  Biosignature gas detection is one of the ultimate future goals for exoplanet
atmosphere studies. We have created a framework for linking biosignature gas
detectability to biomass estimates, including atmospheric photochemistry and
biological thermodynamics. The new framework is intended to liberate predictive
atmosphere models from requiring fixed, Earth-like biosignature gas source
fluxes. New biosignature gases can be considered with a check that the biomass
estimate is physically plausible. We have validated the models on terrestrial
production of NO, H2S, CH4, CH3Cl, and DMS. We have applied the models to
propose NH3 as a biosignature gas on a "cold Haber World," a planet with a
N2-H2 atmosphere, and to demonstrate why gases such as CH3Cl must have too
large of a biomass to be a plausible biosignature gas on planets with Earth or
early-Earth-like atmospheres orbiting a Sun-like star. To construct the biomass
models, we developed a functional classification of biosignature gases, and
found that gases (such as CH4, H2S, and N2O) produced from life that extracts
energy from chemical potential energy gradients will always have false
positives because geochemistry has the same gases to work with as life does,
and gases (such as DMS and CH3Cl) produced for secondary metabolic reasons are
far less likely to have false positives but because of their highly specialized
origin are more likely to be produced in small quantities. The biomass model
estimates are valid to one or two orders of magnitude; the goal is an
independent approach to testing whether a biosignature gas is plausible rather
than a precise quantification of atmospheric biosignature gases and their
corresponding biomasses.
