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Weathering

The Phosphorus Cycle

Phosphorus cycle.png
By Bonniemf Incorporates work by NASA Earth Science Enterprise - Reworked by Bonniemf from the public domain file File:Carbon cycle-cute diagram.svg, CC BY-SA 3.0, Link

We're back once more to cover one of the Earth's biogeochemical cycles!  We've seen how water, oxygen, and carbon all move through our planet, and even covered one of the oft-overlooked cycles with nitrogen.  This time we'll be covering another lesser known element when it comes to life on Earth, and that's phosphorus.  Don't let not hearing much about this element fool you, though, as it's vital to both plants and animals and plays a few important roles in how life took shape on our planet.  Thankfully, while this cycle is a slow mover, it does what it needs to at its own pace and that serves us well!
Let's dive in!

Why is Phosphorus Important

Phosphorus, like the other elements that move in biogeochemical cycles, is integral to how life took hold on Earth.  It's thanks to phosphorus that our cells generate energy the way that they do.  ATP (Adenosine Triphosphate) is a molecule that acts as an energy carrier for living things.  It's held together with phosphorus bonds that release a sizable amount of energy when broken.  It's this behavior that influenced multicellular life to form in the first place, and continues to be the way that life functions today!

In addition to being the pop caps on life's energy drinks, phosphorus is also the bridge responsible for double helix DNA similarly due to its bonding qualities.  Beyond its molecular significance, it is also a component of cell membranes, the walls that protect cells.
Phosphorus is a key ingredient in the molecule ATP, the energy carrier of cellular life.

Check out how with this awesome video!
Protecting cells is just one of phosphorus' more protective qualities.  Stepping further back on the cellular scale, it is also a component in bones and the exoskeletons of most insects, as well as the enamel on mammalian teeth.  The latter of those being the hardest biological material on the planet.  That puts your phosphorus-powered tooth enamel on a scale of hardness greater than steel!  This doesn't mean they're not breakable, hardness and brittleness are not mutually exclusive, but does explain why your teeth aren't scratched by metal cutlery while they can be chipped if you get hit in the face with a fast-moving ball.
Phosphorus is also the reason DNA is shaped as a double helix.

This is possible with a bridge formed by phosphorus bonds!
And if that wasn't enough, humans get one more perk from phosphorus.  Our bodies do a lot of regulation and deal with a variety of acidic materials over the course of this regulation.  One of the pieces of the human biology puzzle that keeps our bodies pH levels balanced is called the phosphate buffer system, which, you guessed it, functions thanks to phosphorus.

Even while researching this topic, I wasn't expecting to find this element putting in this much effort!  Even beyond being an element integral in bonding with other elements necessary for carbon-based life, it goes above and beyond finding ways to be useful to the life it already supports.  You go phosphorus!

Where is Phosphorus

So far we've looked at how phosphorus accumulates itself in the biosphere, but how do we living things get this element in the first place?
Having looked at the other cycles, it probably comes as no surprise that us heterotrophs (organisms that can't harness the sun's energy directly) depend on getting this element from none other than our plant friends.  Plants will take up phosphorus from the soil and use it in photosynthesis, storing what it needs in ATP and glucose (sugar) molecules.

​Exploring the rest of the lithosphere is where we'll find most of the rest of phosphorus.  Among the geology of our planet is a rock rich in phosphorus called apatite.  While phosphorus, in general, is a rare mineral in our universe to the human eye, how it did find our planet also impacted some bodies around our planet too.  Rocks collected from NASA's Apollo program also contained traces of apatite!
While this is also an insignificant reservoir for phosphorus, trace amounts can also be found in the atmosphere when dust containing phosphorus gets dissolved into rainwater and sea spray.  This is such a small fraction of the Earth's phosphorus, however, that the cycle primary factors in its movement between the lithosphere, hydrosphere, and biosphere.  Speaking of...

How Does Phosphorus Move

If a majority of phosphorus is locked in the Earth's geology, how does it move?  It's due to this characteristic that makes this cycle so slow, actually.  Tectonic activity in the Earth's crust slowly pushes phosphorus-carrying rock, such as apatite, above the ocean's surface.  After sufficient exposure to the Earth's elements, these rocks will break down and take hold in the soil.  As the soil is churned by living organisms, some of this particulate phosphorus will rejoin the Earth's geology.  When it doesn't, it's picked up by plants.
Phosphorus is the reason tooth enamel is the hardest biological substance on the Earth and is why steel utensils don't scratch your teeth.

