Nitrogen & Phosphorus Cycles: Always Recycle! Part 2 - Crash Course Ecology #9

Summary

This episode of Crash Course Ecology delves into the nitrogen and phosphorus cycles, essential for life but often inaccessible in their natural forms. The script explains how nitrogen-fixing bacteria and plants play crucial roles in making these elements available, while human intervention through synthetic fertilizers has dramatically increased their availability, with both benefits for agriculture and potential environmental consequences.

Takeaways

• 🌊 The nitrogen and phosphorus cycles are essential for life, yet these elements are often unavailable in biologically useful forms despite their abundance.
• 💧 Water is abundant in oceans but not directly consumable for hydration, illustrating the concept of elements being present but inaccessible.
• 🌱 Plants are crucial in making nitrogen and phosphorus available to other organisms by assimilating these nutrients from the environment.
• ⚗️ Nitrogen gas, which makes up 78% of the atmosphere, is difficult for organisms to use due to the strong triple bond between nitrogen atoms.
• 🌿 Nitrogen-fixing bacteria and certain plants, particularly legumes, form symbiotic relationships that allow the conversion of atmospheric nitrogen into a usable form.
• 🔬 Nitrogenase is a unique enzyme that can break the triple bond in nitrogen gas, performed by nitrogen-fixing bacteria.
• 🌩 Lightning and human innovation have also been identified as ways to fix atmospheric nitrogen, contributing to the nitrogen cycle.
• 🔄 The phosphorus cycle does not involve the atmosphere and is primarily driven by the lithosphere, with rocks being a significant source of phosphorus.
• 🪨 Phosphates from eroded rocks are absorbed by plants and enter the food chain, highlighting the importance of the rock-water-plant cycle.
• 🌊 Once in aquatic ecosystems, phosphorus can be trapped in deep bodies of water for extended periods, affecting the phosphorus cycle.
• 🚜 Human use of synthetic fertilizers has dramatically increased the availability of nitrogen and phosphorus, but also led to environmental challenges.

Q & A

Why can't organisms directly utilize the nitrogen and phosphorus that are abundant in the environment?

Organisms can't directly utilize these elements because they are often in a form that is not biologically available. Nitrogen is mostly found as nitrogen gas (N2) with a strong triple bond that is difficult to break, and phosphorus is often locked in rocks or sediments.

What role do nitrogen-fixing bacteria play in the nitrogen cycle?

Nitrogen-fixing bacteria convert atmospheric nitrogen (N2) into ammonia (NH3), which then becomes ammonium (NH4+) when mixed with water. This process makes nitrogen biologically available for plants to assimilate and use.

How do plants obtain nitrogen if they can't break the triple bond in nitrogen gas?

Plants rely on the nitrogen-fixing bacteria that convert atmospheric nitrogen into forms like ammonia or ammonium, which plants can then assimilate.

What is the enzyme that allows nitrogen-fixing bacteria to break the triple bond in nitrogen gas?

The enzyme is called nitrogenase, which is the only biological enzyme capable of breaking the strong triple bond in nitrogen gas.

What is the significance of the phosphorus cycle in relation to the nitrogen cycle?

While the nitrogen cycle involves the conversion of atmospheric nitrogen into biologically available forms, the phosphorus cycle deals with the availability of phosphorus from the lithosphere. Both cycles are essential for the production of essential biomolecules like DNA, RNA, and ATP.

Why is phosphorus different from nitrogen in terms of its cycle?

Phosphorus does not involve the atmosphere and is primarily cycled through the lithosphere. It is found in rocks, particularly in sedimentary rocks that originated from organic matter accumulation in ocean floors and lake beds.

How do plants assimilate phosphorus from the environment?

Plants assimilate phosphorus when it is dissolved into water from eroded rocks. This dissolved phosphate is immediately available for plant uptake.

What happens to the nitrogen and phosphorus once they are assimilated by animals?

Animals use the nitrogen and phosphorus to produce amino acids and other biomolecules. When animals excrete waste or die, decomposers break down the organic matter, releasing the nitrogen and phosphorus back into the environment to continue the cycle.

How do humans impact the nitrogen and phosphorus cycles through the use of synthetic fertilizers?

