CARBOXYDOTROPHY, METABOLIC MUTUALISM, AND CARBON MONOXIDE FLUXES IN THE ARCHEAN ATMOSPHERE

Carbon monoxide (CO) budgets for the Archean atmosphere generally treat the early biosphere as a major sink for CO, thereby preventing the development of a "CO runaway" atmosphere. Indeed, hydrogenogenic carboxydotrophy (CO + H₂O --> CO₂ + H₂) is an anaerobic metabolism that appears to be geographically widespread in hydrothermal microbial ecosystems. Carboxydotrophs have been isolated that can use CO as sole carbon and energy source in culture medium with headspace CO partial pressures ranging from £ 10^-4 atm to ³ 2 atm. In modern environments, this metabolism was thought to depend on CO supplied as a dissolved and free phase constituent of geothermal fluids. Yet recent determinations of CO consumption in hot spring microbial mats reveal rates that far exceed the rates at which CO is supplied by the hydrothermal systems. Instead, proximal biological production of CO by environmental microbial consortia must be invoked. Recent pure culture work with has shown small but ecologically significant CO production by certain methanogens and sulfate reducers. In the Archean, leakage to the atmosphere of even a small fraction of the biologically produced CO would have exceeded the volcanic outgassing flux with implications for Archean atmospheric chemistry.