Kinetic characterization of Methanobacterium bryantii M.o.H.

H₂ is a key electron donor for many anaerobic microorganisms; thus, keen competition for H₂ occurs among H₂-utilizing microbial groups. Monod kinetic parameters provide essential information for kinetic analysis of competition for H₂. In this study, we estimated Monod kinetic parameter values for a methanogen that consumes only H₂ as its electron donor, Methanobacterium bryantii M.o.H. Utilization of a single electron donor is an advantage in this study, because complications from alternate metabolic pathways are avoided. Using a set of batch experiments designed to provide the best estimates of each parameter, we obtained these values: maximum specific growth rate (μ_max) = 0.77/day, maximum substrate consumption rate (q_max) = 2.36 mol-H₂/gcells/ day, true yield (Y) = 0.325 gcell/mol H₂, fraction of donor electrons to synthesis (f_s°) = 0.03 e^-cell/e^-donor, half-maximum-rate substrate concentration (K_s) = 18 000 nM = 18 μM H₂, and endogenous decay rate (b) = 0.088/day. This self-consistent set of parameters indicates that, when H₂ is not limiting, M. bryantii M.o.H. is a slow grower (low μ_max) compared to other H₂-oxidizing methanogens and sulfate reducers, and this is mainly due to its low true Y, not a low q_max. The relatively high K_s and b values suggest that M. bryantii also may not be a strong competitor when H₂ is limiting.