The Phosphorus Cycle

My AP Biology Thoughts Unit 1 Episode #38

Welcome to My AP Biology Thoughts podcast, my name is Shriya and I am your host for episode 38 called The Phosphorus Cycle. Today we will be discussing what makes up the Phosphorus Cycle, its significance, and how it fits into the greater topic of the Chemistry of Life.

Segment 1: Introduction to The Phosphorus Cycle

• We will begin by discussing what exactly phosphorus is and why it is so important to life as a whole
• Phosphorus is an essential nutrient found in the macromolecules of humans and other organisms including their DNA
• Key part of the phospholipids that form our cell membranes
• In aquatic ecosystems, phosphorus is actually a limiting nutrient (one that is the most scarce) which limits growth
• The phosphorus cycle itself is a very slow process, and most of it exists in nature as the ion PO4 3-
• Phosphorus can be found in fertilizers, and when it is carried off in lakes or oceans, it can cause the overgrowing of algae, or eutrophication which depletes the body of water of oxygen which is detrimental to the ecosystem overall
• It isn’t available for plants to use because most phosphorus is locked up in sediments and rocks, and the phosphorus in the soil isn’t available for plants
• Several reversible pathways are formed which is what the availability of phosphorus in soil depends on:
• Bacteria: It converts plant-available phosphate into organic forms that are then not available to plants
• Adsorption: Inorganic (and available) phosphorus can be chemically bound (adsorbed) to soil particles, making it unavailable to plant
• pH: Inorganic phosphorus compounds need to be soluble to be taken up by plants which depends on the acidity (pH) of the soil, so if soils are less than pH 4 or greater than pH 8, the phosphorus starts to become tied up with other compounds, making it less available to plants
• Many farmers replenish phosphorus through the use of phosphate fertilisers which is obtained by mining deposits of rock phosphate

Segment 2: Example of The Phosphorus Cycle

• Here is a picture of what the Phosphorus Cycle looks like
• First, since phosphate compounds are found in sedimentary rocks, the rocks weather, and the phosphorus they contain slowly leach into the soil and surface water
• These compounds are taken up by plants and then transferred to the animals that eat these plants
• When plants and animals excrete wastes or die, phosphates can be consumed by detritivores or returned to the soil
• Phosphorus-containing compounds may also be carried in surface runoff to rivers, lakes, and oceans, where they are taken up by aquatic organisms
• When phosphorus-containing compounds from the bodies or wastes of marine organisms sink to the floor of the ocean, they form new sedimentary layers
• Over long periods of time, phosphorus-containing sedimentary rock may be moved from the ocean to the land by a geological process called uplift
• When plants and animals die, decomposition results in the return of phosphorus back to the environment via the water or soil

Segment 3: Digging...