ARE ‘BIO-INORGANIC BRIDGES’ FALLING DOWN? THE EVOLUTION OF LIFE AND THE ENVIRONMENT IN THE WAKE OF THE GREAT OXIDATION EVENT

The evolution of Earth’s surface environments and life are inextricably linked through the cycling of major and trace elements, the use of inorganic elements in biological molecules, and the biosphere’s ability to alter these cycles. One of the best examples of this is the biosphere’s role in driving the pervasive oxygenation of Earth’s surface environments following the Great Oxidation Event (GOE) and the resultant impacts on elemental cycling. More subtle, however, may be the way in which the availability of trace elements affects their utilization in biological systems or molecules, such as metalloenzymes. The latter of these two examples is often referred to as the ‘bio-inorganic bridge’ as per Anbar and Knoll (2002), and links changes in the availability of trace elements throughout Earth’s history to biospheric evolution. Examples of the ‘bio-inorganic bridge’ include an Archean decline in nickel abundances resulting in the marginalization of methanogens in the lead up to the GOE, increasing Mo availability for nitrogenase following the GOE, and the proliferation of Zn and Cu metalloenzymes by eukaryotes following Neoproterozoic ocean oxygenation. In the wake of the GOE, it may be expected that increased environmental oxygenation resulted in an increase in the supply of biologically critical trace elements, a possible trigger for eukaryogenesis. Yet, despite geochemical evidence for protracted oxygenation in the Paleoproterozoic, there is a dearth of evidence for the rise of eukaryotes in the immediate aftermath of the GOE. Furthermore, the proliferation of a nickel-based antioxidant enzyme in the Neoproterozoic by cyanobacteria, which runs counter to nickel’s documented availability, raises questions about whether ‘bio-inorganic bridges’ always hold up. Here we examine several examples of ‘bio-inorganic bridges’ that seem to have fallen and the potential underlying reasons which may include alternative geochemical controls on trace metal availability and niche-based competition.

Anbar, A.D., Knoll, A.H., 2002. Proterozoic ocean chemistry and evolution: A bioinorganic bridge? Science 297: 1137-1142.