Water as a Resource
Water as a Resource
The central role played by water in the origin of life has long been acknowledged. As early as the seventeenth century, geologists recognized the extent to which water shaped and reshaped the landforms on the earth's surface. More recently, it has been argued that water plays a fundamental role in the formation of stars (Nisini, "Water's Role in Making Stars"). According to this account of the process of star formation, water may be present in two of its physical states, ice (a solid), and water vapor (a gas) in the earliest stages of the birth of a new star. During this phase, ice on the surface of dust particles contributes to the cooling of circumstellar gases by removing excess energy released during protostellar collapse. At the same time, water vapor serves as a reservoir of oxygen in the warmer sectors of accumulating mass. Certain of the peculiar physical and chemical properties of water to be discussed later make water uniquely qualified to participate in this seminal cosmological event.
Since stars form as the result of the gravitational collapse of dense molecular clouds, almost all of the water present in its earliest stages is found in the ice coats of dust grains. A small amount, however, is produced as vapor through a series of reactions beginning with H3+ and O and resulting in the formation of H2OandO2. As star formation proceeds and gravitational collapse accelerates, increasing amounts of radiation energy are released. Consequently, the ambient temperature rises dramatically, and the temperature of the dust grains attains a level where water is released by evaporation. As the temperature continues to rise, all available atomic oxygen is rapidly transformed to water by the following reactions:
As the process of star formation moves into its later stages, these equations are reversed with the release of OH and atomic oxygen. However, some of the water produced is deposited on dust grains in the form of ice, where it may remain while planets are formed. Thus, newly formed planets have continued access to water during their evolution. Within our own solar system, there is considerable evidence that liquid water was abundant at some earlier stage in the history of Mars. The erosion patterns still visible on the planet's surface bear a striking resemblance to some found on earth. Many astronomers are also convinced that liquid water is still present on Saturn's moon, Titan, and under a deep layer of ice on Jupiter's moon, Europa.
The Reshaping of Landforms
Geologists have long been aware of the role water plays in the reshaping of the earth's surface. The uniformitarian theory of the earth's history explicated by Charles Lyell in the early nineteenth century recognized that the surface of the earth is essentially a chronicle of its reshaping by water erosion. Some of the most informative strata for the reconstruction of past events are those that contain sedimentary deposits. These deposits, formed in lake beds and ocean floors, often contain fossils of species extant at a specific time, and can therefore be used, along with chronometric methods, to estimate the age of a given deposit and its contemporaneity with other deposits bearing similar fossils.
The formation of sedimentary deposits provides one of the bases for determination of relative dating sequences through the comparison of fossil inclusions. However, it is the gradual wearing down of mountains and other landforms by rain and snowfall and the transport of particles of rock and soil to the sea that are continually altering earth's topography. Water erosion, combined with tectonic forces at work beneath the earth's crust, is ultimately responsible for environmental changes that occasionally trigger major evolutionary events. In some areas, such as the Grand Canyon of Arizona, long sequences of the earth's history can be reconstructed through examination of strata exposed by the action of Colorado River water.
When major geological events such as the collision of tectonic plates occur, marine deposits sometimes end up in mountains, while other segments of the sea floor may be returned to the magma layer in zones of subduction. Mountain building may have profound effects on rainfall patterns. Under the appropriate circumstances, alterations in atmospheric conditions associated with major events such as the collision of the plates that joined the Indian subcontinent to the Asian landmass have the potential to trigger significant climatic changes. Changes in rainfall patterns, glaciations, and altered temperature gradients may be implicated in the onset of epochal events such as the Pleistocene "Ice Age," with worldwide effects on the evolution of plant and animal species.
Water and Weather
The possibility of a worldwide change in climate associated with the rapid building of a major mountain range arises from the central role that the oceans of the earth play in the distribution of heat energy received from the Sun. Approximately 70 percent of the earth's surface is covered by water. Because of the tilt in the earth's axis, the waters of the equatorial zone receive the Sun's radiation more directly than areas in the higher latitudes. Consequently, more of the wavelengths of solar radiation that are subject to reflection, scatter, or absorption in the atmosphere actually reach the earth's surface in the tropics. Much of their energy serves to warm the oceans. The energy so received sets up warm ocean currents that flow away from the equatorial zone. Ultimately, the warm currents flowing out of the tropics mix with colder currents originating in the polar regions, but not before heat energy has been widely distributed over the oceans and landmasses of the temperate zones.
