Alkaline vents recreated in two dimensions to study pH gradients, precipitation morphology, and molecule accumulation

Abstract

Alkaline vents (AVs) are hypothesized to have been a setting for the emergence of life, by creating strong gradients across inorganic membranes within chimney structures. In the past, three-dimensional chimney structures were formed under laboratory conditions; however, no in situ visualization or testing of the gradients was possible. We develop a quasi-two-dimensional microfluidic model of AVs that allows spatiotemporal visualization of mineral precipitation in low-volume experiments. Upon injection of an alkaline fluid into an acidic, iron-rich solution, we observe a diverse set of precipitation morphologies, mainly controlled by flow rate and ion concentration. Using microscope imaging and pH-dependent dyes, we show that finger-like precipitates can facilitate formation and maintenance of microscale pH gradients and accumulation of dispersed particles in confined geometries. Our findings establish a model to investigate the potential of gradients across a semipermeable boundary for early compartmentalization, accumulation, and chemical reactions at the origins of life.

Fig. 1.. Mixing of alkaline and acidic solutions in seafloor rock pores is mimicked in a microfluidic flow cell.

Acidic ocean water percolates the narrow cracks and pores of the seafloor and is converted to an ion-enriched and strongly alkaline fluid by interaction with the surrounding rock. When this fluid is exhaled back into the ocean, rapid mineral precipitation happens upon contact with the ocean water. To mimic this scenario in the laboratory, an alkaline sodium-hydroxide solution is pumped at a controlled flow rate into a flat microfluidic chamber prefilled with an acidic iron solution. Microscope imaging with pH-dependent dyes allows the visualization of different morphologies, depending on inflow rate and OH⁻ concentration, and the assessment of emerging pH gradients.

Fig. 2.. Variation of inflow rate and concentration of the alkaline fluid.

Injection of an NaOH solution into an ocean fluid containing 200 mM Fe(II)Cl₂ yields three dominant morphologies. Layers/rings form at high flow and concentrations, whereas diffusive fields persist at low values. In between both regimes, precipitate fingers form, which allow the formation and maintenance of steep pH gradients. At the higher limit, the acidic ocean is almost completely repressed by the precipitation front, and at the lower limit, both solution mix by diffusion due to weak precipitation. Therefore, no gradients can be formed or maintained in both limiting morphologies.

Fig. 3.. Changes in inflow rate and concentration correspond to a timescale shift in morphology formation.

Each of the three time series represents conditions that are dominated by one of the three morphologies. Layers/rings form predominately within the first 20 s, then converge to the finger structures between roughly 30 and 80 s, and finally fade into the flow fields after around 100 s. On the right, an average of 20 pixels in height over the cross-section of the chamber is plotted over time: (A) No steady state is reached and the fluids intermix by diffusion over time (blue areas become wider). (B) Stable gradients are formed between red (acidic) and blue (alkaline) areas and are maintained over time (straight blue stripes at persistent width). (C) In steady state, a dominant flow through the layers forms and fluorescence fades in adjacent structures, indicated by narrowing of the blue (alkaline) areas.

Fig. 4.. Accumulation of fluorescent beads in the different dominant morphologies.

Ten-micrometer fluorescent beads are dispersed into the alkaline fluid to visualize accumulation in layers/rings (A), precipitation fingers (B), and flow fields (C). Strongest local increase of fluorescence is observed in the precipitation fingers (B), suggesting the accumulation of dispersed beads.