There is an increasing interest in the icy moons of the Solar System due to their potential habitability and as targets for future exploratory missions, which include astrobiological goals. Several studies have reported new results describing the details of these moons’ geological settings; however, there is still a lack of information regarding the deep subsurface environment of the moons. The purpose of this article is to evaluate the microbial habitability of Europa constrained by terrestrial analogue environments and sustained by radioactive energy provided by natural unstable isotopes. The geological scenarios are based on known deep environments on Earth, and the bacterial ecosystem is based on a sulfate-reducing bacterial ecosystem found 2.8 km below the surface in a basin in South Africa. The results show the possibility of maintaining the modeled ecosystem based on the proposed scenarios and provides directions for future models and exploration missions for a more complete evaluation of the habitability of Europa and of icy moons in general.

Since the pioneering experimental work performed by Urey and Miller around 70 years ago, several experimental works have been developed for approaching the question of the origin of life based on very few well-constructed hypotheses. In recent years, attention has been drawn to the so-called alkaline hydrothermal vents model (AHV model) for the emergence of life. Since the first works, perspectives from complexity sciences, bioenergetics and thermodynamics have been incorporated into the model. Consequently, a high number of experimental works from the model using several tools have been developed. In this review, we present the key concepts that provide a background for the AHV model and then analyze the experimental approaches that were motivated by it. Experimental tools based on hydrothermal reactors, microfluidics and chemical gardens were used for simulating the environments of early AHVs on the Hadean Earth (~4.0 Ga). In addition, it is noteworthy that several works used techniques from electrochemistry to investigate phenomena in the vent–ocean interface for early AHVs. Their results provided important parameters and details that are used for the evaluation of the plausibility of the AHV model, and for the enhancement of it.

Certain subterranean environments of Earth have naturally accumulated long-lived radionuclides, such as ²³⁸ U, ²³² Th, and ⁴⁰ K, near the presence of liquid water. In these natural radioactive environments, water radiolysis can produce chemical species of biological importance, such as H ₂ . Although the proposal of radioactive decay as an alternative source of energy for living systems has existed for >30 years, this hypothesis gained strength after the recent discovery of a peculiar ecosystem in a gold mine in South Africa, whose existence is dependent on chemical species produced by water radiolysis. In this study, we calculate the chemical disequilibrium generated locally by water radiolysis due to gamma radiation. We then analyze the possible contribution of this disequilibrium for the emergence of life, considering conditions of early Earth and having as reference the alkaline hydrothermal vent theory. Results from our kinetic model point out the similarities between the conditions caused by water radiolysis and those found on alkaline hydrothermal systems. Our model produces a steady increase of pH with time, which favors the formation of a natural electrochemical gradient and the precipitation of minerals with catalytic activity for protometabolism in this aqueous environment. We conclude by describing a possible free-energy conversion mechanism based on protometabolism, which could be a requisite for the emergence of life in Hadean Earth.

Energy metabolism at the emergence of life has been the topic of intense theoretical and experimental study. Alkaline hydrothermal vents (AHVs) may have facilitated energy transfer and carbon fixation at life’s emergence. Specifically, pH separation across vent walls could have been the forerunner to pH separation across cell membranes, with inorganic barriers containing [Ni-]FeS minerals as precursors of metalloenzymes in potentially ancient biological reductive acetyl-CoA Wood–Ljungdahl (WL) and other metabolic pathways. We previously demonstrated pH-gradient-dependent reduction of CO2 to formate by H2 in AHV interface conditions. Here, we address the same problem of CO2 reduction using a macroscale reactor with minerals synthesized via protocols meant to mimic the natural processes of hydrothermal chimney formation. This reactor also allowed us to probe more variables and explore longer experimentation time frames. These results elucidate how different aspects of the hydrothermal–vent interface (e.g., different minerals and/or temperature gradients) affect the observed CO2 electrochemical reduction as well as the flow of electrons under passive vs induced currents and potentials. Using experimental simulations and electrochemistry techniques, we detected two key steps of the WL pathway (CO2 to formic acid and the formation of acetic acid). We explored effects of Ni incorporation in the mineral catalyst, as well as temperature and the effects of these variables on the production of formate. Currents as small as 10 nanoamps to 10 microamps were enough to efficiently carry out CO2 reduction. In this work, we electrochemically explore energy protometabolism in vent–ocean interfaces, specifically focusing on [Ni-]FeS minerals as precursors of metalloenzymes.

Iron (II) sulfide minerals have gained attention in the last decades due to their relevance in hypotheses for the emergence of life on the early Earth around 4 billion years ago. In the submarine vent theory, it has been proposed that those minerals, especially mackinawite, had a key role in prebiotic processes. Those are estimated to be present in a natural electrochemical setting, analogous to a chemiosmotic one, formed in the interface between the early ocean and the interior of the alkaline hydrothermal systems, the early vent-ocean interface. To evaluate this and other hypotheses, voltammetric studies were performed to better understand the electrochemical behavior of minerals under conditions analogous to the vent-ocean interface. The preliminary results presented here indicate that, in the potential range estimated to exist in that interface, mackinawite can transition to other mineral phases and may posibly coexist with other minerals, resulting from its oxidation. This can create a local chemical diversity. In addition, it has been tested a protocol for Ni incorporation in mackinawite structure, resulting in a surface that showed an interesting behavior in the presence of CO₂, although definitive experiments showed necessary for a deeper comprehension of that behavior. Overall, the results are consistent with previous results on electrocatalytical properties of Fe-Ni-S materials for CO₂ reduction, and also could lead to the emergence of a protometabolism on early Earth.

Energy metabolism at the emergence of life has been the topic of intense theoretical and experimental study. Alkaline hydrothermal vents (AHVs) may have facilitated energy-transfer and carbon-fixation at life’s emergence, via a primordial chemiosmotic mechanism. Specifically, pH separation across vent walls could have been the forerunner to pH separation across microbial membranes, with electron-conductive inorganic barriers containing [Ni-]FeS minerals mimicking the active sites of metalloenzymes in potentially ancient biological reductive acetyl-CoA Wood-Ljungdahl (WL) pathway, reverse Krebs and other metabolic pathways. We previously demonstrated pH gradient dependent reduction of CO2 to formate by H2 in AHV interface conditions. Here, we address the same problem of CO2 reduction, however using a macroscale reactor that allowed us to use minerals synthesized via protocols analogous to the alkaline vent mount formation. This reactor also allowed us to probe more variables, and explore longer experimentation time-frames. Thus, this work aimed to investigate the effect of the different aspects of the gradientrich hydrothermal-vent interface on the observed CO2 electrochemical reduction- such as different minerals, temperature gradients, as well as the flow of electrons under passive vs. induced currents and potentials. Using experimental simulations and electrochemistry techniques, We detected two key steps of the WL pathway (CO2 to formic acid, and the formation acetic acid) as well as reactions in the reverse Krebs cycle (fumarate to succinate). Both Ni in the mineral, and temperature as a property, were shown to affect the formation of formate. Extremely small currents were enough to efficiently carry out CO2 reduction. This work develops benchmarks to electrochemically explore the model of protometabolisms in the vent-ocean interface and opens to future analyses on the catalytic or electrocatalytic properties of Fe-[Ni-]S minerals as precursors of metalloenzymes.