  Some atmospheric gases have been proposed as counter indicators to the
presence of life on an exoplanet if remotely detectable at sufficient abundance
(i.e., antibiosignatures), informing the search for biosignatures and
potentially fingerprinting uninhabited habitats. However, the quantitative
extent to which putative antibiosignatures could exist in the atmospheres of
inhabited planets is not well understood. The most commonly referenced
potential antibiosignature is CO, because it represents a source of free energy
and reduced carbon that is readily exploited by life on Earth and is thus often
assumed to accumulate only in the absence of life. Yet, biospheres actively
produce CO through biomass burning, photooxidation processes, and release of
gases that are photochemically converted into CO in the atmosphere. We
demonstrate with a 1D ecosphere-atmosphere model that reducing biospheres can
maintain CO levels of ~100 ppmv even at low H2 fluxes due to the impact of
hybrid photosynthetic ecosystems. Additionally, we show that photochemistry
around M dwarf stars is particularly favorable for the buildup of CO, with
plausible concentrations for inhabited, oxygen-rich planets extending from
hundreds of ppm to several percent. Since CH4 buildup is also favored on these
worlds, and because O2 and O3 are likely not detectable with the James Webb
Space Telescope, the presence of high CO (>100 ppmv) may discriminate between
oxygen-rich and reducing biospheres with near-future transmission observations.
These results suggest that spectroscopic detection of CO can be compatible with
the presence of life and that a comprehensive contextual assessment is required
to validate the significance of potential antibiosignatures.
