Origin and Distribution of Water in the Ecosphere: Water and Prehistoric Life

The Eccentric Liquid

Water is the only inorganic liquid that occurs naturally on earth. It is also the only chemical compound that occurs naturally in all three physical states: solid, liquid and vapour. It existed on this planet long before any form of life evolved but, since life developed in water, the properties of the 'Universal Solvent' or 'Life's natural habitat' or 'Life's preferred habitat' came to exert a controlling influence over the many biochemical and physiological processes that are involved in the maintenance and perpetuation of living organisms. It is therefore in order to discuss briefly the occurrence of water on earth, its distribution and its controlling influence on the development of life.

The Hydrologic Cycle

At present we are left with several puzzles concerning the composition of the atmosphere and the quantity of water in the hydrosphere. It seems safe to assume that the presence of water in the liquid state can only date from a time when the temperature of the earth's crust had dropped to below the critical temperature of water, 374 °C. If all the water that now makes up the oceans had previously existed as a supercritical water atmosphere, then the pressure per square metre of earth surface would have been 25 MPa. During cooling to below the critical point, vast masses of water would have condensed onto the earth's surface and also penetrated deep into rock crevices. Some of this water would immediately have boiled off again, to be recondensed at a later time. The hydrologic evaporation–condensation cycle could thus have begun several billion years ago. It is therefore irrelevant whether the heat of the earth itself or solar radiation initiated the cycle. What is relevant, however, is the scale of the water movement. The total water content of the atmosphere is 6 x 108 ha m (1 ha m = 10000 m3. This is the amount of water which will cover an area of 1 ha to a depth of 1 m. Since the total annual precipitation is 225 x 108 ha m, the water in the atmosphere is turned over 37 times every year. This level of precipitation is equivalent to a water depth of 0.5 m averaged over the earth's surface. Such an averaging is of course meaningless, because the level of precipitation is quite non-uniform in time and space.

Although the hydrologic cycle, shown in Figure 1.1, is a continuum, its description usually begins with the oceans which cover 71% of the earth's surface. The heat of the sun causes water to evaporate. Under the influence of certain changes in temperature and/or pressure, the moisture condenses and returns to earth in the form of rain, hail, sleet or snow – collectively referred to as water of meteoric origin. It falls irregularly with respect to geographical location, with coastal areas receiving more.

Of the average rainfall, about 70% evaporates; the remainder appears as liquid water on or below the land surface. Some water evaporates in the air between the clouds and the land surface. The remaining losses are of two forms: direct evaporation from wet surfaces and transpiration through plants from their leaves and stems. The 30% of water not directly returned to the atmosphere constitutes the runoff and provides our potentially available freshwater supply. Actually, the proportion of the earth's total freshwater resources which participates in the hydrologic cycle does not exceed 0.003%; the remainder is locked up in the Antarctic ice cap. If melted, it would supply all the earth's rivers for 850 years.

Enormously large quantities of water participate in the cycle, as shown in Table 1.1. The oceans constitute by far the largest proportion of our water resources, with the Antarctic ice cap as the major freshwater reservoir. By comparison, all the other contributions are of a minor nature.

Available Water and Global Warming

The most important source of readily available, albeit recycled, fresh water is rain, the distribution of which is quite erratic. As a result of rainfall and percolation from the water table to the topsoil, the total volume of moisture in the soil is 25 000 km3. Plants normally grow on what is considered to be 'dry' land, but this is a misnomer, because even desert sand contains up to 15% of water. It appears that plant growth requires extractable water; thus, an ordinary tree withdraws and transpires about 190 1 per day. Groundwater is an increasingly important source of fresh water. Indeed, less than 3% of the earth's available fresh water occurs in streams and lakes, although the proportion is much higher in the UK. Groundwater hydrology is a relatively young, but rapidly growing, branch of science and technology, mainly as a result of the growth of research in botany, agriculture, chemistry, physics and meteorology.

Now that global warming is believed to cause a major future threat, the numbers in Table 1.1 assume a particular significance. If the Antarctic ice cap were to recede and become subject to partial melting, the water so produced would contribute to a rise in the sea level, in addition to any rise caused by thermal expansion of the oceans. Unfortunately such water which is now part of our freshwater resources, albeit locked up, would become useless for immediate utilisation.

