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Abstract

Methanogenic Archaea are often encountered in habitats that are not entirely anoxic in space or time. Recent biochemical and genomic studies have revealed the capacity of methanogens to reduce molecular oxygen. O2 reduction by Methanobrevibacter species was investigated. Cell suspensions incubated in agar tubes under a headspace of H2–CO2 and increasing concentrations of O2 formed a distinct growth band, which coincided with the oxic–anoxic interface and indicated that the influx of O2 into the band was balanced by its consumption. However, in batch cultures methanogenesis ceased as soon as traces of O2 were added. Focusing on Methanobrevibacter cuticularis, a species colonizing the microoxic gut epithelium of termites, a diffusion-limited setup was used that allowed the exposure of dense cell suspensions to controlled O2 fluxes. Here, Methanobrevibacter cuticularis was capable of simultaneous CH4 production and O2 consumption. Low O2 fluxes (10% of the CH4 production rate) had virtually no influence on methanogenesis [4.5 μmol CH4 (mg dry wt)−1 h−1], whereas higher O2 fluxes (up to 30% of the initial CH4 production rate) caused a reversible decrease in methanogenesis, which was accompanied by a reversible, partial conversion of coenzyme F420 to factor F390. The maximum O2 reduction rate [4.8 μmol O2 (mg dry wt)−1 h−1] that could be maintained over extended time periods (>30 min) was similar to the CH4 production rate under anoxic conditions.