To survive, living organisms need energy. In the animal kingdom, the majority of this energy comes from a system known as the electron transfer chain, which is the final stage of respiration. This is a series of reactions that requires two main parts as fuel: an electron donor and an electron acceptor. The electron donor is just the food that we eat, which contains electron-rich compounds such as sugar that can be harvested for energy. The electron acceptor, in animals, is always oxygen, but bacteria can use a range of other, less common and energetic substances. An exotic option for alien life is chlorine, which is very good at accepting electrons in the same way as oxygen.
For this system to be competitive with oxygen, the alien species would need to live on a world that has more chlorine than ours. The chlorine would need to occur as hydrogen chloride, which dissolves in water as hydrochloric acid. The native life would have to be much more tolerant of acidic conditions than us, but bacteria can survive acids either by maintaining a less acidic environment in their own flesh, or more rarely adapting their whole biology to a higher acidity. Plants would release chlorine gas during photosynthesis, replenishing the chlorine as it reacts with any water to make bleach and hydrochloric acid. The chlorine gas would give the seas and atmosphere a vivid, greenish tinge, and it is very good at absorbing light, so only a little would be required.
The animals would breathe the chlorine gas in the same way as oxygen, releasing energy. If there is much water around, it is likely that the plants would split the water to release oxygen, and animals may breathe the oxygen as well. There would not be an ozone layer formed by the oxygen, due to the chlorine destroying it faster than it can form, so radiation levels may be too high for us. As we exhale a mixture of carbon dioxide and water, the animals would exhale carbon dioxide and hydrogen chloride. There would be a chlorine cycle between animals and plants, similar to the oxygen cycle. The actual equations are:.
Respiration: C6H12O6 + 6H2O + 12Cl2 -> 6CO2 + 24HCl + Energy
Photosynthesis: 6CO2 + 24HCl + Energy -> C6H12O6 + 6H2O + 12Cl2
Note that Silicon-Based Life and any other exotic composition may replace the carbon dioxide and sugar with other molecules, such as silicon dioxide and silicones. It seems that the most efficient form of chlorine respiration, at least with sugars, does not release the chlorine equivalent to carbon dioxide, or carbon tetrachloride. However, it may be that carbon tetrachloride (CCl4) or other chlorinated molecules are released by other metabolic activity. This is not necessary but high chlorine levels will encourage its incorporation into organic molecules. The chlorine can be removed from a molecule and sugar consumed to release energy.
Dechlorination: CCl4 + C6H12O6 + 2H2O -> 4CO2 + 3CH4 + 4HCl + Energy
As the reaction releases more energy than fermentation, muscles might use it as a substitute when elemental oxygen and chlorine are depleted, so that the body can reprocess chlorinated wastes as it burns sugars. This may be a benefit to intense activities such as sprints, as long as the wastes are made and stored in sufficient amounts. When the chlorocarbon liquids, akin to dry cleaning fluid or industrial toxins such as polychlorinated biphenyl, are fully burned, the muscles can resort to fermentation. The methane can be burned with chlorine or oxygen as soon as the intense activity ends, or it might be vented away, or used in a more creative manner.
Game and Story Use
- A world with a lot of chlorine would be a pretty nasty place for an oxygen-breather, but it is possible that a chlorine-breather could survive on a world with just oxygen.
- The temperatures and other conditions need to match up right, or they'll suffer hypothermia or heat stroke soon enough.
- We use oxygen to make collagen, so chlorine won't just be used for respiration. They might need dietary supplements or occasional respirator sessions.
- The chlorine atmosphere is a powerful visual image.
- It means green skies, green seas, steamy acidic mists, and dark, gloomy landscapes etched by acidic tides.
- Just make sure to wear a mask and filter your water if you want to live to see the sights.