字幕列表 影片播放 列印英文字幕 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.