  Chemical disequilibrium in exoplanetary atmospheres (detectable with remote
spectroscopy) can indicate life. The modern Earth's atmosphere-ocean system has
a much larger chemical disequilibrium than other solar system planets with
atmospheres because of oxygenic photosynthesis. However, no analysis exists
comparing disequilibrium on lifeless, prebiotic planets to disequilibrium on
worlds with primitive chemotrophic biospheres that live off chemicals and not
light. Here, we use a photochemical-microbial ecosystem model to calculate the
atmosphere-ocean disequilibria of Earth with no life and with a chemotrophic
biosphere. We show that the prebiotic Earth likely had a relatively large
atmosphere-ocean disequilibrium due to the coexistence of water and volcanic
H2, CO2, and CO. Subsequent chemotrophic life probably destroyed nearly all of
the prebiotic disequilibrium through its metabolism, leaving a likely smaller
disequilibrium between N2, CO2, CH4, and liquid water. So, disequilibrium fell
with the rise of chemotrophic life then later rose with atmospheric oxygenation
due to oxygenic photosynthesis. We conclude that big prebiotic disequilibrium
between H2 and CO2 or CO and water is an anti-biosignature because these easily
metabolized species can be eaten due to redox reactions with low activation
energy barriers. However, large chemical disequilibrium can also be a
biosignature when the disequilibrium arises from a chemical mixture with
biologically insurmountable activation energy barriers, and clearly
identifiable biogenic gases. Earth's modern disequilibrium between O2, N2, and
liquid water along with minor CH4 is such a case. Thus, the interpretation of
disequilibrium requires context. With context, disequilibrium can be used to
infer dead or living worlds.
