Under the Microscope/Prof William Reville: The only planet in our solar system with a huge body of water in direct contact with its atmosphere is our Earth. Also, Earth is the only body in our solar system whose surface temperature allows water to exist in all three states - solid, liquid and gas.
The various oceans on Earth really comprise one single ocean because they are connected to each other and the various continental land-masses sit in this ocean as islands. The ocean is expertly described in Asimov's Guide to Science (Penguin, 1993).
This salty ocean covers 71 per cent of the earth's surface. Ocean volume is about 1,370 million cubic kilometres (326 million cubic miles) which represents about 97.2 per cent of all the water on Earth. The ocean is also the source of Earth's fresh water because 80,000 cubic miles of it evaporate annually as fresh water to fall again as rain or snow. Because it sits on the Earth's surface, the ocean constitutes only a small fraction (1/4,000th) of the mass of Earth.
The ocean is pre-eminently important for life on Earth. Life arose on Earth almost four billion years ago in the ocean. As a direct legacy of this origin, about 75 per cent of the mass of biological organisms is salty water. Also, more than 90 per cent of the mass of life on Earth resides in the ocean. More than 3.5 billion, over half the world's 6 billion population, depend on the ocean for their primary source of food, but over 70 per cent of the world's marine fisheries are now fished up to or beyond their sustainable limits.
Ocean surface water moves in regular ways called currents. Because the earth spins on its axis, the ocean water moves in clockwise circles in the northern hemisphere and in anti- clockwise circles in the southern hemisphere. The warm Gulf Stream is the western part of the clockwise current circle in the North Atlantic. The gulf stream is about 50 miles wide at its origin in the Gulf of Mexico, half a mile deep and moves at up to four miles per hour. The ocean currents tend to even out ocean and continental coast temperatures.
In addition to surface circulation, ocean water also moves vertically. In this way, surface oxygen-rich water sinks to support life in the deep and deep water, nutrient-rich from decomposed bodies of sunken dead organisms, rises to replenish nutrients in surface water.
Water movement is driven by temperature changes. Surface water is cooled in the polar regions, becomes denser, sinks and spreads over the ocean floor. Eventually this cold bottom water rises to the surface again to make room for more sinking water, warms and drifts poleward to cool and sink again.
Society needs a continuous supply of fresh water for drinking, washing, for agriculture and for industry. The average fresh water use per person annually is about 60,000 cubic feet. Only 2.8 per cent of water on Earth is fresh and three quarters of this is locked away in the polar ice caps. Annual rainfall amounts to four million cubic feet of fresh water per person, which is 75 times the amount used by humanity. However, much fresh water falls on ocean or as snow on ice caps. Also, some of the rain that falls on land runs off to sea in rivers and is not used by humans. On the other hand, the human population is steadily increasing and polluting the existing fresh water supplies. Fresh water will eventually become a scarce commodity.
Ocean depth is measured by echo-sounding. Basically a pulse of ultrasonic (above audible frequency) sound wave is directed from a ship towards the ocean bottom. The wave is reflected by the bottom and the length of time between sending the signal and the return of its echo is a measure of the depth of the bottom. The average depth of the Atlantic Ocean is 3,600 metres and the average depth of the Pacific Ocean is 4,200 metres. These average figures disguise the fact that the ocean bottom contains higher mountains and deeper canyons than any on land. The island of Hawaii is on top of a 11,000-metre-high underwater mountain, taller from base to summit than Mount Everest. There are also deep ocean trenches that dwarf the Grand Canyon. In the Pacific Ocean, the Mindanao Trench is 11,524 metres deep.
Life exists in the ocean at all depths, including the deepest reaches of the abyss. Life is richest nearer the surface of the ocean and declines with depth. The density of life two miles deep is 10 times the density at four and a half miles and lower.
The main food chain in the ocean rests on photosynthetic plants that live near the surface and use the energy of sunlight to make food. Most life in the ocean depends on the sun. However, on the deep ocean floor, food chains that do not depend on the sun can exist near hot spots. At these points the hot molten magma of the Earth's interior rises close to the crust and heats the water above. Hot smoky mud squirts upwards, enriching the sea with minerals, particularly sulphur.
Bacteria live here that have evolved to ability to extract energy from chemical reactions that depend on sulphur and heat. Small animals feed on these bacteria and larger animals (worms, crabs, clams) feed on the small animals. This chain of life is independent of the plant life in the surface layer of the sea.
William Reville is associate professor of biochemistry and director of microscopy at University College Cork