 Lakes represent a curious natural source of methane to the atmosphere, as they only cover about 2-3% of Earth's surface, yet are estimated to contribute about 12% to global methane emissions. Therefore, understanding how methane is released from these systems is really important for predicting climate change. Methane release from lakes is controlled by methane-oxidizing bacteria, which sit at interfaces in lakes where methane diffuses upwards from the sediment where it's produced and oxygen diffuses down from the surface. Increasingly, evidence suggests that methane is also oxidized in lakes where oxygen is no longer available, yet the involved bacteria and their metabolic requirements remain largely unknown. Therefore, we investigated methane oxidation in the deep Meromictic Lake Zug in central Switzerland, where water below 160 meters is permanently oxygen-free. With 13 carbon labeling experiments, we could show that methane is oxidized at a similar degree under both oxic and anoxic conditions, yet at all depths only aerobic methane-oxidizing bacteria were present. Additions of oxygen, but also iron and manganese oxides, stimulated methane oxidation and also the activity and growth of these bacteria, therefore suggesting that they are able to switch to using other electron acceptors under oxygen limitation. This has been confirmed to occur in pure cultures of these bacteria, and our study provides first evidence that this also occurs under environmental conditions, thus providing important information about these natural methane filters.