  Whether extant life exists in the martian subsurface is an open question.
High concentrations of photochemically produced CO and H2 in the otherwise
oxidizing martian atmosphere represent untapped sources of biologically useful
free energy. These out-of-equilibrium species diffuse into the regolith, so
subsurface microbes could use them as a source of energy and carbon. Indeed, CO
oxidation and methanogenesis are relatively simple and evolutionarily ancient
metabolisms on Earth. Consequently, assuming CO- or H2- consuming metabolisms
would evolve on Mars, the persistence of CO and H2 in the martian atmosphere
set limits on subsurface metabolic activity. Here, we constrain such maximum
subsurface metabolic activity on Mars using a 1-D photochemical model with a
hypothetical global biological sink on atmospheric CO and H2. We increase the
biological sink until the model atmospheric composition diverges from observed
abundances. We find maximum biological downward subsurface sinks of 1.5e8
molecules/cm^2/s for CO and 1.9e8 molecules/cm^2/s for H2. These covert to a
maximum metabolizing biomass of <10^27 cells or <2e11 kg, equivalent to <1e-4
to 1e-5 of Earth's biomass, depending on the terrestrial estimate. Diffusion
calculations suggest this upper biomass limit applies to the top few kilometers
of the martian crust in communication with the atmosphere at low to mid
latitudes. This biomass limit is more robust than previous estimates because we
test multiple possible chemoautotrophic ecosystems over a broad parameter space
of tunable model variables using an updated photochemical model with precise
atmospheric concentrations and uncertainties from Curiosity. Our results of
sparse or absent life in the martian subsurface also demonstrate how the
atmospheric redox pairs of CO-O2 and H2-O2 may constitute antibiosignatures,
which may be relevant to excluding life on exoplanets.
