Revisiting the bio-inorganic bridge 25 years later

The concept of the bio-inorganic bridge links the evolution of Earth's biosphere to the broad-scale changes in trace metal availability driven by shifts in ocean redox conditions. This framework connects the acquisition of metal enzyme cofactors to evolving environmental conditions over geological time. Various approaches have been taken to building this bridge, integrating insights from microbiology, phylogenomics, ecophysiology, and geochemistry. Much of this work has been framed around a model of Earth's oceans evolving from an Archean anoxic state, through an intermediate sulfidic phase, to the well-oxygenated conditions of the modern world. This perspective predicts corresponding changes in the abundance of key trace elements and highlights their roles in governing primary productivity and the emergence of eukaryotes. That said, geological proxy studies in the intervening years revealed much more complexity to ocean redox evolution, while novel phylogenomic analyses reveal a deeper evolutionary antiquity for several redox-sensitive metalloenzymes. These discoveries require that geobiologists pay close attention to environmental variations in space as well as time. Moreover, increasing awareness that Precambrian trace metal abundances reflect large changes in sources and sinks, as well as in redox conditions, urges closer attention to tectonically influenced fluxes of major nutrients, especially phosphorus, as well as changing weathering fluxes through time. A new understanding of the relationships between Earth's physical history and metalloenzymes awaits.