SPATIAL/TEMPORAL VARIATION IN THE BIOLOGICAL CONTROL OF ENVIRONMENTAL CHEMISTRY

Biotic processes operating over geological time have altered Earth’s chemical environment in profound ways. Biotic creation of a mostly aerobic atmosphere and ocean has altered the redox states of sulfur, iron and many other elements, and made the evolution of large heterotrophs like ourselves possible. These conditions contrast with a chemical environment that would exist through purely physical-chemical processes. The relative strength of geochemical influences increases with volcanic, tectonic, erosion and deposition rates, and with extreme climatic conditions. Biotic functions, in contrast, require specific resources and information. Resources include high-grade energy--primarily light--a relatively narrow range of acceptable temperatures, liquid water, and a sufficient supply of limiting elements. Informational codes are necessary to synthesize biochemical catalysts to drive thermodynamically costly reactions. This information is held in the genetic structure of species and can be locally extinguished.

Given the different conditions favoring geochemical versus biotic influences, the relative importance of each to control of local environmental chemistry should vary spatially and temporally in predictable ways. Geochemical control should dominate where geological processes are relatively rapid such as on actively eroding slopes, where temperature and water resources are limiting such as deserts, and where physical transport processes such as running water are relatively energetic. Biotic control, on the other hand, should dominate at biologically congenial ranges of temperature and liquid water availability, where transport processes are weak, and where disturbances have not reduced the biota. Appropriate indices of the relative importance of geochemical versus biotic controls can be generated to test predictions of those controls based on geographic variables of controlling phenomena. A consciousness of scale-dependent, spatially and temporally varying biogeochemical controls will contribute to a better understanding of the limits and mechanisms of environmental stoichiometry.