Iron-Sulfide-Bearing Chimneys as Potential Catalytic Energy Traps at Life's Emergence

Abstract

The concept that life emerged where alkaline hydrogen-bearing submarine hot springs exhaled into the most ancient acidulous ocean was used as a working hypothesis to investigate the nature of precipitate membranes. Alkaline solutions at 25-70°C and pH between 8 and 12, bearing HS(-)±silicate, were injected slowly into visi-jars containing ferrous chloride to partially simulate the early ocean on this or any other wet and icy, geologically active rocky world. Dependent on pH and sulfide content, fine tubular chimneys and geodal bubbles were generated with semipermeable walls 4-100 μm thick that comprised radial platelets of nanometric mackinawite [FeS]±ferrous hydroxide [∼Fe(OH)(2)], accompanied by silica and, at the higher temperature, greigite [Fe(3)S(4)]. Within the chimney walls, these platelets define a myriad of micropores. The interior walls of the chimneys host iron sulfide framboids, while, in cases where the alkaline solution has a pH>11 or relatively low sulfide content, their exteriors exhibit radial flanges with a spacing of ∼4 μm that comprise microdendrites of ferrous hydroxide. We speculate that this pattern results from outward and inward radial flow through the chimney walls. The outer Fe(OH)(2) flanges perhaps precipitate where the highly alkaline flow meets the ambient ferrous iron-bearing fluid, while the intervening troughs signal where the acidulous iron-bearing solutions could gain access to the sulfidic and alkaline interior of the chimneys, thereby leading to the precipitation of the framboids. Addition of soluble pentameric peptides enhances membrane durability and accentuates the crenulations on the chimney exteriors. These dynamic patterns may have implications for acid-base catalysis and the natural proton motive force acting through the matrix of the porous inorganic membrane. Thus, within such membranes, steep redox and pH gradients would bear across the nanometric platelets and separate the two counter-flowing solutions, a condition that may have led to the onset of an autotrophic metabolism through the reduction of carbon dioxide.

Peptide and RNA contributions to iron–sulphur chemical gardens as life's first inorganic compartments, catalysts, capacitors and condensers

“…Data presented here and elsewhere [18,38] suggest that sulphidic chemical garden structures had the potential to operate as hydrothermal flow-through reactors imagined as the hatcheries of life. A mound of such gardensubiquitous on the early Earth-growing on the floor of the acidulous ocean above an alkaline submarine hydrothermal vent provides a plausible setting for the origin of biological complexity [20,44,85,86].…”

“…How the energy flows were organized in these chemical gardens remains unclear. Perhaps the crenulated pattern-a feature of the chimneys with or without the organic additives-is a clue, the rills and ridges defining the original importing and exporting sites across the inorganic membrane [18]. Such periodic depositions are not unique to these experiments and have been observed by other groups [10,46,48] (cf.…”

“…The ultimate reduction or waste product was probably initially acetate [12][13][14][15], and the womb or omphalos of life a chemical garden-like precipitate over a submarine alkaline spring, especially where it vented into the acidulous, iron-bearing Hadean Ocean [16][17][18]. Much as predicted, a comparable spring-the so-called Lost City-was discovered in the North Atlantic in 1999 [16,19,20].…”

“…Given the existence of alkaline hydrothermal chimneys as probable localities for a geochemically and electrochemically driven emergence of life, they are urgent targets for research into the processes that govern their formation, as well as for empirical testing of their ability to act as flow-through reactors and culture chambers for chemical interconversions leading at least to the onset of metabolism [16]. Thus, we have begun the systematic investigation of metal sulphide and oxide chimneys in the laboratory, with the intention of probing in detail the determinants of their formation and characteristics [18]. Recently, we have shown that the passage of high-pressure alkaline fluid through iron sulphide and basaltic rock is sufficient for the mobilization of appreciable quantities of both sulphide ions as well as silica and that the resultant fluid is sufficient to support the formation of a chimney upon introduction to a ferrous-ironbearing solution [38].…”

“…As judged by EDAX, Fe : S ratios approached unity at some locations (Fe : S = 1.3). Indeed, in materials from similar experiments in which silicate was also added at standard temperature and pressure, mackinawite (FeS) was generated and greigite (Fe 3 S 4 ) was formed at higher temperature (70 • C) as analysed by X-ray diffraction and Raman spectroscopy [18,38,47] (and see [39]). A striking observation from these experiments was the general, although not invariable, appearance of crenulated patterns on both the exterior and interior of the chimneys (Fe : S ∼ 4) with ridges and rills aligned normal to the apparent direction of chimney growth (figure 1b).…”