  Methane (CH4) is a primarily biogenic greenhouse gas. As such, it represents
an essential biosignature to search for life on exoplanets. Atmospheric CH4
abundance on Earth-like inhabited exoplanets is likely controlled by marine
biogenic production and atmospheric photochemical consumption. Such
interactions have been previously examined for the case of the early Earth
where primitive marine ecosystems supplied CH4 to the atmosphere, showing that
the atmospheric CH4 response to biogenic CH4 flux variations is nonlinear, a
critical property when assessing CH4 reliability as a biosignature. However,
the contributions of atmospheric photochemistry, metabolic reactions, or solar
irradiance to this nonlinear response are not well understood. Using an
atmospheric photochemical model and a marine microbial ecosystem model, we show
that production of hydroxyl radicals from water vapor photodissociation is a
critical factor controlling the atmospheric CH4 abundance. Consequently,
atmospheric CH4 partial pressure (pCH4) on inhabited Earth-like exoplanets
orbiting Sun-like stars (F-, G-, and K-type stars) would be controlled
primarily by stellar irradiance. Specifically, irradiance at wavelengths of
approximately 200-210 nm is a major controlling factor for atmospheric pCH4
when the carbon dioxide partial pressure is sufficiently high to absorb most
stellar irradiance at 170-200 nm. Finally, we also demonstrated that inhabited
exoplanets orbiting near the outer edge of K-type stars' habitable zones are
better suited for atmospheric pCH4 buildup. Such properties will valuably
support future detection of life signatures.
