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.
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.