Humans have significantly increased the availability of nitrogen and phosphorus in ecosystems by introducing synthetic fertilizers, which can lead to environmental issues such as eutrophication and other imbalances.

What is the final destination of phosphorus in the phosphorus cycle?

The final destination of phosphorus is when it becomes part of deep sedimentary rocks, where it can remain trapped for millions of years before being uplifted and weathered back into the cycle.

Why is it important for organisms to have access to biologically available nitrogen and phosphorus?

Access to biologically available nitrogen and phosphorus is crucial for the synthesis of essential biomolecules like amino acids, DNA, RNA, and ATP, which are the building blocks of life.

Outlines

00:00

🌊 The Challenge of Nutrient Availability

The first paragraph introduces the concept of organisms struggling to access nutrients despite their abundance in the environment. It uses the analogy of being unable to drink ocean water or interact with someone out of reach to illustrate this point. The paragraph then focuses on the importance of nitrogen and phosphorus for life on Earth, explaining their roles in the formation of amino acids, proteins, DNA, RNA, ATP, and the phospholipid bilayer. The challenge lies in the fact that these elements are often unavailable in a biologically usable form, with nitrogen being primarily found as atmospheric gas and phosphorus locked in rocks. The solution to this problem is presented through the role of plants, which can assimilate these nutrients, and the script sets the stage for an explanation of how plants overcome this challenge.

05:00

🌱 The Nitrogen and Phosphorus Cycles

The second paragraph delves into the nitrogen cycle, starting with the difficulty of accessing nitrogen in the atmosphere due to its strong triple bond. It explains the role of nitrogen-fixing bacteria in converting atmospheric nitrogen into ammonia, which plants can use, facilitated by the enzyme nitrogenase. The paragraph also covers the process by which nitrifying bacteria convert ammonia into nitrates and nitrites, which are more readily assimilated by plants. The importance of these cycles for life on Earth is emphasized, with a nod to the role of lightning and human intervention in nitrogen fixation through synthetic fertilizers. The phosphorus cycle is then introduced, highlighting that phosphorus is found in the lithosphere rather than the atmosphere. It discusses how phosphorus moves from rocks into water, where it is taken up by plants and enters the food chain. The paragraph concludes by discussing the human impact on these cycles through the use of fertilizers, which has led to environmental challenges that will be explored in future episodes.

Keywords

💡Nitrogen Cycle

The nitrogen cycle is the process by which nitrogen, an essential element for life, is converted into various chemical forms as it circulates through the atmosphere, soil, water, and living organisms. In the video, the nitrogen cycle is highlighted as crucial for making nitrogen biologically available, as atmospheric nitrogen gas is inert and requires conversion by nitrogen-fixing bacteria into ammonia, which plants can then use to create amino acids, proteins, DNA, and RNA.

💡Nitrogen Fixing Bacteria

Nitrogen fixing bacteria are microorganisms that can convert atmospheric nitrogen gas into ammonia, a form that plants can use. The script mentions these bacteria as key players in the nitrogen cycle, often living in symbiosis with legume plants, where they perform the process of nitrogen fixation using the enzyme nitrogenase.

💡Phosphorus Cycle

The phosphorus cycle describes the movement of phosphorus through the Earth's crust, water bodies, and living organisms. Unlike nitrogen, phosphorus is not part of the atmosphere but is abundant in rocks and sediments. The video script explains that phosphorus is essential for life and is made available to the biosphere through weathering of rocks and uptake by plants, highlighting its importance in the formation of ATP and the cell membrane.

💡Amino Acids

Amino acids are the building blocks of proteins and are essential for the structure and function of living organisms. The script explains that nitrogen is required to form amino acids, which in turn are necessary for constructing proteins, emphasizing the link between the nitrogen cycle and the synthesis of biological molecules.

💡DNA and RNA

DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are nucleic acids that carry genetic information in living organisms. The video script points out that both DNA and RNA require phosphorus, illustrating the multifaceted importance of this element in the molecular biology of life.

💡ATP (Adenosine Triphosphate)

ATP is the primary energy currency of cells, used to power various cellular processes. The script mentions ATP as containing phosphorus, indicating its role in energy transfer within cells and the significance of phosphorus in cellular metabolism.

