Describe how certain organisms contributed to the dramatic change in oxygen content of the atmosphere?
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Dude plants and algea, planton converting carbon dioxide to oxogen in photosythesis. Plus those weird bacteria in the ocean that can do it without light in darkness.
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HISTORY OF OXYGEN\n\nIt's difficult to think of free oxygen, which makes up 21 percent of earth's atmosphere, as a poisonous gas. Virtually all organisms alive today need oxygen to survive. Even plants, which expel oxygen into the atmosphere, need oxygen to live. There was a time early in earth's history, however, when living organisms not only did not need oxygen, but were killed when exposed to the element. \n\nThree and a half billion years ago, Earth's atmosphere contained almost no free oxygen. Instead, it consisted mainly of carbon dioxide, perhaps as much as 100 times more carbon dioxide than contained in today's atmosphere. During this time, Earth's only life forms were aquatic, one-celled organisms -- primitive forms of bacteria -- that extracted energy from a variety of sources. To most of these organisms, free oxygen was poisonous. \n\nAt about the same time, new types of primitive bacteria evolved. Unlike their predecessors, these bacteria could convert energy directly from the Sun, through photosynthesis. One type, bluish-green organisms called cyanobacteria, used sunlight to convert carbon dioxide and water into glucose (sugar). These organisms expelled oxygen gas -- a by-product of photosynthesis -- into the oceans. \n\nAs the cyanobacteria multiplied and spread around the globe, they released more and more free oxygen into the oceans. This oxygen combined with the molecules of living bacteria, killing them off as it did so. It also combined with other matter, such as the iron dissolved in the ocean. Only after the oxygen combined with all of the matter available did free oxygen begin to accumulate in the oceans. And when the oceans became saturated with oxygen, it began to make its way into the atmosphere. \n\nOrganisms that don't need free oxygen to survive are called anaerobic; all living things that need free oxygen, including us, are aerobic organisms. As the oceans and atmosphere filled with oxygen gas, aerobic life evolved further, at some point adding a nucleus to the cell (to protect the cell's DNA from the harmful effects of oxygen) and eventually to multi-cellular organisms. Anaerobic life, on the other hand, was pushed back to areas where there was little or no oxygen. Today, anaerobic organisms like those that existed more than three and a half billion years ago live in oxygen-free places such as volcanic hot vents, where they get their energy in ways other than from carbon dioxide, water, and sunlight.\n\nIMPORTANCE OF OXYGEN\n\nCirculatory System, or cardiovascular system, in humans, the combined function of the heart, blood, and blood vessels to transport oxygen and nutrients to organs and tissues throughout the body and carry away waste products. Among its vital functions, the circulatory system increases the flow of blood to meet increased energy demands during exercise and regulates body temperature. In addition, when foreign substances or organisms invade the body, the circulatory system swiftly conveys disease-fighting elements of the immune system, such as white blood cells and antibodies, to regions under attack. Also, in the case of injury or bleeding, the circulatory system sends clotting cells and proteins to the affected site, which quickly stop bleeding and promote healing.\n\nRespiratory System, in anatomy and physiology, organs that deliver oxygen to the circulatory system for transport to all body cells. Oxygen is essential for cells, which use this vital substance to liberate the energy needed for cellular activities. In addition to supplying oxygen, the respiratory system aids in removing of carbon dioxide, preventing the lethal buildup of this waste product in body tissues. Day-in and day-out, without the prompt of conscious thought, the respiratory system carries out its life-sustaining activities. If the respiratory system’s tasks are interrupted for more than a few minutes, serious, irreversible damage to tissues occurs, followed by the failu
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imbaord: the only thing sadder than thinking that the notion of the human race being a virus is deep is to steal that notion from the Matrix and pass it off as your own.  Get an original and meaningful thought, please.  Humans are not destroying the world, they're changing it.  Did you know that before the evolution of oxygen-producing cyanobacteria, there was no oxygen in the atmosphere?  Oxygen was destructive.  Reactive metals like iron used to exist everywhere in their reduced forms, but the new surge of oxygen destroyed those metals are turned them all to rust.  Metals precipitated out of the water, turning into a non-biodegradable form of crud that layered all the ocean floors.  The oxygen was destructive to virtually every form of life that existed before that time, and an enormous percentage of all organisms were made extinct.  BUT! Evolution moved on from there, and new organisms that can use oxygen for energy flourished.  The oxygen became such an abundant part of the atmosphere that it rose upwards, was changed by the incident UV light, and became a screen that sheilded the surface from these UV rays.  These two effects made life on land as we know it suddenly possible when before it was not.  Change, even destructive change, isn't necessarily a terrible thing--it's all a matter of perspective.  Evolution is a powerful thing.\n\nViruses are like living things in many ways, but they are not living things, and I don't care about 10 points, but the simple fact is that the virus is non-living.
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My thought is air as well. Air is composed of around 2/3s part of Nitrogen. Oxygen is a much smaller amount
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According to current theory, 4 billion years ago (4 bya) the atmosphere had no free oxygen.\n\nAbout 3 bya, cyanobacteria evolved, which produce oxygen as a waste product of their photosynthesis.  This oxygen was poison for many forms of life, resulting is mass extinctions.\n\n2.5 bya, there was enough free oxygen in the atmosphere for organisms with modern oxygen based metabolisms to evolve.\n\nSo, sometime between 3 bya and 2.5 bya, fire became possible.
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It's depends on the availability of oxygen.  In the presence of O2, bacteria will convert most of the organic material to carbon dioxide gas (CO2).  In the absence of O2, they'll mostly produce methane gas (CH4).
