Study Guide

Ecology: Organisms and Their Environments - The Phosphorus Cycle

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The Phosphorus Cycle

The last important chemical in an ecosystem that we will tackle in this unit is phosphorous. This essential chemical is used by organisms as one of the main components of DNA. Nitrogen and carbon are also part of DNA. Unlike carbon and nitrogen, most of the phosphorous on Earth is stored in soil and rocks in the form of phosphate. Phosphate is one molecule of phosphorous surrounded by four molecules of oxygen, or PO43-. Plants can absorb phosphate directly through their roots. From there, it moves through the community in traditional fashion and back into the ground as animals die and decompose. Just in case you’re wondering, while most plants can’t extract phosphate directly from rocks, this type of phosphate eventually makes its way into the soil as rocks are weathered and eroded.


As is generally the case, too much of a good thing can actually be bad. This is true for nitrogen and phosphorous as well. Most plants and animals cannot survive in environments with excessive amounts of either chemical. Normally, this is rarely a problem. In fact, most habitats don’t have enough nitrogen or phosphorous, and plants will compete fiercely for them. However, extensive use of fertilizers, which are composed of mainly nitrogen and phosphorous, by humans has led to a buildup of these essential chemicals in aquatic environments near areas of intense fertilization. This nutrient buildup is called eutrophication, and when it occurs, bacteria and algae experience intense exponential growth and quickly use all of the available oxygen, killing off all other species in the ecosystem. Sounds bad, and it is. Eutrophication has occurred in small ponds on tiny farms and in huge bodies of water, like the Baltic Sea. No matter the scale, it’s bad news for the ecosystem involved. 


Water, carbon, nitrogen, and phosphorous are obviously not the only chemicals present in an ecosystem, though they are the most abundant. In addition to these essential chemicals, many of the other elements on the periodic table are also lurking nearby. Some of these elements, especially heavy metals like mercury (Hg), can get into the food chains as they are absorbed by producers or consumed by higher trophic levels. Heavy metals like mercury and other inorganic substances, like pesticides, are difficult for most organisms to process and degrade. As such, they can accumulate in the tissues of organisms and persist there until it they are consumed by other organisms. As the substance moves upward on the food chain, it begins to accumulate in higher-order consumers in a process known as biomagnification.

When biomagnification occurs with dangerous elements like mercury, it can become a risk for higher-level consumers like humans. Pregnant women, for example, are advised not to eat certain kinds of fish because of the potential for biomagnification of mercury in aquatic systems. Mercury has been shown to negatively affect the growth and development of unborn children.

Brain Snack

Recently, a NASA scientist discovered a bacterium that could swap the phosphates in its nucleic acids for arsenic, which is one of the most toxic elements known. Read more here.

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