Water and the Development of Life

Studies of the origin of water in the universe and the prehistoric changes that may have occurred in the composition of our atmosphere and hydrosphere make fascinating reading. The search for water has become an important aspect of space exploration. The existence of ice in many cold stars and meteorites is now firmly established. It is also believed that, next to hydrogen, oxygenated hydrogen is the most abundant chemical species in outer space. In principle, wherever ice exists in an extraterrestrial cold environment, there should also be evidence of water vapour, since ice has a finite sublimation pressure. Indeed, water vapour has been detected on our moon, on Mars, and moons of Jupiter and Saturn. However, during the past decade noncrystalline water has also been detected in the photosphere of the sun. By comparison with high-temperature emission spectra of very hot water, the infrared lines observed in sunspot spectra have been assigned to characteristic rotation and rotation–vibration transitions involving H2O molecules and OH radicals. At ca. 3000 K the spectra correspond to approximately equal concentrations of molecules and radicals.

For any discussion of life on earth it is very important to establish when, and how, molecular oxygen first made its appearance. Early prokaryotes had minimal requirements of H, C, N, O, S and P, with a further selective management of Na, K, Mg, Ca, Fe, Mo, Se and Cl for energy regulation and electron transport. There is now little doubt that there existed enough H2S to provide the reducing environment needed for the essential reactions with CO2 to synthesise organic molecules. However, the low solubility of H2S did not make it the ideal provider of hydrogen. Water itself was a much better provider of hydride. Eventually, with the aid of manganese as catalyst, living systems found the means of releasing hydride from water, but with a devastating side effect: the release of molecular oxygen which was the enemy of the reductive cell chemistry of primitive life. Eventually living organisms came to terms with oxygen and were able to gain energy from its breakdown:

O2 + C/H/N compounds [right arrow] N2 + CO2 + energy

Different forms of simple organisms thus came to coexist: some remained anaerobic, while others made use of oxygen and became photosynthetic, while yet others became parasitic, using plants and oxygen as energy sources; they ultimately developed into animals. Free oxygen was produced by the splitting of water, first by high energy radiation and later also by photosynthesis. The original earth atmosphere is believed to have been composed of methane, ammonia, carbon dioxide, nitrogen and water vapour; it had a reducing character. It might also have included hydrogen and helium. An alternative view is that the present atmosphere is due more to the degassing of the earth's interior, e.g. by volcanic eruptions. Some planets still possess their 'original' atmospheres; in this context the current Galileo space probe exploration of Jupiter and its atmosphere takes on a special significance. Hopefully, we shall learn something about the origin of water in the solar system, and particularly about the development of our own atmosphere and hydrosphere. Only in this way will it become possible to relate the properties of water to the development of life processes on earth.

Carbon dioxide was first produced through the erosion and decomposition of minerals. In the presence of free oxygen a methane–ammonia atmosphere is unstable, methane being oxidised to water and carbon dioxide. The generation and consumption of oxygen are finely balanced: the production through photosynthesis amounts to ca. 6.8 x 1015 mol a-1, of which > 99% remains in the atmosphere. Of this, 90% is required for oxidative reactions that accompany the weathering and erosion of rocks. Thus only 3 x 109 mol oxygen remain for an enrichment of the atmosphere. By a combination of the above estimates with the known extension of plant and animal life on earth it is possible to sketch the increase of oxygen in the earth's atmosphere to reach its present value of 23% by weight. This is shown in Figure 1.2; the dramatic increase in the rate of oxygen production dates from the later Palaeozoic era, when a rapid growth of plant life took place which was then followed by a corresponding increase in animal life. Nevertheless, even at that time, life had already existed for three billion years.

Aerobic and Anaerobic Life Forms

Life began in hot water and was characterised especially by a rich diversity of algal species. The simplest prokaryotic forms of life can operate and reproduce with a rudimentary genetic machinery that requires no light. The terrestrial species exist at pH 1; they generate energy by oxidative reactions and thus they require not only sulfur, but also atmospheric oxygen. Species that exist in the submarine hot springs, on the other hand, are generally anaerobic. Apart from an absolute requirement for water, they also need variable amounts of CO2, H2, H2S, CO, CH4 and SO2-4. The nearest relatives of the hyperthermophiles are less heat tolerant (ca. 70 °C) and gain their energy by photosynthesis. The line of development then leads to the microfungi and eukaryotes, which, although they exhibit increasing heat sensitivity (50–60 °C), can withstand temperatures that are still lethal for even the most primitive animals. As the earth cooled down, so evolutionary pressures led to the survival and further development of less heat tolerant organisms. We now talk of 'ambient temperatures' of the order of 0–40 °C and have reached the stage in the cooling down process, where scientists are becoming increasingly interested in the study of psychrophily, because cold has already become the most widespread and lethal enemy of life on earth.

Recent years have witnessed a growing interest by microbiologists, molecular biologists and biochemical engineers in extremophilic organisms, mainly directed towards a better understanding of protein stability under extreme conditions of temperature and pressure. The deep-sea trenches in the Pacific Ocean are a rich source of a variety of such microorganisms. Seen from the perspective of these so-called extremophiles, such a label is surely inappropriate. They would regard their habitat as the natural physiological environment. They would therefore classify any form of life that can survive and reproduce in an oxygen-containing atmosphere at 20 °C as a hyperextremophile!

