Biogeochemical Cycling

All of the major elements found in biological organisms are cycled in a predictable manner, and taken together the various element cycles are known as the biogeochemical cycles. This chapter describes the microbial activities that drive biogeochemical cycling, and that are highly relevant to the field of environmental microbiology. It also documents the impact of human activities on these cycles. The cycling of four major elements is discussed: carbon, nitrogen, sulfur and iron. For carbon, the transfer of carbon dioxide to organic carbon via both plants and microorganisms is reviewed, as well as respiration of organic carbon back to carbon dioxide. For nitrogen, the microbial transformations of nitrogen gas to ammonia and then organic forms of nitrogen are identified, as well as subsequent oxidation to nitrate, and finally reduction back to nitrogen gas or nitrous oxide. In the case of sulfur and iron, microbial oxidation and reduction cycles are outlined.