Formation of Thiophene under Simulated Volcanic Hydrothermal Conditions on Earth-Implications for Early Life on Extraterrestrial Planets?

Volatile organic compounds (VOCs) from diffuse degassing areas: Interstitial soil gases as message bearers from deep hydrothermal reservoirs

The chemical composition of volatile organic compounds (VOCs) in interstitial soil gases from hydrothermal areas is commonly shaped by both deep hydrothermal conditions (e.g., temperature, redox, sulfur fugacity) and shallow secondary processes occurring near the soil-atmosphere interface. Caldara di Manziana and Solfatara di Nepi, i.e., two hydrothermal systems characterized by diverse physicochemical conditions located in the Sabatini Volcanic District and Vicano-Cimino Volcanic District, respectively (Central Italy), were investigated to evaluate the capability of VOCs in soil gases to preserve information from the respective feeding deep fluid reservoirs. Hierarchical cluster analyses and robust principal component analyses allowed recognition of distinct groups of chemical parameters of soil gases collected from the two study areas. The compositional dissimilarities from the free-gas discharges were indeed reflected by the chemical features of soil gases collected from each site, despite the occurrence of shallow processes, e.g., air mixing and microbial degradation processes, affecting VOCs. Four distinct groups of VOCs were recognized suggesting similar sources and/or geochemical behaviors, as follows: (i) S-bearing compounds, whose abundance (in particular that of thiophenes) was strictly dependent on the sulfur fugacity in the feeding system; (ii) C4,5,7+ alkanes, n-hexane, cyclics and alkylated aromatics, related to relatively low-temperature conditions at the gas source; (iii) C2,3 alkanes, benzene, benzaldehyde and phenol, i.e., stable compounds and thermal degradation products; and (iv) aliphatic O-bearing compounds, largely influenced by shallow processes within the soil. However, they maintain a chemical speciation that preserves a signature derived from the supplying deep-fluids, with aldehydes and ketones becoming more enriched after intense interaction of the hypogenic fluids with shallow aquifers. Accordingly, the empirical results of this study suggest that the chemical composition of VOCs in soil gases from hydrothermal areas provides insights into both deep source conditions and fluid circulation dynamics, identifying VOCs as promising geochemical tracers for geothermal exploration.

Nickel-organo compounds as potential enzyme precursors under simulated early Earth conditions

The transition from inorganic catalysis through minerals to organic catalysis by enzymes is a necessary step in the emergence of life. Our work is elucidating likely reactions at the earliest moments of Life, prior to the existence of enzymatic catalysis, by exploring essential intersections between nickel bioinorganic chemistry and pterin biochemistry. We used a prebiotically-inspired acetylene-containing volcanic hydrothermal experimental environment to shed light on the efficient formation of nickel-organo complexes. The simplest bis(dithiolene)nickel complex (C2H2S2)2Ni was identified by UV/Vis spectroscopy, mass spectrometry, nuclear magnetic resonance. Its temporal progression and possible function in this simulated early Earth atmosphere were investigated by isolating the main bis(dithiolene)nickel species from the primordial experimental setup. Using this approach, we uncovered a significant diversity of nickel-organo compositions by identifying 156 elemental annotations. The formation of acetaldehyde through the subsequent degradation of these organo-metal complexes is intriguing, as it is reminiscent of the ability of Pelobacter acetylenicus to hydrate acetylene to acetaldehyde via its bis(dithiolene)-containing enzyme acetylene hydratase. As our findings mechanistically characterize the role of nickel sulfide in catalyzing the formation of acetaldehyde, this fundamental pre-metabolic reaction could play the role of a primitive enzyme precursor of the enzymatic acetylene metabolism and further strengthen the role of acetylene in the molecular origin of life.

C2-addition patterns emerging from acetylene and nickel sulfide in simulated prebiotic hydrothermal conditions

Chemical complexity is vital not only for the origin of life but also for biological evolution. The chemical evolution of a complex prebiotic mixture containing acetylene, carbon monoxide (CO), and nickel sulfide (NiS) has been analyzed with mass spectrometry as an untargeted approach to reaction monitoring. Here we show through isotopic 13C-labelling, multiple reaction products, encompassing diverse CHO and CHOS compounds within the complex reaction mixture. Molecules within the same chemical spaces displayed varying degrees of 13C-labelling, enabling more robust functional group characterization based on targeted investigations and differences in saturation levels among the described classes. A characteristic C2-addition pattern was detected in all compound classes in conjunction with a high diversity of thio acids, reminiscent of extant microbial C2-metabolism. The analysis involved a time-resolved molecular network, which unveiled the behavior of sulfur in the system. At the onset of the reaction, early formed compounds contain more sulfur atoms compared to later emerging compounds. These results give an essential insight into the still elusive role of sulfur dynamics in the origin of life. Moreover, our results provide temporally resolved evidence of the progressively increasing molecular complexity arising from a limited number of compounds.

Formation, stabilization and fate of acetaldehyde and higher aldehydes in an autonomously changing prebiotic system emerging from acetylene

Many essential building blocks of life, including amino acids, sugars, and nucleosides, require aldehydes for prebiotic synthesis. Pathways for their formation under early earth conditions are therefore of great importance. We investigated the formation of aldehydes by an experimental simulation of primordial early earth conditions, in line with the metal-sulfur world theory in an acetylene-containing atmosphere. We describe a pH-driven, intrinsically autoregulatory environment that concentrates acetaldehyde and other higher molecular weight aldehydes. We demonstrate that acetaldehyde is rapidly formed from acetylene over a nickel sulfide catalyst in an aqueous solution, followed by sequential reactions progressively increasing the molecular diversity and complexity of the reaction mixture. Interestingly, through inherent pH changes, the evolution of this complex matrix leads to auto-stabilization of de novo synthesized aldehydes and alters the subsequent synthesis of relevant biomolecules rather than yielding uncontrolled polymerization products. Our results emphasize the impact of progressively generated compounds on the overall reaction conditions and strengthen the role of acetylene in forming essential building blocks that are fundamental for the emergence of terrestrial life.

The Abiotic Formation of Pyrrole under Volcanic, Hydrothermal Conditions-An Initial Step towards Life's First Breath?

Porphyrins, corrins, and tetrapyrroles constitute macrocycles in essential biomolecules such as heme, chlorophyll, cobalamin, and cofactor F430. The chemical synthesis as well as the enzymatic synthesis of these macrocycles starts from pyrrole derivatives. We here show that pyrrole and dimethyl pyrrole can be formed under the simulated volcanic, hydrothermal conditions of Early Earth, starting from acetylene, propyne, and ammonium salts in the presence of NiS or CoS as catalysts.