Chapter 1 Biogeochemical Cycling of Elements – General Considerations

Abstract

Publisher Summary This chapter discusses the general considerations of biogeochemical cycling of elements. The chemical elements, including all the essential elements of protoplasm, tend to circulate in the biosphere in characteristic paths from environment t o organisms and back to the environment. These more or less circular paths are biogeochemical cycles. A biogeochemical cycle is, overall, an endergonic process, which relies ultimately on solar energy. The chemical environment may be influenced by organisms in several ways classified under two general headings: primary processes and secondary processes. Two of the most important primary biogeochemical processes are the accumulation of elements into the cellular structure of organisms and metabolic changes in the oxidation state of elements leading to the accumulation of oxidized or reduced forms of elements in the external milieu. A third process, biomethylation, has been the subject of interest in recent years in view of its implications with respect to the entry of toxic metals into the food chain. It is also a potentially important reaction for mobilizing and transporting certain elements through the environment. Secondary biogeochemical processes arise as the result of non-specific modifications of the physical and chemical characteristics of the environment due to metabolic and other activities of organisms. A biogeochemical cycle results from the integration of several biological, chemical, and, in many cases, physical processes. The chapter discusses the nitrogen cycle illustrating the ways in which organisms cooperate to affect the cyclic turnover of an element, and the selenium cycle demonstrating the interaction between the biosphere and geosphere.