1. Why should you care about phosphorus?
To see why, take the quiz below.
[qwiz qrecord_id=”sciencemusicvideosMeister1961-The importance of phosphorus”]
[h]The importance of phosphorus
[i]In living things, the element phosphorus is found within the phosphate ion. In unit 1 and throughout this course, you’ve seen phosphate represented as PO43-, or as
[q]The molecule below is [hangman]. At number 3 you see three [hangman] groups.
[c]QVRQ[Qq]
[c]cGhvc3BoYXRl[Qq]
[q]In both RNA and DNA, there’s a sugar-[hangman] backbone
[c]cGhvc3BoYXRl[Qq]
[q]The key molecule in membranes is the [hangman]. You can see this in part 1 below, which makes this molecules “head” [hangman], which means “water-[hangman]”
[c]cGhvc3Bob2xpcGlk[Qq]
[c]aHlkcm9waGlsaWM=[Qq]
[c]bG92aW5n[Qq]
[q]In many cell signaling systems the second messenger (at “a”) is [hangman], which is made when adenylyl cyclase removes two [hangman] groups from ATP (5). Letters “b” through “k” show how another process that involves phosphates: a [hangman] cascade.
[c]Y0FNUA==
[c]cGhvc3BoYXRlcw==[Qq]
[c]cGhvc3Bob3J5bGF0aW9u[Qq]
[q][hangman] groups are involved in many metabolic processes. For example, at the start of glycolysis, glucose gets doubly [hangman], creating fructose-1-6 bisphosphate.
[c]UGhvc3BoYXRl[Qq]
[c]cGhvc3Bob3J5bGF0ZWQ=[Qq]
[q]If that’s not enough, phosphate, in the form of calcium phosphate, is the key mineral that gives bones and tooth enamel their hardness and structural strength.
[/qwiz]
2. Understanding the Phosphorus Cycle
As you saw in the quiz above, the phosphorus in living things is typically found in the form of the phosphate ion (PO₄³⁻). This is the same phosphate ion that you’ve come to know as “inorganic phosphate,” or Pi. Pi is what gets attached to ADP to make ATP. And (as you saw above), Pi is one of the substrates involved involved in the phosphorylation reactions that you’ve seen in cell communication, metabolic reactions, and so on.
Where does phosphate come from? That’s what the phosphorus cycle is all about. As with the water cycle, the phosphorus cycle doesn’t involve a chemical transformation. It involves tracing the movement of the phosphate ion between various reservoirs. But unlike the carbon, nitrogen, and water cycles, phosphate rarely enters the atmosphere (and when it does, the conditions are unusual enough that we can ignore them). So understanding the phosphate cycle is about tracking fluxes of phosphate on land, in water, and between land and sea.
In terrestrial ecosystems, phosphate originates from the weathering of phosphate-containing rocks (a). “Weathering” is exactly what it sounds like: it’s the breakdown of minerals in rock caused by rain, snow, freezing, and thawing. This connects the phosphorus and water cycles, since precipitation drives the weathering of rocks and also transports phosphate through runoff into soil and waterways.
In soil, phosphate dissolves in water and is absorbed by plant roots (b). Animals acquire phosphate by eating plants (c), or by eating other animals that ate plants (not shown).
When animals defecate and urinate (f), their waste can enrich the soil with phosphate, making it available again for uptake by plants. When plants and animals die (d), decomposers break down their organic matter, releasing phosphate back into the soil (e).
Weathering also brings phosphate directly into aquatic ecosystems such as lakes, rivers, and oceans. Additional phosphate enters these systems from soil through natural runoff (h). In aquatic environments, dissolved phosphate is absorbed by producers (i), such as algae, and passed to consumers (j) through aquatic food chains. As in terrestrial systems, aquatic animal waste (not shown) contributes to the pool of available phosphate.
Over time, some phosphate settles into sediments (k) on the ocean or lake floor. Once buried, this phosphate is effectively removed from the biological cycle. It remains unavailable until geological processes, such as tectonic uplift or volcanic activity (l), eventually bring it back to the surface — a process that can take tens or hundreds of millions of years.
Since the dawn of agriculture, humans have increased phosphorus input by applying phosphate fertilizers (g). Much of this added phosphate is lost through agricultural runoff (m). This creates two problems which we’ll explore in more detail below, and in the final module in this course.
