The competitive success of Methanomicrococcus blatticola, a dominant methylotrophic methanogen in the cockroach hindgut, is supported by high substrate affinities and favorable thermodynamics

Methanomicrococcus blatticola is an obligately anaerobic methanogen that derives the energy for growth exclusively from the reduction of methylated compounds to methane with molecular hydrogen as energy source. Competition for methanol (concentration below 10 microM) and H(2) (concentration below 500 Pa), as well as oxidative stress due to the presence of oxygen are likely to occur in the peripheral region of the cockroach hindgut, the species' normal habitat. We investigated the ecophysiological properties of M. blatticola to explain how it can successfully compete for its methanogenic substrates. The organism showed affinities for methanol (K(m)=5 microM; threshold<1 microM) and hydrogen (K(m)=200 Pa; threshold <0.7 Pa) that are superior to other methylotrophic methanogens (Methanosphaera stadtmanae, Methanosarcina barkeri) investigated here. Thermodynamic considerations indicated that 'methanol respiration', i.e. the use of methanol as the terminal electron acceptor, represents an attractive mode of energy generation, especially at low hydrogen concentrations. Methanomicrococcus blatticola exploits the opportunities by specific growth rates (>0.2 h(-1)) and specific growth yields (up to 7 g of dry cells per mole of methane formed) that are particularly high within the realm of mesophilic methanogens. Upon oxygen exposure, part of the metabolic activity may be diverted into oxygen removal, thus establishing appropriate anaerobic conditions for survival and growth.