Impact of the Physical Properties of Water on Terrestrial Ecology

The maintenance of life, as we know it, on this planet depends critically on an adequate supply of water of an acceptable quality and also on a well-regulated temperature and humidity environment. The oceans satisfy both these requirements, either directly of indirectly. They constitute our most important reservoirs of water and energy. They exert a profound influence on the terrestrial climate and on the levels of precipitation. A simple calculation serves to illustrate this stabilising effect:

The Gulf Stream is 150 km wide and 0.5 km deep and flows at a rate of 1.5 km-1 from the Gulf of Mexico to the Arctic Ocean, north of Norway. The temperature drop of the water during its passage north is about 20 °C. This temperature drop is equivalent to an energy transfer of about 5 x 1013 kJ km-3, equivalent to the thermal energy generated by the combustion of 7 million tonnes of coal. The above dimensions, coupled with the rate of flow of the current provide for a movement of 100 km3 of water per hour, equivalent therefore to the energy supplied by 175 million tonnes of coal. All the coal mined in the world in one year would be able to supply energy at this rate for only 12 h. The warm ocean currents thus act as vast heat exchangers and are responsible for maintaining a temperate climate over much of the earth's surface. The oceans are able to store such large amounts of energy by virtue of the large (for a substance with a molecular weight 18) heat capacity of water, one of its abnormal physical properties, to be discussed in more detail in several chapters of this book.

Other physical properties of liquid that greatly influence our ecological environment are the low density of ice relative to that of the liquid, and the phenomenon of the negative coefficient of expansion of cold water. Between them, these two properties are responsible for the freezing of water masses from the surface downwards, with obvious implications for the survival of aquatic life.

Structure of the Water Molecule and the Nature of the Hydrogen Bond in Water

The Isolated Water Molecule

Ultimately the macroscopic behaviour of water depends on the details of its molecular structure. Quantum mechanics permits a theoretical analysis of the molecular structure from a knowledge of the masses, charges and spins of the subatomic particles involved. This requires the solution of the Schrödinger equation, the Hamiltonian for the water molecule being given by

H = En + Ee + U(r, R) (2.1)

where the first two terms on the right-hand side represent the kinetic energy of the three nuclei and 10 electrons respectively, and where U(r, R) is the potential-energy function that contains the electrostatic interaction contributions from all pairs of particles, the particle coordinates being given by r and R. Because of the large difference in the masses of the nuclei and the electrons, the first term in equation (2.1) contributes little to the total energy, and thus an approximate, but adequate, solution of the Schrödinger equation can be given by considering the electronic motions in a force field of fixed nuclei; this is known as the Born–Oppenheimer approximation. The solution of equation (2.1) then follows along conventional lines; electronic eigenvalues and eigenfunctions must first be defined. Although there is no exact method for doing this, various approximate treatments are available for estimating the upper limit of the electron energy. It is beyond the scope of this review to discuss molecular orbital calculations for complex molecules (water is a complex molecule in that it is polynuclear). A representation of the total calculated electron density of the ground state of water in the molecular plane is shown in Figure 2.1. This graphically illustrates the asymmetry of the molecule. Although the excited electronic states are of interest for spectroscopy and the elucidation of reaction mechanisms, much less is known about them as regards theoretical predictions.

The solution of the nuclear Schrödinger equation provides information about the internal motions (vibration and rotation) of the molecule, and the theoretical results are consistent with the information provided by infrared spectroscopy. The equilibrium geometry of the isolated H2O molecule indicates that the O–H bond length is 0.0958 nm and the H–O–H angle is 104°27'. The principal vibrations are shown in Figure 2.2. These frequencies are modified in the condensed states, where inter-molecular effects become important.

The calculated electron density distribution surface shown in Figure 2.1 lends support to an earlier, empirical model for the water molecule, the Bjerrum four-point-charge model, which is illustrated in Figure 2.3. The oxygen atom is situated at the centre of a regular tetrahedron with the fractions of charge [+ or -] ηe placed at the vertices of the tetrahedron at distances 0.1 nm from the centre. The van der Waals diameter (d) assigned to this molecule is 0.282 nm, identical with that of neon (water and neon are isoelectronic). According to this representation the vertices carrying positive charge are the positions of the two hydrogen atoms, with the two lone electron pair orbitals carrying the negative charge, directed towards the other two vertices. It is easily seen that if two such molecules are allowed to approach one another, their interaction would have the characteristics of what has become known as the hydrogen bond, owing to electrostatic interactions between the charges.