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The Sulfur Cycle
The sulfur cycle is a poorly understood process by high school students. In fact, beyond the carbon, nitrogen and oxygen cycle, many students are unaware that
other essential elements have a well-defined cycle thoroughout the biosphere. The Winogradsky Column is an excellent way to illustrate this process.
Any diagram of the sulfur cycle will show that the entire cycle is represented by bacteria species that are present in the Winogradsky Column (Madigan et. al.
2000) Each can be isolated and grown to show that, indeed sulfur is cycled through nature just as nitrogen and carbon dioxide.
However, the isolation of each species is difficult due to the fact that they are anaerobes and special media are needed as well as time to effectively culture them.
Each bacterial species that can be found in the Winogradsky column has a role to play in the cycling of sulfur through the system.
Thiobacillus and Beggiatoa are sulfur oxidzers breaking down H2SO4 and H2S
So + 3/2 O + H2O  H2SO4
They use the energy to fix Carbon Dioxide. If there is a significant source of easily degradable carbon then they may be inhibited.
Chromatium and Chlorobium are photoheterotrophs and can oxidize reduced sulfur compounds under anaerobic conditions
H2S So SO4 2-
Desulfvibrio is anaerobic but grows lithtrophically with H2 as the electron donor, sulfate as the acceptor and CO2 as the sole carbon source. Also
nitrate can be substituted as the acceptor. It has a very complex biochemistry.
There are far more reactions that occur and this is just a few of the pathways that exist in the column. There are also more species that inhabit the
environment and each has its own unique contribution to the sulfur cycle.
Iron Chemistry
Sulfur is not the only element being cycled through the column, iron is as well. As the column develops, the mud blackens this is due to the migration of H2S
upward and being replaced by FeS .
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