3. Phosphorus Cycle Flashcards
[qdeck bold_text=”false” qrecord_id=”sciencemusicvideosMeister1961-Phosphorus Cycle Flashcards”]
[h] Phosphorus Cycle: Key Concepts and Terms
[i]
[start]
[q] Identify letters a through f in the diagram below, and describe how each letter relates to a part of the phosphorus cycle.
[a]a – weathering, releasing phosphate from rocks; b – uptake of phosphate from the soil into plant roots (AKA assimilation); c – movement of phosphate into animals; d – death, moving the phosphate in organisms to decomposers; e – decomposition, making phosphate available to plants.
[q] Identify letters g through k in the diagram below, and describe how each letter relates to a part of the phosphorus cycle.
[a]g – phosphate mining, releasing phosphate from rocks for use in agriculture; h – movement of phosphate in soil into lakes, rivers, and oceans; i – uptake of dissolved phosphate by aquatic producers (algae, etc); j – movement of phosphate from aquatic producers to aquatic consumers through consumption; k – movement of organic phosphate into marine sediments, removing phosphate from the phosphorus cycle; m – movement of phosphate from agricultural fields into aquatic ecosystems.
[q]Identify and describe 1 – 5 in the diagram below.
[a]
1. **”Geological uplift” or “tectonic activity” or “volcanic activity”** – phosphate is brought from sediments back to the surface
2. ** Weathering** – Phosphorus is released from phosphate-containing rocks into the soil through weathering.
3. **Consumption by animals** – Animals obtain phosphorus by eating plants or other animals.
4. **Death** – Movement of phosphate from organisms to decomposers.
5. **Assimilation/uptake of phosphate by plants** – Plants absorb phosphate from the soil into their roots.
[q]Identify and describe 6 – 9 in the diagram below.
[a]
6. **Decomposition** – Decomposers break down organic matter, releasing phosphorus back into the soil.
7. **Runoff to terrestrial or aquatic systems** – Phosphate from is brought to land, streams, rivers, lakes or oceans through runoff.
8. **Runoff of phosphate from soil to aquatic system** – Phosphates moves from soil to bodies of water.
9. **Sedimentation** – Phosphates sink and become part of sediments, which over time may form new phosphate rock.
[q] What makes the phosphorus cycle different from the carbon and nitrogen cycles?
[a] Phosphorus rarely enters the atmosphere, so its cycle is mostly studied by tracking phosphate (PO₄³⁻) through soil, water, and organisms.
[q] What is phosphate, and what is its chemical formula?
[a] Phosphate is the form of phosphorus that cycles through ecosystems. Its chemical formula is PO₄³⁻.
[q] Where does phosphate originate in terrestrial ecosystems?
[a] Phosphate originates from the weathering of phosphate-containing rocks (a).
[q] What natural forces cause weathering of rocks?
[a] Rain, snow, freezing, and thawing break down rocks and release phosphate into soil and water.
[q] How does the water cycle contribute to the phosphorus cycle?
[a] Precipitation drives the weathering of rocks and transports phosphate through runoff.
[q] How do plants obtain phosphate?
[a] Plants absorb dissolved phosphate from the soil through their roots (b).
[q] How do animals acquire phosphate?
[a] By eating plants (c), or by eating other animals that ate plants (not shown bel0w).
[q] How does animal waste contribute to the phosphorus cycle?
[a] Feces and urine (f) return phosphate to the soil, making it available for plant uptake.
[q] What role do decomposers play in the phosphorus cycle?
[a] Decomposers break down dead plants and animals (d), releasing phosphate back into the soil (e).
[q] How does phosphate enter aquatic ecosystems?
[a] From direct weathering and through natural runoff from soil (h).
[q] How do aquatic producers obtain phosphate?
[a] They absorb dissolved phosphate from the water (i).
[q] How is phosphate passed through aquatic food chains?
[a] Aquatic producers are consumed by aquatic consumers (j), transferring phosphate.
[q] What eventually happens to some phosphate in aquatic systems?
[a] It settles into sediments on lake or ocean floors (k), removing it from the cycle.
[q] What can bring buried phosphate back into the phosphorus cycle?
[a] Tectonic uplift or volcanic activity (l) can return phosphate to the surface, a process that may take tens or hundreds of millions of years.
[q] How have humans altered the phosphorus cycle?
[a] By applying phosphate fertilizers to crops (g), increasing the amount of phosphate in ecosystems.
