[Cobalt(III)-EDTA]− reduction by thermophilic methanogen Methanothermobacter thermautotrophicus

•Thermophilic methanogen Methanothermobacter thermautotrophicus is capable of reducing Co(III) to Co(II).•Co(III) bioreduction inhibited methanogenesis.•High concentrations of Co(III) are toxic to methanogen cells.

Cobalt is a metal contaminant at high temperature radioactive waste disposal sites. Past studies have largely focused on mesophilic microorganisms to remediate cobalt, despite the presence of thermophilic microorganisms at such sites. In this study, Methanothermobacter thermautotrophicus, a thermophilic methanogen, was used to reduce Co(III) in the form of [Co(III)-EDTA]−. Bioreduction experiments were conducted in a growth medium with H2/CO2 as a growth substrate at initial Co(III) concentrations of 1, 2, 4, 7, and 10 mM. At low Co(III) concentrations (< 4 mM), a complete reduction was observed within a week. Wet chemistry, X-ray absorption near-edge structure (XANES) and electron paramagnetic resonance (EPR) analyses were all consistent in revealing the reduction kinetics. However, at higher concentrations (7 and 10 mM) the reduction extents only reached 69.8% and 48.5%, respectively, likely due to the toxic effect of Co(III) to the methanogen cells as evidenced by a decrease in total cellular protein at these Co(III) concentrations. Methanogenesis was inhibited by Co(III) bioreduction, possibly due to impaired cell growth and electron diversion from CO2 to Co(III). Overall, our results demonstrated the ability of M. thermautotrophicus to reduce Co(III) to Co(II) and its potential application for remediating 60Co contaminant at high temperature subsurface radioactive waste disposal sites.