5 Water, the Spring of Life

5 Water, the Spring of Life André Brack Primitive life probably appeared with the first chemical systems able to transfer their molecular information via self-reproduction and also to evolve. By analogy with con- temporary life, it is generally believed that life on Earth emerged in liquid water from the processing of organic molecules. Schematically, the premises of primitive life can be compared to parts of "chemical robots". By chance, some parts self-assembled to generate robots capable of assembling other parts to form identical robots. Sometimes, a minor error in the building generated more efficient robots, which became the dominant species. The number of parts required for the first robots as well as the nature of these chemical robots are still unknown. The traces of the primitive chemical robots have been probably erased for ever on Earth by the combined action of plate tectonics, the permanent presence of liquid water, the unshielded solar ultraviolet radiation and atmospheric oxygen. Despite its harmful ability to erase fossils, liquid water exhibits interesting peculiarities, which contributed to the birth of life on Earth. It is therefore generally considered as a prerequisite for terrestrial-type life to appear on a planet. Some possible contributions of water to the origin of life are presented here. 5.1 Water as a Diffusion Milieu Assembling parts of the primitive molecular robots must have occurred at a reasonable rate. Solid state life is generally discarded, the parts being unable to migrate and to be easily exchanged. A gaseous phase would allow fast diffusion of the parts, but the limited inventory of stable volatile organic molecules would constitute a severe re- striction. A liquid phase, i.e. as liquid water, offers the best environment for the diffusion and the exchange of organic molecules. Other solvents can be considered such as liquid ammonia, hydrogen sulphide, sulphur oxide, as well as hydrocarbons, organic acids or alcohols. Compared to any of these possible solvents, water exhibits an excel- lent stability towards heat and UV radiation degradation. Liquid water is a fleeting substance that can roughly persist above 0 °C and under a pressure higher than 6 mbars (Fig. 5.1). Salts dissolved in water (brines) depress the freezing point. For instance, the 5.5% (by weight) salinity of the Dead Sea depresses the freezing point of sea water by 2.97 °C. Large freezing point depressions are ob- served for 15% LiCl (23.4 °C) and for 22% NaCl (19.2 °C). Monovalent and divalent salts are essential for terrestrial life, because they are required as co-catalysts in many enzymatic activities. Usually, the tolerated salt concentrations are quite low (<0.5%), 80 A. Brack Fig. 5.1 Phase diagram of water (from http://www.sbu.ac.uk/water/phase.html). because high salt concentrations disturb the networks of ionic interactions that shape biopolymers and hold them together. However, salt-loving microorganisms (both eu- caryotes and procaryotes) known as extreme halophiles tolerate a wide range of salt concentrations (1-20%) and some procaryotes have managed to thrive in hypersaline biotopes (salines, salt-lakes) up to 25-30% sodium chloride. The size of a planet and its distance from the star are two basic characteristics that will determine the presence of liquid water. If a planet is too small, like Mercury or the Moon, it will not be able to retain any atmosphere and, therefore, any liquid water. If the planet is too close to the star, the mean temperature rises due to starlight intensity. Any seawater present would evaporate delivering large amounts of water vapor to the atmosphere, thus contributing to the greenhouse effect. Such a positive feedback loop could lead to a runaway greenhouse: all of the surface water would be transferred to the upper atmosphere where photodissociation by ultraviolet light would break the molecules into hydrogen, which would escape into space, and oxygen, which would recombine into the crust. If a planet is far from the star, it may host liquid water providing that it can maintain a constant greenhouse atmosphere. However, water would provoke its own disappearance. The atmospheric greenhouse gas, CO2 for instance, would be dissolved in the oceans and finally trapped as insoluble carbonates by rock- weathering. This negative feedback would lower the surface pressure and the temperature to an extent that water would be largely frozen. A volcanically active planer could recycle the carbon dioxide by breaking down subducted carbonates. The size of the Earth and its distance from the Sun are such that the planet never experienced either a runaway greenhouse or a divergent glaciation. The origin of primitive Earth’s water remains difficult to define. The source of the primitive terrestrial water can be found in a combination of H2O trapped in the rocks that made the bulk of the planet’s mass ("internal reservoir") and a late