The warming of the oceans in the equatorial zone has the additional effect of evaporating ocean water. Thus, water vapor enters the atmosphere. In the tropics, some of the moisture-bearing warmed air rises until it is sufficiently cooled to form precipitation. The result is the return of fresh water to the surface. The circulation of moisture-bearing atmosphere through the action of thermal gradients and prevailing winds allows some of it to flow to the cooler, drier areas at higher latitudes as well. Thus, the supply of fresh water is continuously replenished on the continental landmasses of both the Northern and Southern Hemispheres. The large number of plant and animal species that require access to fresh water for survival is evidence that the distribution of fresh water through the atmosphere has played a major role in the evolutionary process.
The mixing of ocean waters and the direction of currents produce a complex pattern that is influenced by factors beyond the dissipation of heat energy absorbed in the tropics. The salinity of seawater varies from one area to another according to the amount and composition of the silt emptied into the oceans by rivers and streams. Since very salty water is heavier than fresh water, streams of water flowing away from the mouths of rivers tend to sink. This tendency sets up a network of crosscutting currents that affect the temperature of water at various depths. Temperature and salinity both affect the mix of marine organisms present in the ocean biome (ecological community). Generally, estuaries are some of the richest and most diverse of the marine habitats. Exploitation of fisheries such as the one that formerly produced abundant catches of sardines in the Mediterranean Sea off the mouth of the Nile River provided local human populations with an important source of dietary protein for many millennia. Other areas, such as the zone of nutrient upwelling off the west coast of South America, are illustrative of the way in which the combination of geological factors and ocean currents can create a major source of food for human populations that have learned how to exploit them.
Although the vast majority of the earth's water is too salty to drink for many animals, including humans, evaporation and precipitation provide a constantly renewing source of fresh water. In addition, the atmospheric recirculation of water modifies the climate in the higher latitudes to such an extent that life can exist in areas that would otherwise be uninhabitable. Moreover, along with rainfall, atmospheric circulation plays an important role in the carbon cycle and nitrogen fixation, both essential to the maintenance of life.
The Physical and Chemical Properties of Water
Through its role in the formation of stars, the reshaping of the earth's surface, the absorption and release of solar energy, and the provision of marine and aquatic habitats, water is essential for the existence of life as we know it. However, as important as each of these factors has been in shaping the evolutionary pathways that produced contemporary species, the physical and chemical properties of water have resulted in a much more intimate relationship between water and the processes of life than the mere provision of an appropriate environment. Water molecules and the molecules of living organisms interact in ways that would make the existence of life, as we know it, inconceivable in the absence of water. Understanding the nature of certain properties of water provides important insights into the nature of life itself, since organisms can quite properly be categorized as aqueous chemical systems.
Water has an unusually high boiling point, melting point, and heat of vaporization. Thus, the transition from solid to liquid and liquid to gaseous phases requires a high energy input. Functionally, this attribute makes water an effective heat absorber. The aforementioned capture and distribution of solar energy by the oceans is one result of this attribute. Moreover, the density of water is greatest when it is in its liquid phase. Therefore, ice floats. The formation of a layer of ice on the surface of a lake or pond serves as insulation for the water beneath it, with the result that aquatic life can continue even through the long, cold winters of the higher latitudes. While it is certainly possible for small bodies of water to freeze solid during prolonged periods of severe cold, most large, deep lakes and rivers sustain a thriving population of fish and other aquatic species through even the most severe winters.
Water molecules have a strong attraction for each other. Therefore, water has surprisingly high surface tension and exhibits the properties of cohesion and adhesion. Its ability to form hydrogen bonds is extraordinary. This ability comes into play in a variety of ways during many biochemical reactions. Water's tendency to adhere to a surface and to attract other molecules underlies the biologically important process of capillary action whereby water has the capability of moving against the force of gravity under certain circumstances. An illustration of this capability is seen in the capillary action that pulls water from soil through the roots and ultimately to the leaves of even the tallest trees.
Water is also a very effective solvent. In fact, it is so effective in the dissolution of other substances that it is only rarely present in the pure, unadulterated state. The solvent properties of water have significance in both the geochemical and biochemical context. The nature and rate of erosion occurring in different kinds of rock will produce changes in the physical geography of the land as well as the chemical nature of silt borne by rivers and streams. The dissociation of mineral compounds can be of special significance when the solute is one of a number of salts that dissociate into ions in water. Ionization of salts converts water into an effective conductor of electricity. Many biological processes, including transmission of nervous impulses and contraction of muscle fibers, depend upon a mild salt solution's ability to conduct an electrical current under controlled conditions.
