Jon Wade et al.; PNAS December 21, 2021 118 (51) e2109865118

Iron is an irreplaceable component of proteins and enzyme systems required for life. This need for iron is a well-characterized evolutionary mechanism for genetic selection. However, there is limited consideration of how iron bioavailability, initially determined by planetary accretion but fluctuating considerably at global scale over geological time frames, has shaped the biosphere. We describe influences of iron on planetary habitability from formation events >4 Gya and initiation of biochemistry from geochemistry through oxygenation of the atmosphere to current host–pathogen dynamics. By determining the iron and transition element distribution within the terrestrial planets, planetary core formation is a constraint on both the crustal composition and the longevity of surface water, hence a planet’s habitability. As such, stellar compositions, combined with metallic core-mass fraction, may be an observable characteristic of exoplanets that relates to their ability to support life. On Earth, the stepwise rise of atmospheric oxygen effectively removed gigatons of soluble ferrous iron from habitats, generating evolutionary pressures. Phagocytic, infectious, and symbiotic behaviors, dating from around the Great Oxygenation Event, refocused iron acquisition onto biotic sources, while eukaryotic multicellularity allows iron recycling within an organism. These developments allow life to more efficiently utilize a scarce but vital nutrient. Initiation of terrestrial life benefitted from the biochemical properties of abundant mantle/crustal iron, but the subsequent loss of iron bioavailability may have been an equally important driver of compensatory diversity. This latter concept may have relevance for the predicted future increase in iron deficiency across the food chain caused by elevated atmospheric CO₂.

See: https://www.pnas.org/content/118/51/e2109865118

Fig. 1. The iron content of terrestrial bodies of approximate Mars size reflects the mean redox conditions of core formation (oxygen fugacity, fO₂, shown relative to the iron-wüstite [FeO] buffer). A Mars-like planet forming under conditions around 10× more oxidizing than the Earth possesses a mantle in excess of double the FeO content. In contrast, a Mercury-like planet forming under conditions ∼100× more reducing leaves its mantle denuded in FeO (<0.5 Earth) and also, a host of other transition elements (Cr < 0.5 Earth, Ni ∼ 0.1 Earth, Mo ∼ 0.01 Earth). The arrow represents the upper limit of Mercury’s mantle FeO content (14). The model follows ref. 75 and assumes all planets are of Mars mass, with metallic cores fully equilibrating with the mantle at depths of ∼1/3 of the growing mantle; further details are given in SI Appendix.