[q] What happens to much of the phosphate from fertilizer?
[a] It is lost through agricultural runoff (m).
[/qdeck]
4. Phosphorus cycle quiz
[qwiz qrecord_id=”sciencemusicvideosMeister1961-Phosphorus Cycle Quiz”]
[h]Phosphorus Cycle Comprehensive Quiz
[q]In the diagram below, what letter indicates release of phosphate caused by weathering from rocks.
[textentry single_char=”true”]
[c]YQ ==[Qq]
[f]Q29ycmVjdCEgV2VhdGhlcmluZyByZWxlYXNlcyBwaG9zcGhhdGUgZnJvbSByb2Nrcy4=[Qq]
[c]Kg==[Qq]
[f]Tm8uIFdlYXRoZXJpbmcgaGFwcGVucyB3aGVuIHByZWNpcGl0YXRpb24gYnJlYWtzIGRvd24gcm9jay4gV2hlcmUgY291bGQgdGhhdCBiZSBoYXBwZW5pbmcgaW4gdGhpcyBkaWFncmFtPw==
[q]In the diagram below, letters “c” and “j” both involve movement of phosphate by [hangman] of producers by primary consumers (animals).
[c]Y29uc3Vt cHRpb24=[Qq]
[f]WWVzISBBbmltYWxzIGdhaW4gcGhvc3BoYXRlIGJ5IGNvbnN1bWluZyBwcm9kdWNlcnMu
[q]In the diagram below, what letter shows decomposers breaking down dead organisms and returning phosphate to the soil?
[textentry single_char=”true”]
[c]ZQ ==[Qq]
[f]WWVzISBUaGF0IGFycm93IGNvbm5lY3RzIGRlY29tcG9zaXRpb24gdG8gc29pbCBwaG9zcGhhdGUu[Qq]
[c]Kg==[Qq]
[f]Tm8uIFRoaW5rIGFib3V0IHdoYXQgaGFwcGVucyB0byBwaG9zcGhhdGUgYWZ0ZXIgYW4gb3JnYW5pc20gZGllcy4=
[q]The phosphate ion has this chemical formula: [hangman]₄³⁻
[c]UE 8=[Qq]
[f]VGhhdOKAmXMgY29ycmVjdCDigJQgcGhvc3BoYXRlIGlzIFBP4oKEwrPigbsu
[q]In the diagram below, what letter shows the absorption of phosphate from the soil by plant roots?
[textentry single_char=”true”]
[c]Yg ==[Qq]
[f]Q29ycmVjdCEgVGhhdOKAmXMgd2hlcmUgcGxhbnRzIGFyZSB0YWtpbmcgdXAgcGhvc3BoYXRlIGZyb20gdGhlIHNvaWwu[Qq]
[c]Kg==[Qq]
[f]Tm8uIExvb2sgZm9yIHJvb3RzLg==
[q]In the diagram, which letter shows phosphate settling into sediments, removing it from the cycle?
[textentry single_char=”true”]
[c]aw ==[Qq]
[f]WWVzISBTZWRpbWVudGF0aW9uIGF0IHRoZSBib3R0b20gb2YgbGFrZXMgYW5kIG9jZWFucyByZW1vdmVzIHBob3NwaGF0ZSBmcm9tIGNpcmN1bGF0aW9uLg==[Qq]
[c]Kg==[Qq]
[f]Tm8uIExvb2sgb3JnYW5pYyBtYXRlcmlhbCB0aGF0JiM4MjE3O3MgZmFsbGluZyB0b3dhcmQgdGhlIGJvdHRvbSBvZiBhbiBhcXVhdGljIHN5c3RlbS4=
[q]In the diagram, what letter shows how geological uplift or volcanic activity brings phosphate back to the surface?
[textentry single_char=”true”]
[c]bA ==[Qq]
[f]Q29ycmVjdCEgVGhhdCBzaG93cyB0ZWN0b25pYyBhY3Rpdml0eSBicmluZ2luZyBidXJpZWQgcGhvc3BoYXRlIGJhY2sgdXAgdG8gdGhlIHN1cmZhY2Uu[Qq]
[c]Kg==[Qq]
[f]Tm8uIFRoaW5rIGFib3V0IEVhcnRo4oCZcyBjcnVzdCBiZWluZyBsaWZ0ZWQgb3IgZXJ1cHRlZC4=
[q]In aquatic systems, phosphate is absorbed by [hangman] and enters aquatic food chains.
