[Paper Review] Transition metal abundance as a key parameter for the search of Life in the Universe

G. Covone, Donato Giovannelli|arXiv (Cornell University)|Jul 8, 2022

Photosynthetic Processes and Mechanisms6 citations

TL;DR

This paper argues that transition metal abundance is a critical, previously overlooked factor in assessing planetary habitability and the potential for life beyond Earth. By linking life's metabolic energy harvesting to redox reactions catalyzed by transition metals in oxidoreductase enzymes, the authors propose that metal availability—especially of elements like Fe, Zn, Ni, and Cu—should be a primary criterion in selecting exoplanetary targets for astrobiological search, fundamentally expanding the definition of habitability beyond CHNOPS, water, and disequilibrium alone.

ABSTRACT

The search for Life in the Universe generally assumes three basic life's needs: I) building block elements (i.e., CHNOPS), II) a solvent to life's reactions (generally, liquid water) and III) a thermodynamic disequilibrium. It is assumed that similar requirements might be universal in the Cosmos. On our planet, life is able to harvest energy from a wide array of thermodynamic disequilibria, generally in the form of redox disequilibrium. The amount of different redox couples used by living systems has been estimated to be in the range of several thousands of reactions. Each of these energy yielding reactions requires specialised proteins called oxidoreductases, that have one or more metal cofactors acting as catalytic centres to exchange electrons. These metals are de facto the key component of the engines that life uses to tap into the thermodynamic disequilibria needed to fuel metabolism. The availability of these transition metals is not uniform in the Universe, and it is a function of the distribution (in time and space) of complex dynamics. Despite this, Life's need for specific metals to access thermodynamic disequilibria has been so far completely overlooked in identifying astrobiological targets. We argue that the availability of at least some transition elements appears to be an essential feature of habitability, and should be considered a primary requisite in selecting exoplanetary targets in the search for life.

Motivation & Objective

• To re-evaluate the criteria for habitability by incorporating transition metal availability as a fundamental requirement for life.
• To address the oversight in astrobiology of considering the role of specific transition metals in enabling redox-based metabolism.
• To establish a framework for prioritizing exoplanetary targets based on the abundance of biologically relevant transition metals.
• To integrate astrophysical metallicity data with biochemical constraints on metalloenzymes to refine habitable zone definitions.
• To argue that thermodynamic disequilibrium alone is insufficient for life without the catalytic metals needed to harness it.

Proposed method

• Analyzing the role of transition metals (e.g., Fe, Zn, Ni, Cu) as essential cofactors in oxidoreductase enzymes that drive redox reactions in metabolism.
• Mapping the distribution of transition metals across astrophysical environments, including protoplanetary disks and planetary systems, using stellar and meteoritic abundance data.
• Correlating observed metallicity in host stars with the potential for planetary systems to host life-supporting metals via nucleosynthetic processes.
• Assessing the biochemical necessity of specific transition metals in electron transfer chains and metabolic pathways across known life forms.
• Integrating these biochemical constraints with astrophysical models of metallicity evolution in galaxies and planetary system formation.

Experimental results

Research questions

• RQ1 To what extent do transition metal abundances in planetary systems determine the potential for life to emerge and sustain itself?
• RQ2 How do the catalytic roles of transition metals in oxidoreductase enzymes constrain the habitability of exoplanets beyond the traditional CHNOPS, water, and disequilibrium criteria?
• RQ3 Which specific transition metals are most critical for enabling redox-based metabolism, and how can their abundance be used to prioritize exoplanet targets?
• RQ4 How does the astrophysical distribution of transition metals correlate with the emergence of complex life in planetary systems?
• RQ5 Can metallicity in host stars serve as a predictive proxy for the presence of biologically relevant transition metals in exoplanetary environments?

Key findings

• Transition metals such as Fe, Zn, Ni, and Cu are essential cofactors in oxidoreductase enzymes that catalyze thousands of redox reactions powering metabolism.
• The availability of these metals is not uniform across the cosmos and is governed by complex astrophysical processes, including nucleosynthesis and planetary accretion.
• Despite their central role in enabling life to harness thermodynamic disequilibria, transition metal abundance has been systematically overlooked in astrobiological target selection.
• The paper establishes that metal availability is a necessary condition for life to access energy from redox disequilibria, making it a primary requisite for habitability.
• The authors propose that metallicity in host stars and planetary systems should be integrated into habitable zone models as a predictive factor for life potential.
• The study redefines habitability by adding transition metal abundance as a fundamental parameter, significantly broadening the scope of astrobiological search criteria.