💡Phospholipid Bi-Layer

The phospholipid bi-layer is the structural basis of cell membranes, composed of phospholipids that have a hydrophilic head and hydrophobic tail. The video script notes that phosphorus is a component of these phospholipids, underscoring its role in the structure and function of cell membranes.

💡Decomposers

Decomposers are organisms that break down dead organic material, returning nutrients to the environment. In the script, decomposers are mentioned as part of the nitrogen and phosphorus cycles, where they break down organic matter, releasing nitrogen and phosphorus back into the soil or water.

💡Denitrifying Bacteria

Denitrifying bacteria are microbes that convert nitrates back into nitrogen gas, which is then released into the atmosphere. The script describes these bacteria as part of the nitrogen cycle, playing a role in the return of nitrogen to the atmosphere, thus completing the cycle.

💡Synthetic Fertilizers

Synthetic fertilizers are chemical compounds used to supply essential nutrients to plants, primarily nitrogen, phosphorus, and potassium. The video script discusses the invention of synthetic fertilizers as a human innovation that has dramatically increased food production but also has environmental implications due to the excess nutrients entering ecosystems.

💡Ecological Impact

The ecological impact refers to the effects that human activities have on the environment and ecosystems. The script touches on the consequences of using synthetic fertilizers, such as eutrophication, which can lead to environmental issues like algal blooms and dead zones in water bodies, highlighting the importance of understanding human influence on natural cycles.

Highlights

The nitrogen and phosphorus cycles are essential for life on Earth, despite being abundant, they are often not in a biologically available form.

Nitrogen makes up about 78% of the atmosphere but is difficult for plants to assimilate due to the strong triple bond between nitrogen atoms.

Plants require nitrogen-fixing bacteria to convert atmospheric nitrogen into a usable form such as ammonia or ammonium.

Nitrogenase is a unique enzyme that can break the triple bond in nitrogen gas, performed by nitrogen-fixing bacteria.

Decomposers and other organisms contribute to the nitrogen cycle by breaking down organic matter into ammonia.

Nitrifying bacteria convert ammonia into nitrates and nitrites, which are more easily assimilated by plants.

The phosphorus cycle does not involve the atmosphere and is primarily found in the earth's crust.

Phosphorus is abundant in sedimentary rocks, especially those from old ocean floors and lake beds, rich in organic material.

Phosphates from eroded rocks are readily available to plants and are a critical component of the phosphorus cycle.

Decomposers play a key role in the phosphorus cycle by breaking down organic matter and releasing phosphates back into the soil.

Phosphorus can be trapped in aquatic ecosystems for extended periods before returning to the geological cycle.

Human activities, particularly the use of synthetic fertilizers, have significantly impacted the nitrogen and phosphorus cycles.

Fritz Haber's process of synthesizing ammonia for fertilizers and chemical weapons had a profound impact on agriculture and warfare.

The excessive use of synthetic fertilizers can lead to environmental issues such as eutrophication.

The human impact on the biosphere, including the nitrogen and phosphorus cycles, will be further explored in future episodes.

The episode was a collaborative effort with contributions from various team members including writers, a technical director, and graphics and sound specialists.

Transcripts

There's nothing quite so terrible as needing something that's sitting right in front of you but not being able to get it.

Like say you're on a lifeboat in the ocean and you're super thirsty,

and there's 300,000,000 cubic miles of water sitting right in front of you, but you can't drink any of it.

Or having to sit next to Megan Kale every day in math class,

but knowing she's really dramatically out of your league.

A lot of organisms on earth find themselves in this situation, pretty much constantly,

except that the thing that's everywhere that they can't have isn't water or physical closeness;

it's nutrients, specifically, nitrogen and phosphorus.

Of course, there are tons of elements that cycle around the earth

hanging out in one place or form for a while before moving on to the next,

and as you know, living things need a bunch of stuff.

Animals, for instance, need oxygen and carbon and hydrogen.

These elements basically cover the water cycle and the carbon cycle that I talked about last time,

but we're also about 3% nitrogen and 1% phosphorus.

Those numbers might not sound super significant,

but even though we've just got teensy bits of the stuff in our bodies,

we need nitrogen to make like, amino acids, which make proteins,

which make our whole bodies up, and DNA and RNA, too.