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When oxygen combines with combustible material, such as carbon, a great deal of energy is released.  You and I get our energy from these reactions (and life depends on energy).  However, not all life on earth uses oxygen for energy.  Anaerobic bacteria, for instance; the name means "no air", and oxygen kills them.\n\nAlmost all the oxygen in our atmosphere was originally released by living things, and it was poisonous to the first life!  One of the most remarkable things about using oxygen is that Life adapted to what was originally a deadly poison, until it became a necessity.
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Bacteria are involved in decomposition, fixing Nitrates into the soil and digesting plant matter in the guts of herbivorous animals, for starters. Without this crops wouldn't grow, animals couldn't eat them and dead bodies wouldn't break down.
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Many people have different theories on this one.  Here are a few of the more popular ones:\n\n1) A large meteor hit the earth, causing ash and debris to cloud the atmosphere for years.  This made it impossible for sunlight to penetrate the atmosphere, and all plant life dies.  Herbivours then died, due to a lack of food.  Soon after, carnivours died due to a lack of food (the herbivours.\n\n2) A climate shift caused the earth to raise its temperature by a few degrees.  This caused the above devistation (beginning with plants) by heat, instead of lack of sunlight).\n\n3) Disease.\n\n4) Large dinosaurs creating large amounts of methane (think herds of cows) caused global warming.  See #2 for result.\n\n#1 is the most believed-in theory, but that status changes from year to year.
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Since the first guy did not even bother to read your question...\n\nYea, you've hit one of the big mysteries in Science - the early atmosphere. Lots of people are doing research in this area.  I just read a great book that talks about it: "Genesis: The Scientific Qest for the origins of life." Great book - check it out.
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I think you might be referring to an ice age that occurred in the Proterozoic leading to the pre-Cambrian explosion?  There was also some theory's about volcanoes and the creation of the atmosphere as we know it.  When photosynthetic life evolved they respired O2 which not many of the other (anaerobic)organisms could breath, and over millions of years plants killed the animals.  (hows that for a mind loop?) an animals had to start all over again leading to the pre-Cambrian explosion.  It was an evolutionary explosion because there so many niches left vacant by the anaerobic organisms. so they diversified, only the strongest survived and eventually achieved a level of equilibrium (so to speak). Or another possibility is that i am indeed full of crap.
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Hi there,\nNo plant or for that matter, no organism on earth can work like that 24hrs to produce oxygen. Actually no plant has any "commitment" to produce oxygen at all. Oxygen is actually a byproduct of the mechanism called photosynthesis. Photosynthesis is how plants and other organisms that have the green pigment Chlorophyll, produce their food. Oxygen is relesed as a byproduct in this process consisting of a series of reactions. All plants work like this during to produce food (carbohydrates, proteins, etc) during day time just because they can utilise the sun's light energy as much as possible. But once night falls, these plants stop this process of "food making" and go to "sleep". During thiese hours of night, they respire just like  any other animal. They take in Oxygen and give out CO2. The Peepal tree is no exception. And if they do try working 24x7 as INdian MYths say, they will soon be exhausted, because the energy required for photosynthetic processes is supplied by the process of "respiration". \nPeepal got its false fame in Hindu literature just because it is considered a holy tree- a tree of enlightenment since ancient times.
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Ultraviolet radiation exposure, photosynthesis and the gaseous composition of elements in the atmosphere.
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The discovery of oxygen
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in biology, the exchange of carbon between living organisms and the nonliving environment. Inorganic carbon dioxide in the atmosphere is converted by plants into simple carbohydrates, which are then used to produce more complex substances. Animals eat the plants and are then eaten by other animals. When these life forms die, they decay, breaking down into, among many other things, carbon dioxide, which returns to the atmosphere. Plants and animals also release carbon dioxide during respiration. Animals and some microorganisms require the carbon-containing substances from plants in order to produce energy and as a source of materials for many of their own biochemical reactions; this cycle is vital to them. The process of incorporating carbon dioxide into the molecules of living matter is called fixation. Nearly all carbon dioxide fixation is accomplished by means of photosynthesis, in which green plants form carbohydrates from carbon dioxide and water, using the energy of sunlight to drive the chemical reactions involved. Green plants use carbohydrates to build the other organic molecules that make up their cells, such as cellulose, fats, proteins, and nucleic acids. Some of these compounds require the incorporation of nitrogen (see nitrogen cycle). When carbohydrates are oxidized in cells they release the energy stored in their chemical bonds, and some of that energy is also used by the cell to drive other reactions. In the process of oxidation, or respiration, oxygen from the atmosphere (or from water) is combined with portions of the carbohydrate molecule, producing carbon dioxide and water, the compounds from which the carbohydrates were originally formed. However, not all of the carbon atoms incorporated by the plant can be returned to the atmosphere by its own respiration; some remain fixed in the organic materials that make up its cells. When the plant dies, its tissues are consumed by bacteria and other microorganisms, a process called decay. These microorganisms break down the organic molecules of the plant and use them for their own cell-building and energy needs; by their respiration more of the carbon is returned to the atmosphere. The carbon-containing molecules that an animal derives from consuming other organisms are reorganized to build its own cells or oxidized for energy by respiration, releasing carbon dioxide and water. When the animal dies it too is decayed by microorganisms, resulting in the return of more carbon to the atmosphere. Carbon-containing molecules in wood (or other dry, slow-decaying organic materials) may be oxidized by burning, or combustion, also producing carbon dioxide and water. Under conditions prevailing on earth at certain times, green plants have decayed only partially and have been transformed into fossil fuels–coal, peat, and oil. These materials are made of organic compounds formed by the plants; when burned, they too restore carbon dioxide to the atmosphere.
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Nitrogen is the most abundent element in our atmosphere.  Bacteria gets N2 from the air.
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C and O2 is turned into CO2 by breathing\nCO2 is made into C (food) and O2 (byproduct) by photosynthesis in plants.