Chlorine photosynthesis on exoplanets

[radtech497]

I tend to think that oxygen and chlorine could coexist on some planets, just like methane and chlorine co-existed on Precambrian Earth. Of course the chlorine would be little more than a trace - but if it were continually replenished, it would be significant enough to make the atmosphere toxic to humans.

A nominally Earth-like world could host an algae-like organism that learned to incorporate chlorine ions into its biomolecules in order to present a less-tasty aspect to potential predators. Over time, as predators became more able to tolerate the chlorine, the prey (algae) might develop an ability to produce chlorine gas from those ions, again as a defense against predators. This chlorine gas could accumulate over aeons, eventually reaching perhaps a few percent of the total atmosphere in composition. The atmosphere will likely consist of nitrogen, oxygen, carbon dioxide, chlorine, various chlorinated hydrocarbon vapors, and, probably, more than a hint of phosgene gas. Since any life on the planet breathes oxygen and carbon dioxide (but can tolerate relatively high concentrations of chlorine), the biochemistry is at least vaguely similar; noticeably, carbon compounds will be somewhat chlorinated and will likely resemble plastics (an analogue of cellulose, for example, will be something like polyvinylchloride (PVC)).
All this chlorine in the atmosphere will react with any water on the surface to produce an aqueous solution of hydrochloric acid and hypochlorous acid. All bodies of "water," then, will be composed of a dilute acid and bleach solution. If chlorine gas comprises one percent of the atmosphere, this solution will have a pH of about 4.50 (similar to undiluted vinegar).
So, yes, chlorine and oxygen can coexist on a Gaian world, as long as the chlorine gas did not dominate the atmosphere.

Radtech497