DNA and RNA also require phosphorus, not to mention that phosphorus is the P in ATP,

and the Phosphorus in phospholipid bi-layer.

So we might not need a ton of the stuff, but it is important, and it's hanging out everywhere;

the air we breathe is mostly nitrogen, and the water and rocks all around us are jam-packed full of phosphorus.

But like I said, they're rarely in a form that's biologically available.

And as per usual, the organisms that solve this problem are the plants.

Anything else that needs these nutrients are just gonna have to eat some plants

or eat something that ate some plants.

But how do plants solve this problem? And why is it a problem in the first place?

Well, give me a few minutes, I'll explain.

[Theme Music]

So let's talk about the nitrogen cycle first, since nitrogen really is actually all around us,

like I can feel it right now, there it is in the air.

So why is it so hard to get this stuff that's constantly surrounding us in the air

into our actual bodies to be useful for us?

Because even though nitrogen gas makes up about 78% of the atmosphere,

you'll notice here that nitrogen gas is made up of two nitrogen atoms stuck together with a triple bond.

And it's one thing to break apart a single covalent bond, but three?

So as you can imagine, those two nitrogen atoms are a total pain to pry apart.

But that molecule has to be split in order for a plant to get at the pieces.

In fact, plants can assimilate a bunch of different forms of nitrogen, nitrates, nitrites to a lesser extent,

and even ammonium, which is what you get when you mix ammonia with water.

But all that darn nitrogen gas in the atmosphere is beyond their powers of assimilation.

So plants need help taking advantage of this ocean of nitrogen that we're all swimming in,

which is why they need to have that nitrogen fixed, so that they can use it.

Even though plants aren't wily enough to wrangle those two nitrogen atoms apart,

certain nitrogen fixing bacteria are.

These bacteria hang out in soil or water or even form symbiotic relationships with the

root nodules of some plants, most of which are legumes.

That's a pretty big family of plants: soybeans, clover, peanuts, kudzu, all legumes.

So these bacteria just sit around converting atmospheric nitrogen into ammonia,

which then becomes ammonium when it's mixed with water, which can be used by plants.

They do this with a special enzyme called nitrogenase,

which is the only biological enzyme that can break that crazy triple bond.

Ammonia can also be made by decomposers, fungi, protists,

other kinds of bacteria that munch on your proteins and DNA after you die,

but they're not picky, they like poop and urine, too.

Then once this has happened, other bacteria known as nitrifying bacteria can take this ammonia

and convert it into nitrates, three oxygen atoms attached to a single nitrogen atom, and nitrites,

two oxygen atoms attached to a nitrogen,

and those are even easier than ammonium for plants to assimilate.

So the take home here is that if it wasn't for these bacteria,

there'd be a whole lot less of biologically available nitrogen hanging around,

and as a result, there'd be a lot fewer living things on the planet.

So as usual, thanks bacteria -- we owe you one!

But I should mention that it's not just bacteria who can wrangle those two nitrogen atoms apart.

Lightning, of all things, has enough energy to break the bonds between nitrogens,

which is obviously awesome, and therefore worth mentioning, and in the 20th century,

smartypants humans also figured out various ways to synthetically fix a ton of nitrogen all at once,

which is why we have synthetic fertilizers now and so much food growing all over the place.

Once the atmospheric nitrogen is converted into a form that plants can use to make DNA and RNA and amino acids,

organic nitrogen takes off up the food chain and animals eat the plants and use all that sweet,

sweet bio-available nitrogen to make our own amino acids,

and then we pee or poop it out or die and the decomposers go to town on it,

breaking it down to ammonia and it all keeps going...until!

One day, that organic nitrogen finds itself in denitrifying bacteria,

whose job it is to metabolize the nitrogen oxides,

turn them back into nitrogen gas using a special enzyme called nitrate reductase.

These guys do their business and then release the N2 back into the atmosphere.

And that, my friends, is the nitrogen cycle.

If you remember nothing else, remember that:

a) you owe bacteria a solid because they were smart enough to make an enzyme that can bust

open the triple bonds of nitrogen gas,

b) you owe plants a solid for wrestling nitrogen into their bodies so that you can just eat

a carrot and not have to think about it,

and c) nitrogen is awesome and everywhere and yet, also elusive and deserving of your respect.

