Redox and pH gradients drive amino acid synthesis in iron oxyhydroxide mineral systems

Abstract

Iron oxyhydroxide minerals, known to be chemically reactive and significant for elemental cycling, are thought to have been abundant in early-Earth seawater, sediments, and hydrothermal systems. In the anoxic Fe 2+ -rich early oceans, these minerals would have been only partially oxidized and thus redox-active, perhaps able to promote prebiotic chemical reactions. We show that pyruvate, a simple organic molecule that can form in hydrothermal systems, can undergo reductive amination in the presence of mixed-valence iron oxyhydroxides to form the amino acid alanine, as well as the reduced product lactate. Furthermore, geochemical gradients of pH, redox, and temperature in iron oxyhydroxide systems affect product selectivity. The maximum yield of alanine was observed when the iron oxyhydroxide mineral contained 1:1 Fe(II):Fe(III), under alkaline conditions, and at moderately warm temperatures. These represent conditions that may be found, for example, in iron-containing sediments near an alkaline hydrothermal vent system. The partially oxidized state of the precipitate was significant in promoting amino acid formation: Purely ferrous hydroxides did not drive reductive amination but instead promoted pyruvate reduction to lactate, and ferric hydroxides did not result in any reaction. Prebiotic chemistry driven by redoxactive iron hydroxide minerals on the early Earth would therefore be strongly affected by geochemical gradients of E h , pH, and temperature, and liquid-phase products would be able to diffuse to other conditions within the sediment column to participate in further reactions. life emergence | iron hydroxides | hydrothermal vents | early Earth | gradients T he synthesis of biomolecules, particularly amino acids and their condensation into peptides, from geochemical carbon and nitrogen sources is an important research topic for assessing the role of specific geochemical environments and mineral phases in the emergence of life. One mineral type of interest that would have been abundant in the mildly acidic, iron-rich oceans of the early Earth (1-3) is the iron oxyhydroxides, which can precipitate in a variety of stable or metastable redox states (4, 5). Iron oxides/oxyhydroxides are versatile reactive minerals that can drive redox reactions and concentrate phosphorus species, trace metals, organic molecules, and other anions (6-11). On the early Earth, iron oxyhydroxides and/or green rust would likely have been present in the water column as well as seafloor sediments, playing a fundamental role in elemental cycling and redox chemistry (4, 10). Iron oxyhydroxides would also have been a primary component in alkaline hydrothermal vent mounds and chimneys, which have been proposed as a possible environment for the emergence of metabolism due to their ambient pH, E h , ion/chemical, and temperature gradients (12-15).Seafloor hydrothermal sediments and chimneys are flowthrough gradient systems that combine reactive minerals with organic compounds in a variety of possible reaction conditions. Alkaline vents produ...

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