Hydrogen (H-2) consumption and methane (CH4) production in pure cultures of three different methanogens were investigated during cultivation with 0, 0.2 and 4.21 mu M added nickel (Ni). The results showed that the level of dissolved Ni in the anaerobic growth medium did not notably affect CH4 production in the cytochrome-free methanogenic species Methanobacterium bryantii and Methanoculleus bourgensis MAB1, but affected CH4 formation rate in the cytochrome-containing Methanosarcina barkeri grown on H-2 and CO2. Methanosarcina barkeri also had the highest amounts of Ni in its cells, indicating that more Ni is needed by cytochrome-containing than by cytochrome-free methanogenic species. The concentration of Ni affected threshold values of H-2 partial pressure (pH(2)) for all three methanogen species studied, with M. bourgensis MAB1 reaching pH(2) values as low as 0.1 Pa when Ni was available in amounts used in normal anaerobic growth medium. To our knowledge, this is the lowest pH(2) threshold recorded to date in pure methanogen culture, which suggests that M. bourgensis MAB1 have a competitive advantage over other species through its ability to grow at low H-2 concentrations. Our study has implications for research on the H-2-driven deep subsurface biosphere and biogas reactor performance.