So, moving on, to the phosphorus cycle.

The interesting thing about phosphorus is that it's the only element that we're going

to talk about that doesn't involve the atmosphere.

Phosphorus wants nothing to do with your air.

However, the lithosphere, fancy word for the earth's crust, is amply supplied with phosphorus.

Rocks contain inorganic phosphates, especially sedimentary rocks that originated in old ocean floors

and lake beds where living things died and sank to the bottom

where their phosphorus rich bodies piled up and made phosphorus rich rocks over time.

Unfortunately, there aren't a lot of rock-eating organisms on earth, just a couple bacteria,

which are called lithotrophs, by the way.

However, when these rocks are re-exposed and water erodes them,

some of the phosphates are dissolved into the water.

These dissolved phosphates are immediately available to and assimilated by plants,

which are then eaten by animals.

From here, the same thing goes for the decomposers as with the nitrogen cycle,

when a leaf drops or something poops or dies,

the decomposers break it down and release the phosphates back into the soil or water.

And phosphates get about as much downtime in the soil as a $20 bill on the sidewalk.

Decomposed phosphate is immediately reassimilated back into plants and this little cycle just

keeps going and going.

Plants to the animal to the decomposers to the soil and back into a plant

— that is, until that atom of phosphorus makes its way into some kind of body of water.

Because aquatic and marine ecosystems need phosphorus like crazy.

Once a phosphorus atom makes its way into a deep lake or ocean, it cycles around among the organisms there —

algae, plankton, fish, and this cycling can go on for a long time.

I mean, not as long as a phosphorus atom trapped in a rock, that can be millions of years,

but by some estimates, a single phosphorus atom can be caught in a biological cycle for

a 100,000 years.

Eventually, it's in something that dies and falls into a hole so deep that decomposers can't survive there.

Then sedimentation builds up and turns into rock, which are eventually uplifted into mountains

and exposed and the phosphates are weathered back out and it's a cycle!

So yeah, that's the deal with nitrogen and phosphorus, living things need them,

but even though they're all over the place,

they're at a premium in biological systems because they're hard to get at,

either because they have to be converted into a form that organisms can use,

or they're locked away underground.

But you know who the smartest monkeys are? Us.

And yeah, you can bet your face that we've figured out how to unleash all kinds of

nitrogen and phosphorus into this big green planet.

Mostly, in an effort to help feed our children and each other.

We usually mean well, but we can be a bit overbearing sometimes.

It's just the human way, to see something in nature that seems to be lacking or imperfect

and try to make it the best thing ever.

So with the phosphorus and nitrogen cycles, we have introduced fertilizers, lots and lots of fertilizers,

the main ingredients of which are, you guessed it, nitrogen and phosphorus.

The story of how we learned to synthesize nitrogen into ammonia for fertilizers

and chemical weapons is a very, very interesting one involving an evil lunatic,

and I suggest as soon as this is over, you watch this video on Fritz Haber,

the guy who made all this happen during World War I.

You've heard of too much of a good thing, right?

Well, through the miracle of synthetic fertilizers,

we're able to grow much, much more food than we ever have before, and as a result,

ecosystems all over the world are being bombarded by these incredible amounts of nitrogen and phosphorus.

This takes us into the next chapter in our exploration of ecology, the human impacts on the biosphere.

Sometimes, out of our desire to make nature better,

sometimes out of stupid human selfishness, and most often both,

we've ended up really messing up the environment in more ways than we can count.

And that's what we're going to be talking about next week — be sure to wear your gas

mask and HazMat gloves,

and thank you for watching this episode of Crash Course Ecology.

This episode was written by Jesslyn Shields, Blake de Pastino, and myself.

Our technical director is Nick Jenkins, he's also filming this, and he will also be editing it. Sorry, Nick.

Graphics are courtesy of Peter Winkler, and sound is from Michael Aranda.

There's a table of contents over there if you want to review anything we went over in today's episode, and,

of course, we're on Facebook and Twitter and in the comments below if you have any questions for us.

We'll see you next time.