Introduction to coupled biogeochemical cycles

At the 2009 Annual Meeting of the Ecological Society of America, investigators of ecosystem science and biogeochemistry gathered in a series of sessions sponsored by the National Science Foundation to examine what is known about the coupling of chemical elements in biochemistry and thus in global biogeochemical cycles. As an introduction to the topic, this paper offers some philosophical thoughts about the origins and implications of coupled biogeochemical cycles on Earth. The chemical characteristics of Earth have carried the imprint of biology since the first appearance of life more than 3.5 billion years ago. Movements of about 30 chemical elements essential to life are coupled, so that the behavior of one element can often be used to predict the behavior of other elements - in both the natural and human-perturbed settings of the world. The coupling of biochemical elements stems from basic stoichiometry (the relative quantities of chemical elements in living biomass), the oxidation-reduction or redox reactions (in which one substance is oxidized [loses electrons] and another is reduced [gains electrons]) of metabolic activity, and chelation (in which a metal ion binds to an organic compound). Many anthropogenic environmental problems - climate change, eutrophication, acid precipitation, metal pollution - originate from the release of non-stoichiometric and toxic quantities of chemical elements to the environment. Geoengineering proposals as solutions to certain global environmental problems must be consistent with the underlying constraints of coupled biogeochemical cycles. Biogeochemistry has much to offer to policy makers, and biogeochemists must contribute to current deliberations about how to solve environmental problems.