[c]cHJvZH VjZXJz[Qq]
[f]RXhjZWxsZW50LiBUaGVzZSBpbmNsdWRlIGFsZ2FlIGFuZCBwaHl0b3BsYW5rdG9uLg==
[q]In the diagram, what letter represents the runoff of phosphate from natural soils into aquatic ecosystems?
[textentry single_char=”true”]
[c]aA ==[Qq]
[f]R29vZCBqb2IhIFRoYXTigJlzIHRoZSBuYXR1cmFsIG1vdmVtZW50IG9mIHBob3NwaGF0ZSBmcm9tIHNvaWwgaW50byB3YXRlci4=[Qq]
[c]Kg==[Qq]
[f]Tm8uIExvb2sgZm9yIHJ1bm9mZiBub3QgYXNzb2NpYXRlZCB3aXRoIGZhcm1sYW5kLg==
[q]In the diagram, what letter shows phosphate entering soil through animal waste?
[textentry single_char=”true”]
[c]Zg ==[Qq]
[f]UmlnaHQhIFdhc3RlIGxpa2UgZmVjZXMgYW5kIHVyaW5lIHJldHVybnMgcGhvc3BoYXRlIHRvIHRoZSBzb2lsLg==[Qq]
[c]Kg==[Qq]
[f]Tm8uIExvb2sgZm9yIHdhc3RlIGdvaW5nIGludG8gc29pbCBuZWFyIGxhbmQgYW5pbWFscy4=
[q]Humans increase phosphate levels in ecosystems by applying [hangman] to crops.
[c]ZmVydGls aXplcg==[Qq]
[f]WWVzISBGZXJ0aWxpemVyIGFkZHMgcGhvc3BoYXRlIHRvIHRoZSBzb2lsLg==
[q]In the diagram, what letter shows the application of phosphate fertilizers?
[textentry single_char=”true”]
[c]Zw ==[Qq]
[f]Q29ycmVjdCEgVGhhdOKAmXMgdGhlIGlucHV0IG9mIHBob3NwaGF0ZSBieSBodW1hbnMu[Qq]
[c]Kg==[Qq]
[f]Tm8uIFRoaW5rIGFib3V0IHdoZXJlIGh1bWFuIGludGVydmVudGlvbiBhcHBlYXJzIGluIGZhcm1pbmcgYXJlYXMu
[q]In the diagram, what letter shows phosphate leaving farm fields due to runoff?
[textentry single_char=”true”]
[c]bQ ==[Qq]
[f]UmlnaHQhIFRoaXMgaXMgcGhvc3BoYXRlIGxvc3QgZnJvbSBhZ3JpY3VsdHVyZSBpbnRvIHdhdGVyd2F5cy4=[Qq]
[c]Kg==[Qq]
[f]Tm8uIFRoaXMgcnVub2ZmwqAgb2NjdXJzIHdoZW4gaHVtYW5zIGFkZCBwaG9zcGhhdGUgdG8gYWdyaWN1bHR1cmFsIGZpZWxkcy4gTG9vayBmb3IgYW4gb3V0cHV0IGZyb20gYW4gYWdyaWN1bHR1cmFsIGZpZWxkLg==
[q]The phosphorus cycle is considered to unique among biogeochemical cycles because phosphorus never enters the [hangman].
[c]YXRtb3Nw aGVyZQ==[Qq]
[f]Q29ycmVjdCEgUGhvc3Bob3J1cyByYXJlbHkgZW50ZXJzIHRoZSBhdG1vc3BoZXJlLCB1bmxpa2Ugbml0cm9nZW4gb3IgY2FyYm9uLg==
[/qwiz]
5. On the Horizon: A Phosphorus Crisis
As we’ve seen, phosphorus is essential for life. But a looming phosphorus crisis is heading our way. Here’s why.
-
Modern agriculture depends heavily on phosphorus-based fertilizers, which are made from phosphate rock.
-
Phosphate rock is a non-renewable resource, and global reserves are being depleted.
-
Most phosphate mining happens in a few countries, especially Morocco, raising concerns about geopolitical control and future scarcity.
-
Worse, excess phosphorus from fertilizers often runs off into waterways, causing eutrophication—a process that leads to algal blooms, oxygen depletion, and massive fish kills. Eutrophication is also connected to the nitrogen cycle, and we’ll learn more about it later in this unit.
