Origin of Life: Aliphatic aldehydes in the Earth’s crust – remains of prebiotic chemistry?

The Origin of Life is a question that has not yet been solved in the natural sciences. Some promising interpretative approaches are related to hydrothermal activities. Hydrothermal environments contain all necessary elements for the development of precursor molecules. There are possibly catalytically active surfaces and wide ranges of pressure and temperature conditions. The chemical composition of hydrothermal uids together with periodically uctuating physical conditions should open up multiple pathways towards prebiotic molecules. Already in 2017, we detected prebiotic organic substances, including a homologous series of aldehydes in more than 3 billion years old Archean quartz crystals from Western Australia. In order to approach the question if the transformation of inorganic into organic substances is an ongoing process, we investigated a drill core from the geologically young Wehr caldera in Germany at a depth of 1000 m. Here we show the existence of a similar homologous series of aldehydes (C 8 to C 16 ) in the uid inclusions of the drill core calcites, a nding that supports the thesis that hydrothermal environments could possibly be the material source for the origin of life.


Introduction
Naturally occurring aliphatic aldehydes may play an important role as prebiotic precursor molecules. With growing chain lengths, the so-called fatty aldehydes gain amphiphilic properties and could serve as starting points for a large collection of membrane-forming lipids (Brown et al. 2009, Brown et al. 2011).
The reactivity of the aldehyde head group allows a wide variety of chemical modi cations, including oxidation to fatty acids, reduction to alcohols, the formation of amines and amides and nally ester or ether formation with glycerol (Fritz et al. 2013). They still play a role in the recent biosynthesis of sphingolipids where a long-chain aldehyde is undergoing a condensation reaction with serine (Awasthi et al. 1980). The biochemical formation of long aliphatic chains with a hydrophilic head group is quite complicated and involves a considerable amount of energy, it is not very likely to have occurred as initial development.
On the other hand, early metabolic processes required functional membranes at an early point of time.
Therefore, during the early steps of life, they were very likely depending on an external occurrence of such molecules. All theories on the origin of protocells which are based on initial membrane formation require an abiotic source of amphiphilic components (Deamer et al. 2002, Sakuma et al. 2015, Deamer 2017).
Recently, aliphatic aldehydes have been discovered in carbon-rich chondritic meteorites (Aponte et al. 2019). Aponte et al. analyzed ten different carbonaceous chondrites and, using a novel analytical approach, detected and quanti ed 16 different aldehydes. Among them were butanal, pentanal and hexanal in concentrations around 2-6 nmol/g. Most aldehydes in chondrites exhibit a clear enrichment of 13 C isotopes compared to their biogenic counterparts, a nding which indicates their abiotic formation (Aponte et al. 2019). This result is highly interesting because it represents convincing proof for an abiotic (in this case, extraterrestrial) origin of such potentially amphiphilic compounds which are possible starting points for membrane-forming lipids. The analytical method used to analyze the aldehydes was based on derivatization with (S,S)-(-)-1,4-dimethoxy-2,3-butanediol ((S,S)-DMB-diol). The analysis ended after 95 min and the derivatized hexanal was detected at a retention time of approximately 93 min; therefore, any longer-chain aldehydes present were not observable.
Moreover, there are also clear indications for a terrestrial abiotic formation of aldehydes. A complete family of homologous aliphatic aldehydes of even and uneven chain lengths was detected inside Archean quartz crystals . Inclusions in these Archean quartz crystals grown in a hydrothermal environment contained eleven different species from heptanal up to heptadecanal. This terrestrial evidence is even more intriguing concerning early membrane formation because of the wide and continuous distribution of chain lengths and the fact that the chain functionalization occurs almost exclusively on the methyl end groups of the chain. Due to the selected analytical conditions, the analysis Thus, it could be shown that a prebiotic presence of short-and long-chain aldehydes is not exclusively due to meteorite impacts on the early Earth but also due to hydrothermal formations, probably based on Fischer-Tropsch type reactions.
Altogether, the abiotic formation of aldehydes may be a common feature of planetary bodies in -at leastour solar system. At this point, one has to ask the question if it is an ongoing process that occurs up to the present day. If it is, one should be able to nd evidence for abiotically formed aliphatic aldehydes in recent hydrothermal environments. To minimize contamination by metabolites from microbial life in a corresponding search, sampling should occur outside of densely populated environments. On the other hand, it should focus on materials that are in close contact with hydrothermal uids and that could be able to collect possible organic products over an extended period of time.
For that purpose, a drilling project was started in the phonolite/trachyte complex of the Wehr caldera . At approximately 1 km of depth, carbon dioxide is expected to undergo a phase transition from the supercritical to the sub-critical gaseous phase. During that transition, the carbon dioxide essentially loses its capability to act as a hydrophobic solvent (Schreiber et al. 2012). As a consequence, this leads to the precipitation of mostly hydrophobic products at this point, forming an accumulation zone of corresponding organic compounds in this depth range (Mayer et al. 2017). In the following, we want to report on analytical data obtained from uid inclusions in the solid core material from 1 km depth.

