and nutrient amended coal well.
A community with low complexity has the potential for methanogenesis from coal but does not lead to successful continued methane production
Using geochemical, 16S rRNA gene amplicon and metagenome sequencing, we established that the microbial community in a nutrient and acetate amended Australian coal well had the potential to break down coal to methane (Fig. 3). The methane production data (Fig. 1A) indicated that methane was generated by acetate conversion rather than by coal degradation. It is puzzling that the microbial community primarily consisted of complex organic compound degraders, which would not have an apparent role in the conversion of acetate to methane. The second key observation is that nutrient amendment lead to an increase in microbial cell counts by four orders of magnitude, which did not result in direct methane production from coal. Degradation of dead microbial biomass could also play a role, but is unlikely to serve as a major organic compound source for methanogenesis.
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