Wade and three co-authors, reported in early December, 2021 that 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.
The authors described 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.
A planet’s suitability to sustain life is the combination of a number of chemical and physical characteristics, including planetary size and stellar metallicity. Among these is the requirement for the presence of liquid water and hence, the planet’s position in the circumstellar habitable zone. The initial development of life on Earth is also linked to iron. Cells utilize iron in numerous and highly conserved processes, including DNA replication, intermediate metabolism, and gene expression, exploiting the chemical properties of iron that facilitate redox reactions and bond formation in multiple orientations (1⇓–3). That iron plays such a fundamental role across the gamut of biology suggests that its utilization is ancestral and hence, may reflect environmental conditions at the initiation of life (here: Reference #4).
See Complete Paper: https://www.pnas.org/content/118/51/e2109865118
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