Table 1.

Description of microcosms.

Group	Years	Description
Set A (high H2)	All	200 kPa H2:CO2 (80%:20%)
Set B (low H2)	All	200 kPa N2:CO2 (80%:20%), 1 ml of headspace replaced with 1 ml of H2:CO2
Set C (no H2)	All	200 kPa N2:CO2
Set D (high H2 + NH4+)	2012–2013	200 kPa H2:CO2 plus 4.7 mM NH4Cl (2012) or 47 μM NH4Cl (2013)
Set E (no H2, tryptone added)	2014–2015	200 kPa N2:CO2 plus 0.5% tryptone and 0.01% yeast extract

Each 60 ml serum bottle contained 25 ml of low-temperature diffuse hydrothermal fluid that was incubated in pairs at 55°C and 80°C.