Scientists warn that if we don’t develop better ways to recycle and manage phosphorus, we could face a future where crop yields plummet due to phosphorus shortages—threatening global food security.
If you want to learn more about the phosphorus crisis,
- read this article from Scientific American, or
- this one from National Geographic (they require an email)
6. The Phosphorus Cycle: Checking Understanding
Here a few free-response prompts to see how well you’ve mastered the material above.
[h]Phosphorus Cycle Free Response Questions
[q multiple_choice=”false”]Describe what’s happening in letters m, i, j, and k of the diagram below.
[c]U2hvdyBzYW1w bGUgYW5zd2Vy[Qq]
[f]bTogcGhvc3BoYXRlIGZyb20gYWdyaWN1bHR1cmUgKGZyb20gZmVydGlsaXplcikgaXMgZW50ZXJpbmcgYSBsYWtlIG9yIHRoZSBvY2VhbiAoYW4gYXF1YXRpYyBlY29zeXN0ZW0p
aTogdGhlIHBob3NwaGF0ZSBpcyBhYnNvcmJlZCAob3IgdGFrZW4gdXAsIG9yIGFzc2ltaWxhdGVkKSBieSBhcXVhdGljIHBsYW50cyAob3IgYWxnYWUsIG9yIHByb2R1Y2Vycyk=
Cg==ajogVGhlIHBob3NwaGF0ZSBtb3ZlcyBmcm9tIHRoZSBhcXVhdGljIHByb2R1Y2VycyB0byBmaXNoIHRocm91Z2ggY29uc3VtcHRpb24=
[Qq]k: organic remains and wastes leave aquatic systems as they fall down into marine sediments.
[q multiple_choice=”false”]Use the diagram below to identify and describe three fluxes in the phosphorus cycle. For each flux, list the number and explain how it moves phosphorus from one reservoir to another.
[c]U2hvdyB0aG UgYW5zd2Vy[Qq]
[f]
CkhlcmUgYXJlIGV4cGxhbmF0aW9ucyBvZiBhbGwgOSBmbHV4ZXMgKG1vdmVtZW50cyBvZiBwaG9zcGhvcnVzKSBzaG93biBpbiB0aGUgZGlhZ3JhbTo=
MS4gKiomIzgyMjE7R2VvbG9naWNhbCB1cGxpZnQmIzgyMjE7IG9yICYjODIyMDt0ZWN0b25pYyBhY3Rpdml0eSYjODIyMTsgb3IgJiM4MjIwO3ZvbGNhbmljIGFjdGl2aXR5JiM4MjIxOyoqIOKAkyBwaG9zcGhhdGUgaXMgYnJvdWdodCBmcm9tIHNlZGltZW50cyBiYWNrIHRvIHRoZSBzdXJmYWNl
CjIuICoqIFdlYXRoZXJpbmcqKiAmIzgyMTE7IFBob3NwaG9ydXMgaXMgcmVsZWFzZWQgZnJvbSBwaG9zcGhhdGUtY29udGFpbmluZyByb2NrcyBpbnRvIHRoZSBzb2lsIHRocm91Z2ggd2VhdGhlcmluZy4=
[Qq]
3. **Consumption by animals** – Animals obtain phosphorus by eating plants or other animals.
4. **Death** – Movement of phosphate from organisms to decomposers.
5. **Assimilation/uptake of phosphate by plants** – Plants absorb phosphate from the soil into their roots.
6. **Decomposition** – Decomposers break down organic matter, releasing phosphorus back into the soil.
7. **Runoff to terrestrial or aquatic systems** – Phosphate from is brought to land, streams, rivers, lakes or oceans through runoff.
8. **Runoff of phosphate from soil to aquatic system** – Phosphates moves from soil to bodies of water.
9. **Sedimentation** – Phosphates sink and become part of sediments, which over time may form new phosphate rock.
Each of these arrows represents a flux that transfers phosphate from one reservoir (such as rocks, soil, water, or living organisms) to another.
[q multiple_choice=”false”]List three reasons why phosphorus is considered essential for living organisms?
