design: For shotgun metagenomic sequencing, 90 mL of the original formation water and 30 mL of the enrichments were filtered and DNA was extracted as described above to maximize DNA template input. DNA concentrations for the original formation water, the amended and the unamended enrichments were measured with Qubit dsDNA HS Assay Kit (Life Technologies, Paisley, UK) and a total of 170 ng, 35 ng and 44 ng DNA was obtained for each sample, respectively. Metagenomic sequencing was carried out by NU-OMICS (Northumbria University, UK), on an Illumina MiSeq platform using the Nextera XT library preparation kit and the V3 600 bp sequencing kit following manufacturers instructions.