THE OXYGEN CYCLE allows for the regeneration of freely available diatomic oxygen (O2) in the atmosphere. Oxygen accounts by volume for approximately 21 percent of the atmosphere, is reactive with myriad inorganic and organic substances, and is vital to living organisms for aerobic respiration and energy production. The cycle involves any source of oxygen within the world, and is not limited to the oxygen animals must breathe to sustain life; any compound containing an atom of oxygen is considered part of the oxygen cycle. Furthermore, the cycle is composed of many distinct biological and geological chemical reactions that together allow oxygen initially consumed and lost from the atmosphere to be released back into the atmosphere.
These reactions take place among the three different primary reservoirs, or storage areas of all of Earth's oxygen. These storage areas are varied and differ in physical and chemical form. The lithosphere, which contains the vast majority of the Earth's total oxygen, comprises the entirety of the Earth's crust and the uppermost portion of the mantle (tectonic plates can be viewed as lithospheric plates); in this reservoir, oxygen is bound in the form of rocks and minerals, primarily in silica (SiO2) and alumina (Al2O3). The second reservoir is the biosphere, in which all living matter resides, including bacteria, plant life, animals, and human beings. The oxygen bound in this reservoir is found in the macromol-ecules of life, including nucleic acids, carbohydrates, proteins, and water. The last oxygen reservoir is the atmosphere, which is composed of approximately 20.95 percent oxygen gas, .038 percent carbon dioxide (CO2), and water vapor (H2O).
All of the reactions that drive the oxygen cycle occur between any two different reservoirs of oxygen. However, two chief reactions account for most of the activity in the use and regeneration of oxygen on Earth; these are cellular respiration and photosynthesis, both of which occur between the atmospheric reservoir and the biospheric reservoir. Other notable reactions contributing to the cycle include the commonplace reaction between the atmosphere's free oxygen and the lithosphere in the form of the oxidation of minerals and carbon dioxide emissions (for example, from volcanic eruptions). The biosphere and lithosphere interact in the oxygen cycle through weathering and absorbed soil nutrients for organisms and deposition of organism shells and bones into the lithosphere.
The primary action of the oxygen cycle occurs through photosynthesis and cellular respiration. Together, they represent the ability of diatomic oxygen gas to be used and produced by living organisms from the oxygen present in the air. Cellular respiration is the process by which an organism consuming food generates energy to sustain life. Most organisms use oxygen to maximize the amount of energy the food can yield, using oxygen as an electron acceptor in the electron transport chain, and using oxidative phosphorylation. Photosynthesis involves combining atmospheric carbon dioxide with water to generate carbohydrate sources and oxygen gas. The reactions are complementary:
Cellular Respiration: C6H12O2 + O2 -> CO2 + H2O and
Photosynthesis: CO2 + H2O -> C6H12O2 + O2
There are other reactions involving oxygen that are vital for life that occur within a single reservoir. The ozone-oxygen reactions allow for stratospheric ozone to absorb ultraviolet light radiated from the sun, in addition to the visible light photosynthesis requires, which can cause damage to DNA, and break other important chemical bonds. The net reaction for the generation and breakdown of ozone by the absorption of ultraviolet light is:
O3 (ozone) + UV Light Energy -> O2 + O (reactive radical)
O (reactive radical) + O2 (atmospheric oxygen gas) -> O3 (ozone)
Other important reactions include the dissolving of oxygen in bodies of water, which allows for aquatic life to undergo aerobic cellular respiration and the evaporation of different bodies of water.
The oxygen cycle is not an isolated system; rather, it has intimate links with many other geological and biological cycles, such as the carbon cycle and the hydrologic cycle. Additionally, other smaller cycling systems, such as the nitrogen and sulfur cycles, require oxygen from the atmosphere, living organisms, or the ground to make their vital nutrients available for uptake and binding to useful substrates. For example, the oxidation of sulfur creates sulfur dioxide, which may then react with water vapor to form sulfuric acid, which can delivered to, and taken up by, plant roots; the sulfur can be used to make cysteine, an important structural amino acid.
Current scientific theories and opinions indicate that the oxygen cycle itself has not always been native to the Earth. In the far geological past, when the Earth was first cooling and forming, the atmosphere contained virtually no free oxygen and was primarily composed of hydrogen and helium gas. As conditions changed, a second type of atmosphere formed, composed of volcanic emissions, still largely devoid of diatomic oxygen gas. While life first developed on Earth, oxygen was released as a waste product from the most ancient forms of life, cyanobacteria. The oxygen cycle, thus, could not have been native to the Earth and must have developed as the Earth matured, because oxygen seemingly was not present. An interesting side note is that if the regeneration of free atmospheric oxygen stopped by cessation of photosynthesis and other oxygen cycle reactions, current estimates indicate that at current rates of oxygen consumption, it would take approximately 5,000 years for the Earth to deplete the atmosphere.
SEE ALSO: Atmospheric Boundary Layer; Atmospheric Component of Models; Atmospheric Composition.
BIBLIOGRAPHY. Rebecca Harman, Carbon-Oxygen and Nitrogen Cycles: Respiration, Photosynthesis, and Decomposition (Heinemann, 2005); Hans Lambers and Miquel Ribas-Carbo, eds., Plant Respiration: From Cell to Ecosystem (Advances in Photosynthesis and Respiration) (Springer, 2005).
