Biosphere
The biosphere is the space on or near the Earth's surface which contains and supports living organisms. It is typically subdivided into the lithosphere, atmosphere, and hydrosphere. The lithosphere is the Earth's surrounding layer composed of solid substance such as soil and rock, the atmosphere is the surrounding gaseous envelope, and the hydrosphere refers to liquid environments such as lakes and oceans which lie between the lithosphere and atmosphere. The biosphere's creation and continuous existence results from chemical, biological, and physical processes. To study these processes a multi disciplinary effort has been employed by scientists from fields such as chemistry, biology, geology, and ecology.
History
The term biosphere was first used by Austrian geologist Eduard Suess (1831-1914) in 1875 to describe the space on Earth which contains life. The concept introduced by Suess had little impact on the scientific community until it was resurrected by Russian scientist Vladimir Vernadsky (1863-1945) in 1926 with the publication of La biosphere. In this work, Vernadsky extensively developed the modern concepts which recognize the interplay between geology, chemistry, and biology in biospheric processes.
Requirements for life
For organisms to live appropriate environmental conditions (e.g., temperature, moisture, etc.) must exist, and the organisms must be supplied with energy and nutrients. In a biosphere such as the Earth's where no external nutrient supply exists, nutrients contained in dead organisms or waste products from living cells must be transformed back into compounds which living matter can reutilize for the biosphere to continue to support life. Mineral sources of nutrients are also important.
Energy is needed for the functions which organisms perform, such as growth, movement, waste removal and reproduction. It is the only requirements for life which is supplied from a source outside the biosphere. This energy, in the form of light or solar radiation received from the Sun, is captured and stored by plants in a process called photosynthesis. Photosynthesis refers to the light induced chemical reaction between carbon dioxide and water which produces oxygen and large carbon compounds called organic molecules. Energy is stored in the chemical bonds of organic molecules and can be released in the process of respiration; the enzymatic chemical reaction between organic molecules and oxygen to form carbon dioxide,water and energy. Organic molecules may also be used for growth since they are the major component of most tissues. Plants and some microorganisms are the only organisms which can form organic molecules by photosynthesis. Heterotopic organisms like humans rely on plants for their energy needs.
The major elements or chemical building blocks which comprise all living organisms are carbon, oxygen, nitrogen, phosphorus and sulfur. Organisms are able to acquire these elements only if they occur in useable chemical forms termed nutrients. In a process called the nutrient cycle the elements are transformed from one chemical form to another and back to the original form. The different chemical forms in which carbon occurs illustrate this process. Carbon occurs in the gaseous molecule carbon dioxide or in the organic molecules which compose living and dead organisms. Gaseous carbon dioxide is transformed to solid organic compounds (simple sugars) by the process of photosynthesis as mentioned previously. When organisms grow they deplete the atmosphere of carbon dioxide. If this process were to continue without carbon dioxide being resupplied at the same rate that it is consumed eventually
the atmosphere would no longer contain carbon dioxide and organisms could no longer use the energy supplied by the Sun to sustain life. Fortunately, carbon dioxide is returned to the atmosphere at the same rate that it is consumed when organisms respire their own stores of organic molecules, when microorganisms respire tissue from organisms which have died in a process known as decomposition, or when wildfires occur.
Biosphere evolution
During the Earth's long history, life forms have drastically altered the chemical composition of the biosphere and at the same time the biosphere's chemical composition has influenced which life forms inhabit the Earth. In the past, the rate that nutrients were transformed from one chemical form to another was not always the same as the transformation back to the original chemical form. This has resulted in a change in the relative concentrations of chemicals in the biosphere. For example, when life first evolved approximately 3.8 billion years ago, the atmospheric carbon dioxide concentration was much greater than what we find today and there was virtually no free oxygen. The decrease in carbon dioxide and increase in atmospheric oxygen which occurred over time was due to photosynthesis occurring at a faster rate than respiration. The carbon which was present in the atmosphere as carbon dioxide now lies in fossil fuel deposits and limestone rock.
Scientists believe that the increase in atmospheric oxygen concentration influenced the evolution of life. It was not until oxygen reached high concentrations such as we find on the Earth today that multicellular organisms like ourselves could have evolved. We require high oxygen concentrations to accommodate our high respiration rates and would not be able to survive had the biosphere not been altered by the organisms which came before us.
Current developments
Most research concerning the biosphere is being done to determine the effects which human activities have on our environment. Pollution, fertilizer application, land use, and fuel consumption affect nutrient cycles and may damage components of the biosphere such as the ozone layer which protects us from ultraviolet radiation. Fertilizer application increases the amount of nitrogen, phosphorus and other nutrients which organisms can use for growth. These excess nutrients damage lakes as demonstrated by large algal blooms and fish kills. Fuel consumption and land clearing increases carbon dioxide levels in the atmosphere and may cause global warming as a result of carbon dioxide's excellent ability to trap heat.
Recent interest in long term manned space operations has spawned research into the development of artificial biospheres. Extended missions will require that nutrients be cycled in a space no larger than a building. The Biosphere 2 project which received a great deal of press in the early 1990s should provide insights into the feasibility of such biospheres as well as the function and evolution of our own.
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