In this video, we're going to talk about the 4th biogeochemical cycle in our lesson, which is the phosphorus cycle. You can see this diagram of the phosphorus cycle behind me, which has these two different colored arrows. The yellow arrows represent terrestrial cycling of phosphorus and the reddish arrows represent aquatic cycling of phosphorus. It's important to recall that phosphorus is yet another essential element to all life, as phosphorus is a component of the phosphate groups and nucleotides that make up nucleic acids like DNA and RNA with their sugar phosphate backbones. Phosphorus is also a component of molecules such as ATP, the energy molecule that's essential to so many life processes.

Phosphorus is required for life as we know it, and the phosphorus cycle is important because it helps to recycle phosphorus and ensure that phosphorus is available to all organisms, regardless of what trophic level they're in and regardless of what ecosystem they're in, whether it be terrestrial or aquatic ecosystems. This makes the phosphorus cycle vital to life as we know it. So that being said, let's go ahead and wipe this slate clean so that we can approach this one step at a time. One of the biggest takeaways that you should get of the phosphorus cycles is that unlike the water cycle, carbon cycle, and nitrogen cycle that we discussed in previous lesson videos, the atmosphere is not a reservoir of phosphorus. The atmosphere on earth does not have phosphorus in it.

This means that the phosphorus cycle is typically going to occur in local areas, and there's the cycling of phosphorus is typically going to be local. Whereas with the other cycles that have atmospheric components, those components can move around the earth freely, and that allows the cycling of those other cycles to be more on a global scale. But again, with phosphorus, since it's not part of the atmosphere, it's going to occur more locally. Instead of the atmosphere being the most significant reservoir, the most significant reservoir of phosphorus is going to be these phosphate containing sedimentary rocks, like this rock that you can see right here. Through weathering, exposure to rain and wind and things of that nature, it can actually release the phosphates that are stored in these sedimentary rocks into the soil, and the phosphorus is going to be released mainly in the form of phosphates, PO43−, which can be readily assimilated by primary producers like this plant here.

Once the plants have obtained their phosphorus, the phosphorus can move up the food chain via consumption to other organisms. And then eventually, these organisms are going to die and decompose, and the decomposition process carried out by organisms like fungi and bacteria can return the phosphorus back to the soil in the form of these phosphates, PO43−. This here completes this little terrestrial cycling that we've got going on here. So now let's focus our attention over on the aquatic cycling that we have on the right. And since I'm right in the middle here, let me move out of the way so that you can see this a little bit more clearly.

There we go. When it comes to aquatic cycling, there are several ways that the phosphate can get into the aquatic system and serve as dissolve PO43−, dissolved phosphate. It could occur through weathering of rocks where, again, the weather, exposure to weather, can release the phosphates from those phosphate containing rocks, and rivers and streams can then allow the phosphates to run off into these aquatic ecosystems. Or this process known as leaching can allow for phosphates to move from the soil directly into the aquatic ecosystems. Once the phosphates are dissolved in the aquatic ecosystems, then notice that we have these set of arrows that are number 2, 3, and 4, which correspond with the numbers 2, 3, and 4 that we have over here in the terrestrial ecosystem.

They represent assimilation, consumption, and decomposition. These dissolved phosphates can be assimilated via primary producers such as phytoplankton, then they can be consumed and make their way up the food chain to other organisms such as fish. Then eventually these organisms will die and decompose, and the decomposition process returns the phosphates back to the water. Once these phosphates are in the water, they can undergo a process known as sedimentation, which can reform the rocks, those phosphate containing rocks. These phosphate containing rocks can be re-exposed to the atmosphere via geologic uplift, which can change the shape of the surface of the earth. And again, reexpose those rocks to the atmosphere to complete this cycle.

This here concludes our lesson on the phosphorus cycle, and moving forward, we'll be able to apply these concepts and problems. So I'll see you all in our next video.