The plasmid DNA was subjected to overnight digestion at 37 °C with exonuclease V Rec BCD (New England Biolabs, Massachusetts, USA) to remove chromosomal DNA. PCR reactions using universal primers covering the V4 region of 16S rRNA gene were performed to check for chromosomal DNA contamination. The primers used for bacteria were F5′-CCTACGGGNGGCWGCAG-3′ (Bac_341F) and R5′-GGATTAGATACCCBDGTAGTC-3′ (Bac_785R)27 (link); and for archaea F5′-CCCTAYGGGGYGCASCAG-3′ (Arc_340F) and R5′-ATTAGAKACCCSNGTAGTCC-3′ (Arc_806R)28 (link). The plasmid DNA was purified using the SureClean Plus kit (Bioline, London, UK).
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