The explanation for the extraordinary properties of water can be found in its previously mentioned unparalleled ability to form hydrogen bonds. Structurally, an individual water molecule is made up of two hydrogen atoms covalently bonded to one oxygen atom. That is, each hydrogen atom shares a pair of electrons with the oxygen atom. As a result, the molecule has a nonlinear shape and has electrical polarity because of a net positive charge on the hydrogen atoms and a net negative charge on the oxygen atom, creating a dipole moment. Water can serve as both a hydrogen donor and a hydrogen acceptor during hydrogen-bond formation, with each water molecule having the potential to form four hydrogen bonds. The intermolecular attraction inherent in this property accounts for water's high boiling point, melting point, heat of vaporization, and surface tension. The hydrogen bonds between water molecules are relatively weak and are continually broken and reformed with different partners. In liquid water, the average lifetime of the hydrogen bond between two water molecules is only 9.5 × 10-12 seconds (Garrett and Grisham, 1999). As a result, there is dynamism in a water solution that intensifies the diffusion of the products of dissociation. This continuous movement of water molecules enhances its solvent effect. Water also forms hydrogen bonds with a wide range of polar solutes. This permits nonionic but polar molecules such as sugars also to dissolve effectively in water.
Certain nonpolar substances, categorized as hydrophobic, interact with water much differently than the hydrophilic ones discussed up to this point. When such substances are dissolved in water, the surrounding water molecules reorganize to encapsulate the solute molecules in a hydration shell. Hydrophobic molecules trapped in these structures are attracted to each other. The tendency of fats and oils, both hydrophobic in nature, to form closed aggregations when submerged in water arises from this tendency. On the other hand, the tendency for oil to spread out into a thin film when poured on the surface of water reflects its hydrophobic nature in a situation where the lowest free energy state occurs when each molecule has minimum contact with neighboring water molecules. The composition of cell membranes, which characteristically combine a hydrophobic phospholipid bilayer with hydrophilic protein and carbohydrate receptors, provides an example of the manner in which the presence of an aqueous environment can be used to govern the movement into and out of the cell's interior.
Life Requires the Presence of Water
All living organisms require the presence of water to satisfy their metabolic needs. Even in such simple organisms as bacteria, metabolic processes requiring maintenance of solute concentrations inside the cell membrane that differ from those outside are essential to survival. In more complex organisms, the ability of water to absorb and diffuse solutes, create a potential electrical charge, and diffuse heat energy involves a variety of tissues, organs, and organ systems. These systems must communicate and support each other to satisfy the demands created by greater size and differentiation. Each organism, whether simple or complex, is dependent upon the unique properties of water to metabolize, survive, and reproduce. So intimate is the relationship between water and the structure and function of living organisms that the phenomenon of life as found on this planet is inconceivable in the absence of water. There is no concrete evidence of life anywhere else at this time. However, there is good evidence that water is abundant in the universe. Therefore, it is likely that if living organisms are ever found elsewhere, they will share many characteristics with those found here on earth, because of the inextricable relationship between life and water.
See also Biodiversity ; Evolution ; Fish ; Nutrients .
BIBLIOGRAPHY
Brown, James H., and Lomolino, Mark V. Biogeography. 2d ed. Sunderland, Mass.: Sinauer, 1998.
Calvin, Melvin. Chemical Evolution: Molecular Evolution Towards the Origin of Living Systems on the Earth and Elsewhere. New York: Oxford University Press, 1969.
Garrett, R. H., and C. M. Grisham. Biochemistry. 2d ed. Fort Worth, Tex.: Saunders, 1999.
Hochachka, Peter W., and Somero, George N. Biochemical Adaptation: Mechanism and Process in Physiological Evolution. Oxford: Oxford University Press, 2002.
Hunten, Donald M. "Clues to the Martian Atmosphere." Science 294 (2001): 1843–1844.
Kauffmann, Guinevere, and Frank van den Bosch. "The Life Cycle of Galaxies." Scientific American 286, no. 5 (2002): 46–58.
Krasnopolsky, Vladamir, and Paul D. Feldman. "Detection of Molecular Hydrogen in the Atmosphere of Mars." Science. 294 (2001): 1914–1917.
Martini, Frederic. Fundamentals of Anatomy and Physiology. Englewood Cliffs, N.J.: Prentice Hall, 1989.
Nisini, Brunella. "Water's Role in Making Stars." Science 290 (2000): 1513–1514.
Ridley, Mark. Evolution. 2d ed. Cambridge, Mass.: Blackwell Scientific Publications, 1996.
Strickberger, Monroe W. Evolution. 3d ed. Sudbury, Mass.: Jones and Bartlett, 2000.
William A. Stini