Evolution of the biogeochemical cycles

Background/Question/Methods

Results/Conclusions The coupling of biogeochemicaly cycles on Earth is dependent largely on microbially driven redox reactions. Six major elements, H, C, N, O, S, and P comprise the major building blocks for all biological macromolecules. The biological fluxes of the first five are largely driven by microbially catalyzed, thermodynamically constrained, redox reactions. These involve two coupled half cells, leading to a linked system of elemental cycles. On geological time scales, resupply of C, S and P are dependent upon tectonics, especially volcanism and rock weathering. Thus, biogeochemical cycles have evolved on a planetary scale as a set of nested abiotic acid-base and redox reactions that sets lower limits on external energy required to sustain the cycles. These reactions fundamentally altered the surface redox state of the planet. Feedbacks between the evolution of microbial metabolic processes and communities, with the surrounding geochemistry are what create the average redox condition of the oceans and atmosphere. The biological oxidation of Earth is driven by photosynthesis, which is the only known energy transduction process that is not directly dependent on preformed bond energy. The fluxes of electrons and protons can be combined with the six major elements to construct a global metabolic map for Earth. The genes encoding the machinery responsible for the redox chemistry of half-cells form the basis of the major energy-transducing metabolic pathways. The contemporary pathways invariably require multimeric protein complexes (i.e., the microbial “machines”) that are often highly conserved at the level of primary or secondary structure. These complexes did not evolve instantaneously, yet the order of their appearance in metabolism and analysis of their evolutionary origins are obscured by lateral gene transfer and extensive selection. These processes make reconstruction of how electron transfer reactions came to be catalyzed extremely challenging. In this lecture, I will discuss how the major elemental cycles evolved and how the became so robust as to withstand huge environmental perturbations over geological time.