John Byun Harvard University
THE PACIFIC OCEAN—named the "peaceful sea" by Ferdinand Magellan, a Portuguese explorer leading a Spanish expedition—is the largest ocean in the world, covering 65.3 million sq. mi. (169.2 million sq. km.), encompassing 32 percent of the total surface of the Earth, and holding 46 percent of the Earth's water. Altogether, there are 25,000 islands in the Pacific, the vast majority south of the equator, which bisects the ocean.
Global warming and climate change pose many real threats to the Pacific Ocean. The major focus of much attention around the world has been on the rising water levels, which is likely to inundate many of the low-lying Pacific Islands. Independent countries such as Fiji, Kiribati, the Federated States of Micronesia, Nauru, Palau, Samoa, and Tuvalu risk losing the vast majority of their land if the rising world temperature continues to raise the water level of the ocean. Atolls in French Polynesia and in Wallis and Futuna are also under threat. In addition to those places, all the countries in the Pacific have an increased risk of flooding, which could lead to permanent soil loss, as well as an increased risk of the prevalence of insect-borne diseases such as malaria and dengue fever as mosquitoes find further breeding grounds. The rising sea levels also threaten mangrove swamps in many areas, including off the northeastern coast of Australia, and in many Pacific Islands, with 13 percent of the world's mangrove swamps at risk of being lost.
For this reason, many of the countries in the Pacific have been at the forefront of urging countries around the word to embrace the Kyoto Protocol and limit carbon dioxide emissions. The Republic of Nauru, the country with the highest per capita rate of carbon dioxide emissions in the Pacific, went as far as adding a long addenda to the Kyoto Protocol, arguing that it did not feel that the protocol went far enough. Two U.S. territories in the Pacific, Guam and American Samoa, have considerable carbon dioxide emissions. The Solomon Islands, Papua New Guinea, and Vanuatu have, respectively, the lowest rates of carbon dioxide emissions in the Pacific, at rates similar to that of many African countries.
Other problems in the Pacific Ocean regarding global warming focus on the marine flora and fauna. The area most dramatically affected has been the bleaching of coral reefs around the Pacific, with studies by the International Ocean Institute of the University
of the South Pacific in Fiji conducting surveys of coral reefs in the southwest Pacific as part of the International Coral Reef Initiative. In many cases, the damage to coral reefs has come from overpopulation, and through overexploitation through tourism, but even many reefs located in remote parts of the Pacific have experienced bleaching, showing that the damage can be ascribed as much to global warming as to other problems.
As well as coral reefs, there have been significant changes to the marine life, especially the fish in the Pacific. The most dramatic changes have been the reduction in the diversity of fish shoals, as well as the decline in the number of fish, the latter probably as much from overfishing as from global warming. However, there still remain large numbers of tuna fish and also some cluepoids in the central part of the Pacific Ocean, as well as sardines and jack mackerel along the coast of Chile, anchovy off the coast of Ecuador and Peru, mackerel and Saury off the Pacific coasts of Mexico and the United States, and sardine and salmon off the Pacific coast of Canada.
Some 4 percent of the ozone in the Earth's stratosphere is lost each decade, and a hole has appeared over Antarctica, leading to a higher risk of skin cancer from ultraviolet light in places such as Australia, New Zealand, Chile, and southern Argentina. Although there has been a great focus on their effects on humans, the ultraviolet rays have also been linked to the reduction in the plankton population in the southern part of the Pacific Ocean. The removal of much of the plankton has major effects on the food chain throughout the Pacific, especially on the whale population, which has been growing following a moratorium on commercial whaling in 1986, although Japan continues whaling for ostensibly "scientific" reasons.
One last major area of problems in the Pacific Ocean through global warming and climate change has been changes in the ocean currents, which have been caused by the rise in the temperature of the water. Although few Polynesians travel long distances in traditional canoes, as they did about 1,000 years ago during the populating of many of the islands, the currents are very important, not just for shipping, but also for the movement of marine life such as shoals of fish. The warmer temperature and changes in the current seem to have had major effects on the spawning process of some fish species, and this may be responsible for a decline in the population of certain fish.
Although there is a serious worry about global warming and its effects on the Pacific Ocean, one report in 1997 by scientists from the Lamont-Doherty Earth Observatory at Columbia University claims that the vast size of the Pacific Ocean has led to the dissipation of many of the effects of global warming and climate change, and might account for the fact that the world's temperature has only risen half the level of that in some projections.
SEE ALSO: Alliance of Small Island States (AOIS); Floods; Marine Mammals; Oceanic Changes; Sea Level, Rising.
BIBLIOGRAPHY. Hodaka Kawahata and Yoshio Awaya, Global Climate Change and Response of Carbon Cycle in the Equatorial Pacific and Indian Oceans and Adjacent Land Masses (Elsevier, 2006); John Morrison, Paul Ger-aghty, and Linda Crowl, eds., Science of Pacific Island Peoples (Institute of Pacific Studies, 1994); Eric Shibuya, "Roaring Mice Against the Tide: The South Pacific Islands and Agenda-Building on Global Warming," Pacific Affairs (v.69/4, 1996-97); Hans von Storch and Ann Smallegauge, The Phase of the 30- to 60-Day Oscillation and the Genesis of Tropical Cyclones in the Western Pacific (Max-PlanckInstitut für Meteorologie, 1991); Clive Wilkinson, ed., Status of Coral Reefs of the World (Australian Institute of Marine Science, 2000).
JUSTIN CORFIELD Geelong Grammar School, Australia
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