Like piants these bacteria then convert this sugar into the other compounds needed for their growth. Some animais Iiving at the "hydrothermai vents” survive by fiitering free 1iving bacteria of this type out of the water and eating them. Others however have deveIoped cioser reiationships with the bacteria caiied symbiosis. In these cases the bacteria Iive inside or right on top of the animai's tissues. The animaIs harvest the hydrogen Suifide needed as fuei by the bacteria from the water and produce carbon dioxide by their own respiration (breathing) and deIiver both of these to the bacteria. In return the bacteria provides the animaI with the sugars and other compounds that they need to grow and reproduce. In this system bacteria are anaiogous to piants, the animais that eat them or Iive in symbiosis with them are the "herbivores" and animais that eat them are "carnivores". The resuIt is a very diverse community of animaIs Tiving in compIete darkness at the bottom of the ocean and in some deep Takes, such as Lake BaikaI in Russia.

Once it was reaiized that any kind of energy, not just sunIight, might be used to convert carbon dioxide to sugar, scientists began to Took for other communities that use different chemicaI compounds as energy sources. In 1988 in the Gqu of Mexico the found another type of community - the "methane seep” community. Methane is another energy rich chemicai. When it is mixed with oxygen-rich compounds, a great deaI of energy is formed - a fact we use when cooking and heating our homes. Bacteria are abie to utiIize this energy reIease by reacting methane and carbon dioxide under controTIed conditions within their ceiis. The energy reIeased takes the piace of sunIight in providing the power needed to "stick" carbon diOxide moiecuies together to form sugar. Again, these— bacteria form the basis for quite rich food chains in the areas where they are found. For exampie, rich communities of cIams that live in symbiosis with these types of bacteria have been found above the naturai gas deposits off SabIe IsTand, as we11 as on the sIopes of the Grand Banks of Newfoundiand.

Many other chemicaI compounds that are energy rich, such as petroIeum, and many nitrogen and squur compounds have aiso been found to support popuiations of bacteria, which in turn form food sources for Iarger marine animais. These aiternatives, though, appear to be much Iess important than the hydrogen squide and methane type communities described above. Moreover, a11 of these -”chemica1 energy” mechanisms for making sugar are Tess efficient than is photosynthesis (the mechanism by which piants make sugar). Thus on Iand, where Tight is pIentifuI, these types of communities are often absent, or of very 10w importance. However, in the oceans and in some fresh water Takes and swamps, the use of chemicai energy to make sugar can be the most important if not the entire basis of the food chains found there.

Much remains to be 1earned about these "chemosynthetic" food chains. Many new types of animaI have been found in these communities and the ways that their metaboiisms work are often strange and new. Scientists are very excited about these processes and they hope that we may be able to Iearn much from these organisms that wiII increase our abiIity to understand our environment. Moreover, answers to certain medicaI probiems may Tie in better understanding of these unique organisms. FinaIIy, some theories suggest that Iife itseIf may have had its beginnings in systems simiiar to hydrothermai vents. Investigations of the systems and the animais that Iive there may even heip to answer the burning question of "where did we come from?".

Editor's Note: This articTe is a foTIowup 0n Louise's January taIk to the

Society in which she used indes to eprain and show how Iife survives near underwater high temperature vents. As there is no Tight at this depth and she

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