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aquatic eco systems is very important in the world.The sulfur cycle as diagrammed in figure 4 -2 illustrates the main features of a bio ecosystem geochemical cycle in a specific ecosystem the cycling and reservoir pools, the chemical forms of the elements and the organisms involved are all shown in the diagram. Sulfate (so4) in the water ecosystem is the principal available form that is reduced by autotrophic plants and incorporated into proteins, sulfur being an essential constituent of certain amino acids (path 1, in figure ecosystem 4 -2). When animals excrete, or the bodies of plants and animals are de composed by heterotrophic microorganisms, sulfate may be returned to the water (path ) or hydrogen sulfide (H2S) is released (path 2). Some of the H2S is then reconverted to sulfate by specialized sulfur bacteria (paths 4,5,6). Some of these bacteria are called chemosynthetic organisms, because they obtain their own energy from the chemical oxidation of inorganic compounds (in this case oxidation of sulfide to sulfur , and so on) instead of from light as do photosynthetic organisms or from organic matter as do heterotrophic organisms. The green sulfur bacteria are photosynthetic, but since H2S is oxidized rather than H2S, as the regular photosynthesis, free oxygen is not released. An equation for this type of bacterial photosynthesis is obtained by substituting H2S for H2O
