Which describes a way in which phosphorus may enter into the phosphorus cycle?

A. Earthquakes release phosphorus.

B. Phosphorus is carried in the rain that was evaporated from lakes.

C. Human use of fertilizers release phosphorus into the groundwater.

D. Phosphorus is transpired.

Phosphorus Cycle. Welcome1To1The1Jungle at English Wikipedia [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

The correct answer is C. Human use of fertilizers release phosphorus into the groundwater.

Phosphorus is an important component of many biological molecules. For instance, the energy carried by ATP is contained in the phosphate bonds and in the cell membrane phospholipids.

Thus, all living organisms need phosphorus to stay alive and function. The phosphates are released back into the soil when plants and animals decay, but much of the mineral also makes its way to the sea where it may accumulate in ocean sediments.

Of all the minerals found on earth, phosphorus is the least common and not easily available. This is because it is usually bound up with other minerals in bedrock.

Biochemical reactions are needed to make the phosphates available for living organisms. In fact, this is a mineral that is very important in determining the biological productivity of a particular region.

In the past, most phosphorus became available because of tectonic processes and mechanical and chemical weathering. Today increased human activity has served to input more phosphates into the cycle through the use of enriched fertilizers and human impacts on soil erosion.

Phosphorus cycle

There are various steps involved in the phosphorus cycle. The first step involves the rocks containing the mineral being exposed through tectonic activity.

These rocks are then subject to mechanical and chemical weathering which then makes the mineral components more readily available as the rock breaks down into soils. Biochemical and geochemical reactions are needed to make the phosphorus available as dissolved orthophosphates.

Plants are able to take up these dissolved minerals, which are then taken up by animals when they feed on the plants. The phosphorus is then returned to the soil when the plants and animals die and decay, and the mineral then ends up in rivers and eventually is deposited as sediments.

Bacteria play an important role in the cycle as they help make the phosphates soluble and available for plants to take up from the soil. Bacterial species that seem to be most important in doing this are those of the bacterial genera: Rhizobium, Bacillus, and Pseudomonas.

The amount of phosphorus available has varied considerably over geological time in accordance with tectonic events such as the uplift of the Himalayan region and subsequent weathering.

Today human activity has impacted the availability of this mineral. The use of fertilizers has had a large impact as have such activities as deforestation, soil erosion, and even sewerage effluent.

Importance of phosphorus

Phosphorus is an essential component of all living organisms. In fact, cell membranes consist mostly of phospholipids which are made of lipids and phosphates.

The phosphate heads of phospholipids allow cell membranes to be in contact with water since the lipid fatty acids are hydrophobic and repel water.

In addition, it is found in the energy molecule of cells, which is adenosine triphosphate (ATP). In this molecule, the energy is stored in the high energy phosphate bonds.

It is therefore involved in energy transfer and other phosphorylation reactions that occur in a cell. This means that cellular respiration, photosynthesis, and many signal transduction reactions depend on phosphorus.

DNA and RNA nucleotides contain a sugar-phosphate backbone to which nitrogen bases are attached. Our genome and ability to make proteins all rely on these nucleic acids.

Protein synthesis requires that RNA be made from a DNA template which of course requires nucleic acids to be readily available.

Human impacts

Humans have greatly modified the environment through activities such as deforestation and agriculture. Plant growth depends heavily on minerals such as nitrogen, potassium, and phosphate and so humans have manufactured fertilizers to add these nutrients to the soil.

The fertilizers add these extra minerals to the soil to help plants to grow. These can contain a mix of nutrients or may only contain one nutrient; for instance, some consist only of superphosphates. However, these substances added to the soils end up in rivers and lakes where they can cause eutrophication.

The use of fertilizers for agriculture is not the only way that excessive amounts of nutrients can enter aquatic ecosystems. In fact, deforestation can impact soil nutrients and runoff as well since it is a disruption of the soil surface.

Effluents from sewerage and industry may also add too many nutrients and minerals to rivers which are then carried into lakes and dams.


Eutrophication is when an excess of nutrients in water leads to algal blooms and plant growth. This, in turn, can cause oxygen levels in the aquatic ecosystems to be substantially reduced causing fish and other organisms to die.

The algal blooms, in particular, blue-green algae, can also produce a foul odor and may even be toxic. The process of eutrophication does occur naturally over time as ecosystems undergo succession.

However, human activity has vastly accelerated the rate at which the process occurs. This human-induced type of eutrophication is often called cultural eutrophication to differentiate it from the natural process.


Abundant cyanobacteria (blue-green algae) block light which is needed by other plants and animals. Fish predators that rely on vision to catch prey may be unable to do so, and the oxygen levels in the water may drop severely when these algae are present.

Another result of the vast algal blooms is that the carbon availability and pH of the water may be adversely affected. This all has consequences for the water quality and ability to support a diversity of living organisms.

Cyanobacteria can in some cases be toxic to wildlife. For instance, blue-green algae of the genera Cylindrospermopsis, Anabaena, and Microcystis have been found to be toxic. In fact, livestock has been killed after drinking water containing these toxic organisms.

In addition, researchers have found that such species are also not good as a food source for the zooplankton. This suggests that these algal species are more harmful than useful in food chains.


  1. MF Chislock, E Doster, RA Zitomer, AE Wilson (2013). Eutrophication: causes, consequences, and controls in aquatic ecosystems. Nature Education Knowledge.
  2. GM Filippelli (2008).  The global phosphorus cycle: past, present, and future. Elements.
  3. WH Schlesinger (2003). Treatise on geochemistry, vol. 8. USA: New York, Elsevier Publishers.
  4. MS Khan, A Zaida (2011). Phosphate solubilizing microbes for crop improvement. USA: New York, Nova Science Publishers.
  5. Editors of Encyclopedia Britannica (2018). Phosphorus cycle. Retrieved from Encyclopedia Britannica.


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