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Microbes, especially bacteria, play an important role in oxidative and reductive cycle of sulfur. The oxidative part of the cycle is mediated by photosynthetic bacteria in the presence of light energy and chemosynthetic forms in the absence of light energy. At the end of the anaerobic food chain in bacteria they serve to purify the system of sulfide and other metabolic end products. In the process sulfur is returned to the system as sulfate. In transition zones from anaerobic to aerobic, photosynthetic bacteria can form a food source to protozoans and microzooplankton. Chemosynthetic sulfur-oxidizing bacteria are the dominant bacterial forms that support thriving ecosystems in hydrothermal vents. Scientists are seeking evidences from such extreme environment for similar life on other planetary bodies. The reductive cycle on the other hand is mostly driven by the sulfate/sulfur-reducing bacteria which use sulfate as the electron acceptor in anaerobic respiration to produce sulfide. Their close association with other microbes can have profound geochemical influence. Their metabolic activity dictates the availability of trace metals to other forms of life. While sulfide gets precipitated, phosphate gets released into the systems. Nitrogen fixation by these anaerobes also adds to the nitrogen economy of the environment they inhabit. In sediments of continental shelves that hold the reserve of gas hydrates, these microbes can modulate the concentration of methane in such ecosystems. Most importantly, the interaction with DMSP, an osmolyte from phytoplankton, can have wideranging climatic implications. The main reactions of the sulfur cycle involving living organisms are closely related to the carbon cycle. The amount of carbon involved in the fluxes of the sulfur cycle through biogenic processes varies depending on the type of organisms undertaking the metabolism of the sulfur compounds. The estimates suggest that the anthropogenic sulfur fluxes to the atmosphere and hydrosphere have reached a level comparable with that of natural fluxes. The natural sulfur flux from the lithosphere, its main reservoir, is compensated by the reverse flux of sulfur compounds to the lithospheric sediments of the ocean. Further, there is also indication that by the end of this century the anthropogenic sulfur fluxes could notably increase all over the world.
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