Page 88 - 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
to eventually convert acetate to methane. As methane production stopped after acetate depletion, these data also suggest that the methane-producing microbial community was not able to thrive without external addition of acetate (Fig. 1A) even though the microbial biomass had increased by several orders of magnitude (Fig. 1B). Stoichiometric analysis indicated that about 25% of the acetate was converted to methane, suggesting presence of syntrophic acetate oxidation, which has been previously observed in organic carbon-rich petroleum reservoirs (Mayumi et al. 2011). Sulfate-dependent acetate oxidation was unlikely to occur due to the rapid depletion of sulfate from 1 mM at the start of the experiment (Table S1) to below the detection limit after 3 months (< 0.02 mM). The decrease in methane after 19 months in the amended coal wells is probably due to escape of methane from the coal well rather than active degradation of methane. We analyzed the sequencing data for aerobic or anaerobic methanotrophs but did not find any indication (16S rRNA gene and marker genes pmoA and mcrA) of aerobic or anaerobic methane oxidation.
To get insight into the functioning of the microbial food web and as indication as to why methane production stopped after 15 months, we sampled the coal well water at 17 and at 25 months to analyze the microbial communities and their functional potential. Phylogenetic analyses were performed on both 16S and 18S rRNA genes and fungal ITS (Fig. 2, Table S2). Functional analyses targeted genes encoding proteins involved in complex organic compound degradation, metal reduction, electron transfer reactions and volatile fatty acid metabolism (Table S2).
The fungal community has the potential for coal biosolubilization
Fungi represented 15% of the combined 16S/18S rRNA gene reads detected in the metagenome data set (Fig. 2). The sequences were most related to species within the Agaricomycetes class that contains well-characterized wood-rotting fungi (Ohm et al. 2014). Additional identification based on fungal ITS sequences supported identification up to the Basidiomycota division, and most reads (89%) were closely related to Basidiomycota sp. MEL 2363319 (KP311433) (99% nt identity).
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