Results And Discussion
The focus of the analyses was on the detection of aldehydes in the calcite samples of the drill core. Figure 1 shows the result of the calcite sample in comparison to those of the aldehyde standards, the hexane and procedure blanks.
The organic content in calcite samples show at least minor concentrations of aldehydes in the uid inclusions of the calcite in the core sample. All measurements are triple measurements and the aldehydes were identi ed with standards. The quanti cation was made by external calibration (Supplementary Information D and E). Table 1 shows the results of the measurements. Further information such as mass spectra are shown in the Supplementary Information E. By means of a six-point external calibration procedure, it was possible to determine the concentration of the detectable aldehydes in the calcite sample. Certainly, it has to be taken into account that this is not a representative sample, since the drill core cannot represent a homogeneous environment. One calcite sample was analyzed in three replicates and the concentration of each aldehyde in the sample was calculated between approximately 10 and 600 µg/kg (details in Table 2  Durham et al. described a supercritical Fischer-Tropsch synthesis using a potassium-promoted iron-based catalyst and supercritical hexane, which allows the production of large amounts of long chain aldehydes with more than ten carbon units (Durham et al. 2014). Whether this reaction is also possible in supercritical CO 2 has not yet been clari ed, but in our opinion, it is likely. While alcohols and carboxylic to organic compounds, such as aldehydes, is an ongoing process that continues to occur today, we believe that our results demonstrate that this is in fact the case.

Materials And Methods
The drill core sample A drill core from the Wehr caldera (N 50° 43', E 7° 22' East Eifel volcanic eld, Germany) was taken out of the depth of 950 m to 968 m. The Wehr volcano erupted twice, the most recent eruption took place 150,000 years ago (Bogaard 1989). The eruption formed a collapse structure in the Devonian basement (Wörner 1988), which was lled up with various layers of tephra and sediments. The hydrothermal processes are closely linked to volcanic activity, which is also evident in the Devonian basement. In particular, Quaternary hydrothermal calcite built is of relevant importance here. There are uid inclusions inside the hydrothermal calcites which have to be examined for organic compounds. For this purpose, samples of hydrothermal calcite from a drilling core were taken. A hydrothermal solution has penetrated into cracks in the ne-grained Devonian rocks (clayish shale, silt stone, ne-grain sandstone) and led to the crystallization of idiomorphic calcites ( Figure 2). Further information about the geological framework is shown in the Supplementary Information A.

Sample Preparation
The calcite samples were taken mechanically by fragmentation of the drill core under protected conditions. After the cleaning procedure of the sample surface with hexane, the washed calcite sample was cooled with liquid nitrogen and mortared with a mortar and a pestle and extracted three times with 10 mL hexane. The solution of the grinding step was collected in a Te on tube. The mortared calcite (powder) was immediately scraped out of the mortar with the pestle and collected in the same Te on tube. Then it was centrifuged with an Eppendorf Centrifuge 5804 R for 10 min with 3000 rpm at 14°C. The supernatant was enriched with a Büchi Syncore apparatus for seven hours with 200 rpm at 40°C to 1000 µL. Then 500 µL of the 1000 µL sample were taken for the analysis of volatile organic compounds with higher vapour pressure (HVOCs). The residual 500 µL were enriched to dryness and resolved with 100 µL hexane for the analysis of volatile organic compounds with a lower vapour pressure (LVOCs).  Light-colored calcite in the fragmented drill core.

Supplementary Files
This is a list of supplementary les associated with this preprint. Click to download. 211001FinalNatureGeoscienceSupplementaryInformationOriginofLifeYG.docx