  Recent observations of several planet-hosting M dwarfs show that most have
FUV/NUV flux ratios 1000 times greater than that of the Sun. Here we show that
the atmospheric oxygen contents (O2 and O3) of potentially habitable planets in
this type of UV environment could be 2~3 orders of magnitude greater than those
of their counterparts around Sun-like stars as a result of decreased photolysis
of O3, H2O2, and HO2. Thus detectable levels of atmospheric oxygen, in
combination with the existence of H2O and CO2, may not be the most promising
biosignatures on planets around stars with high FUV/NUV ratios such as the
observed M dwarfs.
