gene, expressed as a ratio to the 16S rDNA copy genes (Fig. 8). mcrA is ubiquitous among known methanogens and was used in several studies as a biomarker for the effectiveness of methanogenesis (Morris et al., 2014). Methanogens, as obligate anaerobes, require a redox potential of less than 300 mV for growth; otherwise, denitrifying, sulphate-reducing or iron-reducing bacteria may