Synchronized chaotic targeting and acceleration of surface chemistry in prebiotic hydrothermal microenvironments

Abstract

Porous mineral formations near subsea alkaline hydrothermal vents embed microenvironments that make them potential hot spots for prebiotic biochemistry. But, synthesis of long-chain macromolecules needed to support higher-order functions in living systems (e.g., polypeptides, proteins, and nucleic acids) cannot occur without enrichment of chemical precursors before initiating polymerization, and identifying a suitable mechanism has become a key unanswered question in the origin of life. Here, we apply simulations and in situ experiments to show how 3D chaotic thermal convection-flows that naturally permeate hydrothermal pore networks-supplies a robust mechanism for focused accumulation at discrete targeted surface sites. This interfacial enrichment is synchronized with bulk homogenization of chemical species, yielding two distinct processes that are seemingly opposed yet synergistically combine to accelerate surface reaction kinetics by several orders of magnitude. Our results suggest that chaotic thermal convection may play a previously unappreciated role in mediating surface-catalyzed synthesis in the prebiotic milieu.

Keywords: chaos; hydrothermal vents; prebiotic biochemistry; thermal convection.

Fig. 1.

Hydrothermal conveyor based on chaotic thermal convection. (A) Mineral formations near off-ridge alkaline hydrothermal vents lining the ocean floor contain embedded pore networks with microenvironments that impose thermal and geometric conditions robustly capable of sustaining internal convective flow fields. Image below depicts a cross-section of material retrieved from the Lost City vent illustrating its internal void morphology. (Scale bar, 2.5 cm.) Reproduced with permission from ref. . (B) These vent systems embed characteristic porosity and thermal gradients aligned with chaotic thermal convection. Length scales and thermal gradients associated with thermophoretic focusing are illustrated for comparison.

Fig. 2.

Regimes of targeted surface enrichment and accelerated adsorption kinetics. (A) Computational simulations enable enrichment, quantified in terms of a focusing index f, to be parametrically plotted in terms of the Ra, h/d, and thermal gradient. (A, i–iii) Depiction of representative flow trajectories (Left) and sidewall adsorption profiles corresponding to 300 tracers randomly dispersed in the bulk (Center, individual realizations; Right, vertical distribution histogram). Images below the parametric plot display experimentally obtained sidewall adsorption profiles with (A, iv) fluorescently tagged 1-µm-diameter carboxylated microspheres, and (A, v and vi) methylene blue dye [Left, side-view photograph of cylindrical pore; Right, vertical intensity profile of normalized (A, iv) green and (A, v and vi) blue color channel intensity]. (B) Computational simulations incorporating a kinetic surface adsorption model enable the time-resolved surface accumulation of chemical species on the pore sidewalls to be quantified. Adsorption kinetics are accelerated by up to 1,000-fold in the presence of thermal convection compared with molecular diffusion alone, and this enhancement correlates with a transition from periodic to disordered flow trajectories (B, vii–ix). Pore geometries in A and B are not depicted to scale to facilitate comparison between them.

Fig. 3.

Chaotic thermal convection synchronizes targeted and accelerated surface enrichment. (A) Computationally simulated parametric plot of the figure of merit f × (τ_diff/τ_conv) reveals a regime at intermediate Ra and h/d, and at modest thermal gradients, where simultaneous targeted and accelerated surface enrichment is achievable (f is scaled such that its magnitude ranges from zero to unity). This sweet spot, spanning orders of magnitude in thermal and geometric conditions, is characterized by chaotic advection. The expanded panel at the left depicts the chaotic nature of the flow at h/d = 2, both qualitatively in terms of midplane Poincaré sections (Left), and quantitatively in terms of Lyapunov exponent spectra (Right). Symbols at the left of each panel match the corresponding states in the parametric plot. (B) Simulations also quantify the strength of the chaotic flow component by increased values of the Lyapunov exponent λ within the sweet-spot regime, compared with states at lower Ra or higher h/d where periodic trajectories predominate.

Fig. 4.

Chaotic advection accelerates interfacial transport under hydrothermally relevant conditions. (A) Computationally simulated interfacial Sherwood number averaged across a reaction interface spanning the top pore surface (Sh_av) continually decays under periodic flow but achieves an order of magnitude greater plateau value under chaotic advection [h/d = 2, Ra = 1.2 × 10⁵ (periodic); h/d = 2, Ra = 1.2 × 10⁷ (chaotic); and h/d = 6, Ra = 1.2 × 10⁷ (periodic)]. (B) Experimental observations obtained by recording motion of 10-µm fluorescent microspheres are in agreement with simulated flow trajectories under both chaotic (d = 6.4 mm) and periodic (d = 1.4 mm) states at h/d = 2 (Movie S1). (C) Simulated projections of the solute mass fraction x_f,A along a midvertical plane in each pore geometry (arrows indicate that the reaction interface is located at the upper surface) reveal that chaotic advection (Top) enables a thin interfacial boundary layer to be maintained as solute species are continually homogenized in the bulk. Molecular diffusion (no flow, Bottom) is also depicted as a control. Symbols match the flow states depicted in Fig. 3.

Fig. 5.

In situ probe reveals accelerated surface electrochemistry under chaotic thermal convection. (A) Addressable Cu electrodes patterned on a glass substrate affixed to the top surface of a pore-mimicking cylindrical flow cell enable electrochemical dissolution to be viewed from above (drawing not to scale). (B) Anodic dissolution occurs slowly at neutral pH, but is accelerated under alkaline conditions (images of the anode taken after a 3-V potential was applied for 1 min, no convective flow imposed). (C) In the absence of convection, the anode surface is not visibly changed when a 3-V potential is applied in an aqueous solution containing a 100-base-pair double-stranded DNA ladder (1 μg/mL, pH = 7). (D) But, dissolution progresses rapidly when a convective flow is imposed that continuously transports negatively charged DNA toward the anode where the electrophoretically compacted film stabilizes a local pH gradient, favoring Cu dissolution. (E) Experimentally obtained video recordings of the electrodes were analyzed to quantify anodic dissolution at the upper pore surface as a function of time (symbols). These data were used as inputs to a simulated kinetic predictive model (lines). (Insets) Images of the anode corresponding to each condition. A control experiment under convective flow (Ra = 3.4 × 10⁵, h = 4 mm) in the absence of DNA is also shown (open green symbols). Scale: all electrodes are 500 µm wide. (F) Kinetic model of electrode dissolution yields predicted values of τ_diff/τ_conv (G) in agreement with the parametric map in Fig. 2B.