Description:
<jats:p>As biogas from anaerobic digestion becomes an increasingly attractive biofuel, the need to improve the quality of biogas has come to the fore. Biological upgrading focuses on adding enough hydrogen to an anaerobic biogas reactor to allow methanation of the remaining carbon dioxide by methanogenic Archaea (<jats:italic>in situ</jats:italic> upgrading). Alternatively, biogas and hydrogen can be mixed in the absence of feedstock, in a reactor operated exclusively to facilitate methanogenesis (<jats:italic>ex situ</jats:italic> upgrading). This novel technology can encounter inhibition at high loading rates of hydrogen: however, in contrast to anaerobic digestion, the dynamics of this thermophilic functional microbial community are sparsely characterised. High-resolution 16S rDNA community profiles from four anaerobic biogas upgrading reactors were constructed to determine how feedstock, hydrogen, and CO<jats:sub>2</jats:sub> influence biomethanation. Presence/absence of a feedstock led to large differences between <jats:italic>in situ</jats:italic> and <jats:italic>ex situ</jats:italic> communities, determining the dominant methanogen genera, and encouraging distinct populations of hydrolysing and fermenting Firmicutes. Although high hydrogen flow rates (∼37 L/day) caused a collapse in methanogenic Methanothermobacter populations <jats:italic>in situ</jats:italic>, <jats:italic>ex situ</jats:italic> hydrogen rates greatly exceeded these levels (∼400 L/day) without collapse of Methanobacterium, despite some observed instability and proliferation of likely homoacetogens. Subsequent reduction of hydrogen rates <jats:italic>ex situ</jats:italic> (259 L/day) appeared to create a niche for hydrogen production, indicated by increased abundance of various syntrophic fermenters known to supply biogenic hydrogen. In either upgrading setup, instability due to increased hydrogen levels manifested as a disruption of fermenting and hydrolysing populations prior to disruption of methanogens.</jats:p>