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The Biosphere

Chemical Cycling

Many biologically important chemicals, such as carbon, nitrogen, and phosphorus, are transferred between organisms and the environment in a cycle.
Energy transfer is a one-way flow in ecosystems, with the Sun continuously replenishing the energy that is captured by autotrophs, organisms that make their own food. Energy comes from the Sun, flows through the ecosystem, and is eventually lost as heat. Elements, on the other hand, are not replenished, so they must be recycled in an ecosystem. Atoms from the air, water, and soil are taken up by organisms and incorporated into their bodies. When an organism is consumed, the atoms are transferred to the body of the consumer. Eventually, the atoms are released back into the environment through excretion or decomposition. Organisms and the environment pass the atoms back and forth in biogeochemical cycles. A biogeochemical cycle is a cyclic transfer of a chemical, such as carbon, between organisms and the environment. The proportion of time spent in biotic (living) and abiotic (nonliving) forms varies depending on the element. For example, carbon is passed back and forth between the processes of photosynthesis and cellular respiration. There are specialized organisms whose role it is to convert an element into a form that is usable by other organisms, and other organisms are able to convert the chemical back into its free form. Three important elements that cycle through ecosystems are nitrogen, phosphorus, and carbon. Nitrogen is a component of proteins (molecules that speed up reactions), phosphorus is present in DNA (molecules that contain genetic code), and carbon forms the basis of all biological molecules.

Nitrogen Cycle

Nitrogen from the air is "fixed" by bacteria into forms organisms can use. These organisms later release the nitrogen into the soil as waste or as the organisms decompose. Nitrogen eventually returns to the air if not taken up by other organisms.
Nitrogen is a component of amino acids (building blocks of proteins), proteins, and DNA. Nitrogen is plentiful, as 78% of Earth's atmosphere is made of nitrogen gas. However, most organisms cannot use nitrogen in this form. Some bacteria, such as those that live on the roots of legume plants, are able to convert atmospheric nitrogen gas into ammonia, a form that is more usable by plants, through a process called nitrogen fixation. Ammonia is a form that organisms can use. Oxygen disrupts this process, so nitrogen fixation must occur in protected, airtight structures called heterocysts. Nitrogen-fixing bacteria complete this conversion in the soil or in specialized tissues of plants that will then use the ammonia. The availability of nitrogen in soil is a limiting factor in plant growth. After plants take up the "fixed" nitrogen, other organisms can obtain it by consuming the plants. Nitrogen is excreted from animals in urine and feces. When an organism dies, decomposers such as bacteria and fungi break down proteins into individual amino acids. Specialized bacteria process the nitrogen, turning it into nitrates, where it can be stored in the soil. Some of the soil nitrates are picked up by plants that are able to use nitrogen in this form. Other nitrates are processed by nitrogen fixers and returned to plants. Some soil nitrates are processed by denitrifying bacteria that can convert the nitrates into nitrogen gas and release it back into the atmosphere. In addition, human industrial practices release nitrogen oxides into the atmosphere as air pollution.
There is an abundant of nitrogen in Earth's atmosphere. But it is unusable by most organisms because of the chemical structure of nitrogen. This is why atmospheric nitrogen must be "fixed" by bacteria before it can be used by plants for growth. Decomposers break down the nitrogen in organic waste which allows nitrogen gas to return to the atmosphere.

Phosphorus Cycle

Phosphorus is removed from rock by water and then taken up by plants along with the water. Animals eat the plants, then release phosphorus as waste or upon death and decomposition. Some phosphorus from these remains is dissolved by water, while the rest of the remains eventually solidify into rock and begin the cycle again.

Phosphorus is a major component of the cell membrane, ATP (the biological unit of energy), and DNA. Environmental phosphorus is mostly in the form of calcium phosphate, found in rocks and soil. Phosphate is released from eroding rocks and leaches into local waters. Phosphorus does not have a gaseous form, so there is no phosphorus in the atmosphere. Plants absorb phosphates from the soil or water and incorporate them into their tissues. Phosphorus is transferred to heterotrophs when they consume plants. Animals excrete phosphorus in their wastes. When an organism dies, its phosphorus is released by decomposition. Bacteria then convert organic phosphorus back to phosphate.

In freshwater lakes, phosphorus is available in small amounts. However, agricultural fertilizers and other industrial chemicals contain phosphorus. Runoff from farms and industrial pollution can cause an increase in available phosphorus. Too much of a nutrient can cause problems, including a rapid growth, or bloom, of photosynthetic algae in a lake. This turns the lake green, blocking light to deep waters. After the extra algae die, they are consumed by bacteria that use oxygen in the process. As the oxygen levels decrease, animals in the lake begin to die. This algal bloom caused by excessive nutrients, leading to a drop in dissolved oxygen is called eutrophication.
Phosphorus emerges into the soil from natural sources such as the breakdown of rocks or human sources such as runoff from farming. There is no atmospheric source of phosphorus. When this phosphorus is released, it enters a body of water and seeps into the soil. Plants absorb phosphorus which allows animals to obtain phosphorus in their bodies after they eat the plants. Decomposers break down phosphorus and return it to the soil allowing the cycle to continue.

Carbon Cycle

Carbon is taken up from the atmosphere by organisms that undergo photosynthesis. Other organisms eat them, and release carbon dioxide back into the atmosphere as waste. Humans have increased the amount of carbon in the atmosphere by burning carbon-rich fossil fuels, such as coal and oil.

Carbon forms the basis of all biological molecules. Atmospheric carbon is in the form of carbon dioxide. Plants take up carbon dioxide and convert it into organic carbon through photosynthesis. All organisms convert organic carbon into energy usable by the cell through cellular respiration, releasing carbon dioxide as a byproduct. Heterotrophs (organisms that eat other organisms for food) obtain organic carbon by consuming plants or other heterotrophs. Carbon is "stored" in living organisms because it is kept out of the atmosphere. When organisms die, some of their carbon is returned to the atmosphere by decomposers.

Some dead organisms are converted into fossil fuels, such as coal, gas, and oil, over geologic time. Typically this carbon is trapped and remains outside of the carbon cycle. Small amounts of carbon from these sources may slowly reenter the cycle as water interacts with them.

However, modern human behavior has made a practice of burning large amounts of fossil fuels, releasing carbon back into the atmosphere as carbon dioxide. Factories, cars, and fuels to heat human homes all produce carbon dioxide. Burning wood has a twofold impact because it removes an organism capable of taking up carbon dioxide and releases more carbon dioxide into the atmosphere when the wood is burned. Carbon cycles in aquatic environments include the same biological processes. Large amounts of carbon are stored in limestone from ancient ocean creatures buried millions of years ago. As these rocks erode, carbon is released into aquatic environments. In addition, carbon dioxide can also diffuse into water directly.
Carbon is exchanged between the processes of photosynthesis and cellular respiration. Autotrophs convert carbon dioxide into organic compounds through photosynthesis. Heterotrophs obtain organic compounds from eating autotrophs, and undergo respiration, releasing carbon dioxide. Some dead organisms turn into fossil fuels over geologic time. When fossil fuels are burned, carbon dioxide is released.