Biosphere
The biosphere is the space on and near Earth's surface that contains and supports living organisms and ecosystems. It is typically subdivided into the lithosphere, atmosphere, and hydrosphere. The lithosphere is the earth's surrounding layer composed of solid soil and rock, the atmosphere is the surrounding gaseous envelope, and the hydrosphere refers to liquid environments such as lakes and oceans, occurring between the lithosphere and atmosphere. The biosphere's creation and continuous evolution result from physical, chemical, and biological processes. To study these processes a multi-disciplinary effort has been employed by scientists from such fields as chemistry, biology, geology, and ecology.
The Austrian geologist Eduard Suess (1831–1914) first used the term biosphere in 1875 to describe the space on Earth that contains life. The concept introduced by Suess had little impact on the scientific community until it was resurrected by the Russian scientist Vladimir Vernadsky (1863–1945) in 1926 in his book, La biosphere. In that work, Vernadsky extensively developed the modern concepts that recognize the interplay between geology, chemistry, and biology in biospheric processes.
For organisms to live, appropriate environmental conditions must exist in terms of temperature, moisture, energy supply, and nutrient availability.
Energy is needed to drive the functions that organisms perform, such as growth, movement, waste removal, and reproduction. Ultimately, this energy is supplied from a source outside the biosphere, in the form of visible radiation received from the Sun. This electromagnetic radiation is captured and stored by plants through the process of photosynthesis. Photosynthesis involves a light-induced, enzymatic reaction between carbon dioxide and water, which produces oxygen and glucose, an organic compound. The glucose is used, through an immense diversity of biochemical reactions, to manufacture the huge range of other organic compounds found in organisms. Potential energy is stored in the chemical bonds of organic molecules and can be released through the process of respiration; this involves enzymatic reactions between organic molecules and oxygen to form carbon dioxide, water, and energy. The growth of organisms is achieved by the accumulation of organic matter, also known as biomass. Plants and some microorganisms are the only organisms that can form organic molecules by photosynthesis. Heterotrophic organisms, including humans, ultimately rely on photosynthetic organisms to supply their energy needs.
The major elements that comprise the chemical building blocks of organisms are carbon, oxygen, nitrogen, phosphorus, sulfur, calcium, and magnesium. Organisms can only acquire these elements if they occur in chemical forms that can be assimilated from the environment; these are termed available nutrients. Nutrients contained in dead organisms and biological wastes are transformed by decomposition into compounds that organisms can reutilize. In addition, organisms can utilize some mineral sources of nutrients. All of the uptake, excretion, and transformation reactions are aspects of nutrient cycling.
The various chemical forms in which carbon occurs can be used to illustrate nutrient cycling. Carbon occurs as the gaseous molecule carbon dioxide, and in the immense diversity of organic compounds that make up living organisms and dead biomass. Gaseous carbon dioxide is transformed to solid organic compounds (simple sugars) by the process of photosynthesis, as mentioned previously. As organisms grow they deplete the atmosphere of carbon dioxide. If this were to continue without carbon dioxide being replenished at the same rate as the consumption, the atmosphere would eventually be depleted of this crucial nutrient. However, carbon dioxide is returned to the atmosphere at about the same rate that it is consumed, as organisms respire their organic molecules, and microorganisms decompose dead biomass, or when wildfire occurs.
During the long history of life on Earth (about 3.8 billion years), organisms have drastically altered the chemical composition of the biosphere. At the same time, the biosphere's chemical composition has influenced which life forms could inhabit its environments. Rates of nutrient transformation have not always been in balance, resulting in changes in the chemical composition of the biosphere. For example, when life first evolved, the atmospheric concentration of carbon dioxide was much greater than today, and there was almost no free oxygen. After the evolution of photosynthesis there was a large decrease in atmospheric carbon dioxide and an increase in oxygen. Much of carbon once present in the atmosphere as carbon dioxide now occurs in fossil fuel deposits and limestone rock.
The increase in atmospheric oxygen concentration had an enormous influence on the evolution of life. It was not until oxygen reached similar concentrations to what occurs today (about 21% by volume) that multicellular organisms were able to evolve. Such organisms require high oxygen concentrations to accommodate their high rate of respiration.
Most research investigating the biosphere is aimed at determining the effects that human activities are having on its environments and ecosystems. Pollution, fertilizer application, changes in land use, fuel consumption, and other human activities affect nutrient cycles and damage functional components of the biosphere, such as the ozone layer that protects organisms from intense exposure to solar ultraviolet radiation, and the greenhouse effect that moderates the surface temperature of the planet.
For example, fertilizer application increases the amounts of nitrogen, phosphorus, and other nutrients that organisms can use for growth. An excess nutrient availability can damage lakes through algal blooms and fish kills. Fuel consumption and land clearing increases the concentration of carbon dioxide in the atmosphere, and may cause global warming by intensifying the planet's greenhouse effect.
Recent interest in long-term, manned space operations has spawned research into the development of artificial biospheres. Extended missions in space require that nutrients be cycled in a volume no larger than a building. The Biosphere-2 project, which received a great deal of popular attention in the early 1990s, has provided insight into the difficulty of managing such small, artificial biospheres. Human civilization is also finding that it is challenging to sustainably manage the much larger biosphere of planet Earth.
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