[c]U2hvdyBzYW1w bGUgYW5zd2Vy[Qq]
[f]UGhvc3Bob3J1cyBpcyBuZWVkZWQgdG8gbWFrZSBpbXBvcnRhbnQgYmlvbG9naWNhbCBtb2xlY3VsZXMgbGlrZSBBVFAsIEROQSwgUk5BLCBhbmQgcGhvc3Bob2xpcGlkcy4gSXQgcGxheXMgYSBrZXkgcm9sZSBpbiBlbmVyZ3kgdHJhbnNmZXIsIGdlbmV0aWMgaW5mb3JtYXRpb24sIGFuZCBjZWxsIG1lbWJyYW5lcy4=
[q multiple_choice=”false”]What makes the phosphorus cycle different from the carbon and nitrogen cycles?
[c]U2hvdyBzYW1w bGUgYW5zd2Vy[Qq]
[f]VW5saWtlIGNhcmJvbiBhbmQgbml0cm9nZW4sIHBob3NwaG9ydXMgcmFyZWx5IGVudGVycyB0aGUgYXRtb3NwaGVyZS4gSXQgY3ljbGVzIG1vc3RseSB0aHJvdWdoIHJvY2tzLCBzb2lsLCB3YXRlciwgYW5kIGxpdmluZyB0aGluZ3Mu
[q multiple_choice=”false”]Describe the role of decomposers in the phosphorus cycle.
[c]U2hvdyBzYW1w bGUgYW5zd2Vy[Qq]
[f]RGVjb21wb3NlcnMgYnJlYWsgZG93biBkZWFkIHBsYW50cyBhbmQgYW5pbWFscyBhbmQgcmVsZWFzZSBwaG9zcGhhdGUgYmFjayBpbnRvIHRoZSBzb2lsLCBtYWtpbmcgaXQgYXZhaWxhYmxlIHRvIHBsYW50cy4=
[q multiple_choice=”false”]How have humans altered the phosphorus cycle?
[c]U2hvdyBzYW1w bGUgYW5zd2Vy[Qq]
[f]SHVtYW5zIGFwcGx5IHBob3NwaGF0ZS1iYXNlZCBmZXJ0aWxpemVycyB0byBmaWVsZHMsIGluY3JlYXNpbmcgdGhlIGFtb3VudCBvZiBwaG9zcGhhdGUgaW4gZWNvc3lzdGVtcy4gTXVjaCBvZiB0aGlzIHBob3NwaGF0ZSBydW5zIG9mZiBpbnRvIHdhdGVyLCBkaXNydXB0aW5nIGFxdWF0aWMgZWNvc3lzdGVtcy4=
[q multiple_choice=”false”]What is the phosphorus crisis, and why is it a concern?
[c]U2hvdyBzYW1w bGUgYW5zd2Vy[Qq]
[f]UGhvc3BoYXRlIHJvY2sgaXMgYSBub24tcmVuZXdhYmxlIHJlc291cmNlLCBhbmQgd2XigJlyZSBydW5uaW5nIG91dCBvZiBpdC4gU2luY2UgYWdyaWN1bHR1cmUgZGVwZW5kcyBvbiBwaG9zcGhhdGUgZmVydGlsaXplcnMsIGEgc2hvcnRhZ2UgY291bGQgdGhyZWF0ZW4gZ2xvYmFsIGZvb2QgcHJvZHVjdGlvbi4=
[q multiple_choice=”false”]Describe what’s happening in letters m, i, j, and k of the diagram below.
[c]U2hvdyBzYW1w bGUgYW5zd2Vy[Qq]
[f]bTogcGhvc3BoYXRlIGZyb20gYWdyaWN1bHR1cmUgKGZyb20gZmVydGlsaXplcikgaXMgZW50ZXJpbmcgYSBsYWtlIG9yIHRoZSBvY2VhbiAoYW4gYXF1YXRpYyBlY29zeXN0ZW0p
aTogdGhlIHBob3NwaGF0ZSBpcyBhYnNvcmJlZCAob3IgdGFrZW4gdXAsIG9yIGFzc2ltaWxhdGVkKSBieSBhcXVhdGljIHBsYW50cyAob3IgYWxnYWUsIG9yIHByb2R1Y2Vycyk=
Cg==ajogVGhlIHBob3NwaGF0ZSBtb3ZlcyBmcm9tIHRoZSBhcXVhdGljIHByb2R1Y2VycyB0byBmaXNoIHRocm91Z2ggY29uc3VtcHRpb24=
[Qq]k: organic remains and wastes leave aquatic systems as they fall down into marine sediments.
[/qwiz]
7. What’s Next
- Continue to the next tutorial, Topic 8.2, Part 6, Food Chains and Food Webs