Simultaneous methanogenesis and oxygen reduction by Methanobrevibacter cuticularis at low oxygen fluxes

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. O-2 reduction by Methanobrevibacter species was investigated. Cell suspensions incubated in agar tubes under a headspace of H-2-CO2 and increasing concentrations of O-2 formed a distinct growth band, which coincided with the oxic-anoxic interface and indicated that the influx of O-2 into the band was balanced by its consumption. However, in batch cultures methanogenesis ceased as soon as traces of O-2 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 O-2 fluxes. Here, Methanobrevibacter cuticularis was capable of simultaneous CH4 production and O-2 consumption. Low O-2 fluxes (10% of the CH4 production rate) had virtually no influence on methanogenesis [4.5 mu mol CH4 (mg dry wt)(-1) h(-1)], whereas higher O-2 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 F-420 to factor F-390. The maximum O-2 reduction rate [4.8 mu mol O-2 (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.