Finally, there are some organisms which use sulfate as their oxidizing agent, the sulfate being reduced in this process to sulfide. Although there are countless individual biological organisms which utilize the alternatives to atmospheric oxygen, they are less familiar to us in everyday life because they are all microorganisms. Some of them exist only in places where oxygen cannot penetrate, because they die upon exposure to air. Others, like the sulfate reducers are able to function in the presence of air. Oxygen does not kill them, but they tend to thrive better in its absence because there are no aerobic organisms in competition with them.
Sulfate is a very common natural component of the Earth's environment.
Gypsum, from which wallboard and plaster of Paris are made, is the natural rock calcium sulfate. Calcium sulfate is slightly soluble in water. Many rivers and lakes contain small quantities of sulfate as a result, and the oceans are quite abundant in sulfate. Consequently, there are plenty of ecological riches that can be explored by sulfate reducing bacteria. Chemically, sulfate consists of one sulfur atom surrounded by four oxygen atoms.