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  • 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 million cubic miles of water

  • sitting right in front of you, but you can't drink any of it.

  • Or having to sit next to Meghan Kale every day in English 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 phosphorous.

  • 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, 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% phosphorous.

  • Those numbers might not sound super significant,

  • but even though we've just got teensy bits of this stuff

  • in our bodies, we need nitrogen to make amino acids, which make

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

  • DNA and RNA also require phosphorous, not to mention that phosphorous

  • is the P in ATP, and the phospho- in phospholipid bilayer.

  • So, we might not need a ton of this 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 phosphorous.

  • 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 going to 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.

  • 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 around 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, and 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 or

  • 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 called

  • nitrifying bacteria can take this ammonia and convert

  • it into nitrates, 3 oxygens atoms attached to a nitrogen atom,

  • and nitrites, 2 oxygens attached to a nitrogen

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

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

  • there'd be a whole lot less biologically available nitrogen

  • hanging around, and as a result, there'd be a lot fewer

  • livings 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, RNA and amino acids,

  • organic nitrogen takes off up the food chain.

  • Animals eat the plants and use all that sweet,

  • sweet bioavailable 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 into ammonia,

  • and it just keeps going...until one day that organic nitrogen

  • finds itself in denitrifying bacteria, whose job it is

  • to metabolize the nitrogen oxides and 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 could 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 phosphorous is that it's the only

  • element we're going to talk about that doesn't involve the atmosphere.

  • Phosphorous wants nothing to do with your air!

  • However, the lithosphere, fancy word for the Earth's crust,

  • is amply supplied with phosphorous.

  • 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

  • phosphorous-rich bodies piled up

  • and made phosphorous-rich rocks over time.

  • Unfortunately, there aren't a lot of rock-eating organisms

  • on Earth, just a couple of 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 phosphate back into the soil or water.

  • And phosphates get about as much downtime in the soil as a

  • 20 dollar bill on the sidewalk.

  • Decomposed phosphate is immediately re-assimilated

  • 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 phosphorous atom makes it's 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 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.

  • 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

  • onto 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 of 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 phosphorous.

  • This takes us to 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 DePastino 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 that 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.

There's nothing quite so terrible as needing something

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氮磷循環。循環利用!第二部分--生態學速成班 #9 (Nitrogen & Phosphorus Cycles: Always Recycle! Part 2 - Crash Course Ecology #9)

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    Chi-feng Liu 發佈於 2021 年 01 月 14 日
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