Smith Lecture: Reconstructing the Wiring Diagram of Earth's Biogeochemical Cycles
The biogeochemical cycles of H, C, N, O and S are coupled via biologically catalyzed electron transfer (redox) reactions. The metabolic processes responsible for maintaining these cycles evolved over the first ~2.4 billion years of Earth’s history in prokaryotes and form a global electronic circuit with several feedbacks. The penultimate transition was the evolution of oxygenic photosynthesis in cyanobacteria, which did not lead immediately to large scale accumulation of the gas but fundamentally altered the nitrogen cycle. The interaction between the oxygen cycle and the nitrogen cycle in particular led to a negative feed-back, in which increased production of oxygen led to decreased fixed inorganic nitrogen in the oceans. This feedback, which is supported by isotopic analyses of fixed nitrogen in sedimentary rocks from the late Archean, continues to the present; however, once sufficient oxygen accumulated in Earth’s atmosphere to allow nitrification to out-compete with denitrification, a new, stable electron “market” emerged and ultimately spread via lateral gene transfer to eukaryotic host cells, allowing the evolution of “complex” (animal) lifeforms. The resulting network of electron transfers led a gas composition of Earth’s atmosphere, which is far from thermodynamic equilibrium (i.e., it is an “emergent” property) and can be used as a guide to search for the presence of life on terrestrial planets outside of our solar system.