section_path: Astrobiology/Oxygen
section_name: Oxygen
paragraph_text: Oxygen
  
  Molecular oxygen (O) can be produced by geophysical processes, as well as a byproduct of photosynthesis by life forms, so although encouraging, O is not a reliable biosignature. In fact, planets with high concentration of O in their atmosphere may be uninhabitable. Abiogenesis in the presence of massive amounts of atmospheric oxygen could be difficult because early organisms relied on the free energy available in redox reactions involving a variety of hydrogen compounds; on an O -rich planet, organisms would have to compete with the oxygen for this free energy.