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