THE EVOLUTION OF SYMBIOTIC NITROGEN FIXATION IN MARINE ECOSYSTEMS

Nitrogen fixation appears to have arisen once in Earth's history and the genes encoding the heterodimeric enzyme responsible for the reaction, nitrogenase, were spread across both bacterial and archeal lineages via horizontal gene transfer. These genes have never been found in any eukaryote. Regardless, many eukaryotic organisms obtain fixed inorganic nitrogen by forming symbiotic associations with nitrogen fixing prokaryotes. In the upper ocean, such a strategy has been found in some species of diatoms and in corals, but is hardly wide-spread; the end result is that fixed inorganic nitrogen fundamentally limits productivity in the oceans. I will examine the basic reasons for the lack of abundant nitrogen fixers in the oceans, and compare the outcomes with terrestrial and lacustrine ecosystems in the context of how nitrogen fixation emerged as one of the major controls on the biologically mediated fluxes of carbon and oxygen throughout the history of Earth.