Seawater–mantle rocks interactions: a cocktail of molecules suitable for the origin of life?

Four fundamental questions are still arousing human curiosity: the origin of the universe, the origin of life, the occurrence of Mankind and the occurrence of conscience.

The problem of the origin of life can only be tackled indirectly because of the absence of any fossils witnessing the different steps that occurred.

• The oldest way, but still valid, to deal with the issue is to analyse the primitive Earth conditions in order to bring together the various scenarios that could have led to the start of life.
• The second one has been supported since the development of Palaeontology, the theory of the evolution of life and mainly the occurrence of molecular biology. It consists of going back to the past to determine the first living systems (or Last Universal Common Ancestor "LUCA").

It is obvious that these two approaches are complementary and both aim at filling the gap that separates mineral from living.

The origin of life issue is bound to the question of  "what is life ?". Scientists have long been debating on what actually characterises life, its soul, and this debate still raises lots of controversy. However a consensus seems to stand out: life would be "a system made out of biomolecules and able to self replicate and evolve".

Schematically, the main steps to get there could have been:

• pre-biotic conditions enabling the synthesis, so-called abiotic, of organic molecules from mineral matter only. Especially, amino acids, RNA and DNA constituents precursors and lipids that will be at the origin of cells membranes;
• Molecules organisation into self-replicating systems;
• set up of a selectivity process, to be the root of later evolution.

Which primitive molecules have enabled the formation of the initial bio-molecules package involved in a living system ? How deep sea hydrothermal systems could contribute in fuelling the debate ? That is what will be discussed here.

Hydrothermal systems have raised a large interest among the scientists since their discovery in the 70's.

Soon in 1980, Corliss proposed the idea of the likely occurrence of abiotic synthesis in such environments. It was then supported by Holm, Simoneit and Shock between 1992 and 1995.

In 1993 a new type of hydrothermal systems, named ultramafic- hosted, was discovered and relaunched the question of the origin of life. Their specificity is the occurrence of mantle outcroppings that are thus exposed to seawater. As a result, seawater permeates the mantle through cracks, circulates, alters the peridotites that mainly form the mantle and is eventually expelled back to the ocean through chimneys as "hydrothermal fluids".

This process of peridotites alteration is called serpentinisation and produces a large amount of dihydrogen (H₂). This is of great interest as it is a source of energy for further chemical reactions and responsible for the high reducing conditions observed. Furthermore a lot of carbon sources are available down there such as carbon dioxide (CO₂), which means that both fundamental elements (H and C) for the production of organic compounds are available at about 350°C and more than 2000m deep! As such, the high concentration of methane (CH₄) measured in hydrothermal fluids is explained by the reduction of CO₂ by H₂ according to the Fischer-Tropsch reaction. This process is more than likely to be the pathway for generating most of the organics spotted in hydrothermal fluids so far.

The question of stability of pre-biotic molecules in hydrothermal conditions (high pressure and 50 to 200°C) is also essential.

The behaviour of molecules in supercritical (high pressure and temperature) fluids is not well known yet because of a lack of thermodynamic data and the difficulty of generating the same conditions in the lab. In other words, anything is possible and it is difficult to assess that a reaction that doesn't occur in mild conditions will not be favoured in a supercritical fluid. To be noted, many organic molecules have actually been spotted in hydrothermal fluids ! The ultramafic property of some fluids (pH~12 found at Lost City vent field) is also decreasing the stability. However, it has recently been shown that boron, at high pH, stabilises sugars (fundamental building blocks in living organism) for months versus minutes without boron.

 

Finally, even though pre-biotic molecules would be stable/stabilised, the majority of macromolecules typical of the cell organisation are degraded under conditions existing in a pure hydrothermal fluid related to serpentinisation. This does highlight the fact that the "reactor" where pre-biotic molecules would have formed is definitely other than the one where life has developed. However, the wide range of temperature, pressure and fluids compositions at the vent sites could be favourable to the emergence of life especially relying on pre-biotic molecules, which synthesis would be driven by the serpentinisation process.

Huge pressure and temperature, high concentration in H₂ and sulphur, lack of oxygen and light… as unfriendly as hydrothermal environments look, the formation of pre-biotic molecules is more than likely to occur in these conditions. Various groups of organic compounds, such as hydrocarbons, have been indeed detected in hydrothermal fluids. Some could have played a role in the origin of life even ! It remains, now, to figure out where they are from and how they form…