Chlorine in Nature

Chlorine is a highly reactive element which practically does not exist by itself in nature, but only in combination with other elements. In some rare situations, such as in human tears, free chlorine as such can be detected.

It is one of the most common elements in nature, where it is even more plentiful than carbon. Among the inorganic chlorinated substances, common salt is of course the best example. Key natural sources of organic chlorinated substances - organochlorines - are the oceans, forest fires and fungal activity.

Surprising sources

Natural organochlorines are produced as a result of natural chlorinated substances reacting with organic material in the environment.
They have some surprising sources and properties:

• a Chinese folk medicine plant contains five natural organochlorine compounds
• an Ecuadorian tree frog produces a chlorinated alkaloid, with pain-killing properties several hundred times more powerful than morphine
• a natural organochlorine antibiotic, vancomycin, is a key defence against hospital Staphylococcus infections
• and some natural organochlorinated products exhibit potent antibacterial and anticancer properties

A large family

Organohalogens - organic compounds containing chlorine, bromine, iodine or fluorine - have been made by nature throughout the earth's history.

Deposits have been found in lignite samples (woody textured rock) of 15 million years old and bituminous coal of 300 million years old.

More than 4,000 organohalogen substances are known today, of which many occur in living organisms and of which more than half contain chlorine. Chlorine-based compounds are made by marine organisms (sponges, corals, sea slugs, jellyfish and seaweeds), plants, seeds, fungi, lichens, bacteria, freshwater algae and insects.

Seaweeds produce at least 400 organohalogens, and certain phytoplankton and seaweeds produce chlorinated metabolites tentatively identified as trichloroethylene and perchloroethylene, compounds well known as chlorinated solvents.These naturally-occurring organohalogens play an essential role in the survival of these organisms.

Wherever you go...

The oceans release some three million tonnes of methyl chloride into the atmosphere every year. In addition, some 5,000 - 15,000 million tonnes of inorganic chlorine go into the marine atmosphere each year as a fine mist of sea salt. Most of this inorganic chlorine is returned to the ocean surface, but 3-35% remains as "inorganic chlorine vapour", including hydrogen chloride and probably other inorganic compounds including molecular chlorine, hypochlorous acid and nitryl chloride.

Part of this vapour is ultimately converted into chlorine atoms which may react with natural organic compounds to give organochlorines.

On land, some 300 natural chlorine-based compounds are known to be produced by terrestrial fungi and plants. These range from the chloromethane released by wood-rotting fungi to complex chlorinated polyacetylenes made by plants of the Aster family. In terrestrial plants, organochlorines often act as anti-feedants to protect the plant from attack by insects or other herbivores.

Last update : 09/2011