​Thanks phosphorus!
As described a bit in the previous section, autotrophs like plants will take up phosphorus from the soil and in turn be consumed by the organisms that eat those plants.  Solid waste from these organisms, as well as their decaying remains, will eventually reintroduce phosphorus to the soil.  This is the most active part of the phosphorus cycle and can continue for centuries and millenia.

The hydrosphere is primarily responsible for moving this phosphorus in the Earth's topsoil over the land and eventually back to the ocean by way of runoff.  This process is very slow-going, but will eventually bring phosphorus back to the sea, working its way down to the sea floor where it will settle and rejoin the rock where its journey began.  Or, at least when you consider the sea floor rock formations as the beginning of the journey.  Fun thing about cycles is they have no real destination, just a constant state of presence.
The phosphorus cycle moves with geologic time, taking thousands to millions of years to complete full cycles

How Humanity Disturbs the Cycle

As humanity uses resources found within the planet for uses beyond what the planet would naturally do on its own, we have to recognize that these activities have side effects.  While it probably feels like the same song and dance between these pages detailing biogeochemical cycles, we have to understand the humans don't perform in a vacuum and any way we use resources on this planet could (and usually does) have a downstream effect.  Quite literally as with the case of phosphorus.

Similar to nitrogen, the main way humans interact with the phosphorus cycle is by mining it for use in fertilizer.  In fact, 80% of all mined phosphorus is used in creating fertilizers for our global agriculture industry.  Though, simply mining and creating fertilizer with phosphorus isn't the core problem.  It's what comes after the fertilizer is used.
If we recall our human impact section from the nitrogen cycle post, we do a pretty great job of shuttling excess fertilizer into streams that make their way into the ocean.  Turns out that how we treat phosphorus isn't where the similarities to nitrogen end either.  Phosphorus is also a eutrophication element.  Eutrophication occurs when a body of water experiences an overabundance of a nutrient that the ecosystem can use for rapid algal growth followed by an immense algal decay resulting in anoxic (oxygen-depleted) ecosystems.
These oxygen-less freshwater and marine ecosystems form what are known as dead zones where no life can take hold.  The surplus of nitrogen and phosphorus also acidifies the water, which can spread to adjacent ecosystems.  Beyond death and destruction, turning these areas adjacent to our homes into aquatic wastelands has health risks to both us and our neighboring animals out of the water.

A potential silver lining with phosphorus is that it isn't as reactive as nitrogen.  It primarily travels in particulate and dissolved forms, avoiding the atmosphere.  In other words, the vectors for its movement are more limited, thus potentially more controllable.  There have been a number of studies looking into how we can incorporate more wetlands adjacent to eutrophication zones to pull excess nitrogen and phosphorus out of the aquatic ecosystems and back into the soil.  
Eutrophication is the process by which a body of water becomes entrenched with nutrients, can cause rapid algae growth and decay, ultimately draining the water of all available oxygen
These approaches have their limitations, but it's important that we continue to look into ways to combat our own actions' effects on our planet.  Understanding the downstream influence of our actions is the first step to figuring out how to solve the problem.  I'm sure it feels like I belabor the point of human interference with our planet's cycles, but it is abundantly important that we understand how we interact with and affect our home.  By spreading this knowledge, more people recognize that we're not just an observer of these systems, but an active participant.  Participants that can learn and change how we interact with our planet so we can live in such a way the ensures we have a home for millennia to come.
~ And, as always, don't forget to keep wondering ~
Prismatic Planet
Sources
* Phosphorus Cycle and Stores:
https://en.wikipedia.org/wiki/Phosphorus_cycle​. Retrieved 2021-04-11.
https://en.wikipedia.org/wiki/Apatite​. Retrieved 2021-04-11.
* ATP and Cellular Energy:
https://www.youtube.com/watch?v=QImCld9YubE. Retrieved 2021-04-11.
* Human Phosphate Buffer System:
https://www.euroformhealthcare.biz/medical-physiology/phosphate-buffer-system.html. Retrieved 2021-04-11.
* Nitrogen and Phosphorus Eutrophication:
https://www.intechopen.com/books/monitoring-of-marine-pollution/nitrogen-and-phosphorus-eutrophication-in-marine-ecosystems. Retrieved 2021-04-11.

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