accreting veneer of extraterrestrial material ("external reservoir"). The formation of the internal reservoir is a natural consequence of the accretion of the planet. A volatile-rich veneer may 5 Water, The Spring of Life 81 have replaced the atmosphere produced by the planet’s early accretion which was sub- sequently blown off by the giant impact between the Earth and a Mars-sized planetary embryo that generated the Moon and caused the early atmosphere to undergo massive hydrodynamic loss. The abundance and isotopic ratios of the noble gases neon, argon, krypton, and xenon suggest that meteorites alone or in combination with planetary rocks could not have produced the Earth’s entire volatile inventory and that a signifi- cant contribution from icy planetesimals was required [1, 2]. The surface temperature of the early Earth was very much dependent upon the partial pressure of CO2 in the atmosphere. The dominant view in recent years has been that the early atmosphere was a weakly reducing mixture of CO2, N2, and H2O, combined with lesser amounts of CO and H2 [3, 4]. An efficient greenhouse effect induced by a high CO2 partial pressure was the most likely mechanism capable of compensating for the faint young Sun, the luminosity of which was about 30% lower than today. The dominant sink for atmospheric CO2 was silicate weathering on land and subsequent formation and deposition of carbonate sediments in the ocean. However, a negative feedback system probably maintained the Earth’s surface temperature above freezing: if the temperature were to drop below freezing, silicate weathering would slow down and volcanic CO2 would accumulate in the atmosphere. Walker [5] suggested that the CO2 partial pressure could have been as high as 10 bars prior to the emergence of the conti- nents because of limited silicate weathering and storage in continental carbonate minerals. Under these conditions, the early Earth could have been as hot as 85 °C. Recent analysis of samarium/neodymium ratios in the ancient zircons indicate that the conti- nents grew rapidly, so a dense CO2 atmosphere may have been restricted to the first few hundred million years of the Earth’s history. 5.2 Water as a Selective Solvent According to its molecular weight, water should be a gas under standard terrestrial conditions by comparison with CO2, SO2, H2S, etc. Its liquid state is due to its ability to form hydrogen bonds. This is not restricted to water molecules, since alcohols ex- hibit a similar behavior. However, the polymeric network of water molecules via H- bonds is so tight that the boiling point of water is raised from 40 °C (temperature in- ferred from the boiling point of the smallest alcohols) to 100 °C. Hydrogen bonds are formed between water molecules and organic molecules, providing that the latter con- tain -OH, -NH, -SH groups in addition to carbon and hydrogen. As a consequence of this affinity, many CHONS organic molecules are soluble in water. Hydrocarbons cannot form hydrogen bonds with water. As a consequence, they are much less soluble in water. In addition to the H-bonding capability, water exhibits a large dipole moment (1.85 debye) as compared to alcohols (<1.70 debye). This large dipole moment favors the dissociation of ionizable groups such as -NH2 and -COOH leading to ionic groups, which can form additional H-bonds with water molecules, thus improving their solu- bility. Water is also an outstanding dielectric (e = 80). When oppositely charged organic groups are formed, their recombination is unfavored, because the attraction force of reassociation is proportional to l/e. This is also true for mineral ions, which have 82 A. Brack probably been associated with organic molecules since the beginning of life’s adven- ture. 5.3 Water as a Clay Producer Clay minerals are formed by water alteration of silicate minerals. As soon as liquid water appeared in the surface of the primitive Earth, clay minerals probably accumu- lated and became suspended in the primitive ocean. The importance of clay mineral in the origins of life was first suggested by Bernal [6]. The advantageous features of clays for Bernal were (i) their ordered arrangement, (ii) their large adsorption capacity, (iii) their shielding against sunlight, (iv) their ability to concentrate organic chemicals, and (v) their ability to serve as polymerization templates. Since the seminal hypothesis of Bernal, many prebiotic scenarios involving clays have been written and many prebiotic experiments have used clays. So far, the most impressive results have been obtained by Ferris at al. [7]. Oligomers up to 55 long were obtained for both nucleotides and amino acids in the presence of montmorillonite for nucleotides and of illite for amino acids. 5.4 Water Structures the Biopolymers Prebiotic organic molecules can be classified into two groups, CH containing molecules (hydrocarbons) and CHONS containing molecules.

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