Geochemical Agents
During the middle of the 19th century, interest was mostly on the role of microorganisms in fermentation and disease. Pasteur's contributions on fermentation clearly showed that microorganisms can serve as specific agents in chemical transformations.
The credit for establishing the role of microorganisms in transformation of nitrogen and sulphur goes to Winogradsky and Beijerinck. Winogradsky (1856-1953) found that there are several distinct groups of bacteria each characterized by their ability to use a particular inorganic energy source. For example, sulphur bacteria oxidize inorganic sulphur and sulphur compounds while nitrifying bacteria oxidize reduced inorganic nitrogen compounds.
