MAKING SUGAR FROM CARBON DIOXIDE: THE BASIS FOR ALL FOOD CHAINS by L0uise Hootton

Most of us are famiiiar with the idea that piants are abIe to buiid sugars from carbon dioxide using energy coIIected from suniight. This reaction is described by the equation:

Water + carbon dioxide + suniight ----> sugar + oxygen

The sugar produced is converted into the different compOunds needed for growth, such as protein and fats. Animais, which are unabie to coiiect the energy of the sun, must then eat the materiais made by these piants in order to survive. For many years everybody, incIuding scientists, beiieved that a11 Iife on earth was dependant upon piants and their abiiity to harvest the energy of the sun.

In the eariy 1970's an exciting discovery caused to rethink this evaIuation. In 1977 geoiogists and bioiogists were eprOring the area at the bottom of the sea in which new ocean crust is being made. Instead of the usuai sparse animai popuiations found on most of the deep ocean roor, scientists found a very high number of animais at these "mid-ocean ridges” - many to many to be surviving on the meager amounts sinking to these depths from the surface. There must therefore be another source of food for these animais... one not dependant upon the presence of Tight. Quickiy scientists reaiized that in these "vents", animais were utiiizing the chemicai energy avaiiabIe in these unusuai - environments in order to convert carbon dioxide to sugar.

At these "mid-ocean ridges“ seawater is heated by ciose contact with the underiying moiten rock. This water can reach temperatures above 300 degrees C. without boiiing because the pressure of the overiying water is about 150 times that of our atmosphere. Water this hot is abie to dissoive many chemicais from the rocks through which it passes. When this water reaches the surface of the rocks and exits into the coId water (4 degrees C.) of the sea bottom, it coois quickiy. Many chemicais, such as the iron and manganese turn back into soIids, buiiding "chimney" Iike structures around the exiting water. However, some chemicais, such as hydrogen suifide, stay in soiution. Hydrogen suifide is a substance that contains a great deaT of chemicai energy which can be Iiberated by reactions with compounds such as oxygen or carbon dioxide according to the pattern:

Hydrogen suifide + oxygen-rich compounds ----> water + suifur + energy (e.g. oxygen, carbon monoxide)

This reaction wiiT take pIace automaticaiiy in seawater, in which case the energy wiii be "Iost" as heat. However, some bacteria have evoived the abiiity to mediate this reaction by mixing hydrogen squide and carbon dioxide under controIIed conditions within their ceIIs and harvesting the energy that is given off. They use this energy to buiid sugar moIecuIes from carbon dioxide. Hydrogen atoms from the hydrogen suifide moIecuIe and the energy harvested from this reaction take the piece of water and suniight in the conversion of carbon dioxide to sugar in piants so that the reaction inside the bacteria can be written:

Carbon dioxide + hydrogen suifide ----> sugar + suifur