Biogeochemical Cycling and Global Climate Change

1:12new territories. I mean, we are now seeing plague diagnosed in places like Idaho and Wyoming. Which is just staggering. It is a profound and honestly pretty unsettling shift. But to really understand why the Earth systems are changing so rapidly, we actually have to look much, much smaller than insects. Right. We have to look at the microbes themselves. Which perfectly sets the mission for today's deep dive. We are opening up our source notes on Chapter 28 of Prescott's Microbiology.

1:44Yes, the 12th edition. Right. And we're going to look at something called biogeochemical cycling. Right. But I want to promise you right now, this isn't just a deep dive about tiny bugs sitting in a petri dish in a lab somewhere. No, definitely not. This is about how microbes literally run the Earth's engine. I mean, they control the climate, the soil, the water. And we're going to explore how human activities are throwing this incredibly delicate mass balance completely out of whack.

2:11Yeah. And I know biogeochemical cycling is a hefty piece of vocabulary. A mouthful. It really is. But if we break it down, it's fundamentally just the sum of all the microbial, physical, and chemical processes that drive the flow of elements. Things like carbon, nitrogen, and sulfur. Exactly. The flow of those elements between the sediments, the water, and the atmosphere. Yeah. Because without these elemental transformations, without this constant flow of nutrients from one chemical state to another, all life on Earth would simply cease to exist.

2:43Just full stop. Full stop. Microbes are the invisible gears turning the entire planetary system. Okay. Let's unpack this. If microbes are the gears, what are the actual rules of the machine? Right. Because to understand how they cycle elements on a global scale, we first have to understand the local chemical rules that dictate well which microbes live where and honestly what they even eat. Right. And the absolute most important rule in any local environment is something called redox potential.

3:11Okay. Or yeah, as it's abbreviated. That stands for oxidation reduction potential. Yeah. I am going to need you to translate that into plain English for me. Fair enough. At its core, redox potential is just a measure of a system's tendency to either accept or donate electrons. Okay. With electrons being the currency of energy here. Precisely. You can think of the environment's redox state as the local economy.

3:35It dictates which oxidized compounds are available for microbes to use as terminal electron acceptors. And the terminal electron acceptor is basically just the chemical a microbe uses to breathe, right? Yeah. Like how we use oxygen. Yes, exactly. But when oxygen isn't available, microbes have to use something else to breathe, which is what we call anaerobic respiration. Right. And the visual in chapter 28 paints this perfect picture to understand how this works in practice.

4:04So if you don't have the book in front of you, just imagine you're in a submarine. I love this analogy. Yeah. You're diving down into a column of marine sediment, like ocean mud. You start at the top, which is the oxic zone. Right. Plenty of oxygen there. Exactly. There's plenty of oxygen, so microbes use it to breathe. It yields the absolute most energy. But as you dig deeper into the muck, you cross into the anoxic zone, the oxygen completely runs out.

4:28And what's fascinating here is how structured the microbial community becomes once that oxygen is gone. It's not just a random jumble of bacteria down there. The distribution of microbes is strictly dictated by the free energy yield of whatever chemical they can use next. Okay. I want to try an analogy here. It's like a corporate ladder of energy efficiency. Okay. Let's hear it. Oxygen is the CEO. It gives the highest energy paycheck.

4:54So the microbes at the top of the mud are thriving. But when you go deeper and the CEO leaves the room, the microbes have to report to the VP. Which in this case is nitrate. Right. Nitrate offers a slightly lower energy paycheck. That is a brilliant way to look at it. And it continues all the way down that sediment column. Right. When the nitrate gets used up, you go deeper and different microbes are surviving by reducing manganese.

5:18Okay. Below them, they're breathing iron, then sulfate. And finally, right at the very bottom of the barrel where the energy payout is the absolute lowest, you hit methanogenesis. Microbes surviving by making methane. Exactly. It really is a perfect vertical zonation. The deeper you go, the worse the paycheck. But nature always finds a way to bend the rules. Right. The text mentions an incredible example of nature's ingenuity here.

5:44Oh. Cable bacteria. Oh, cable bacteria are phenomenal. They belong to the disulfoabalbeche family. Okay. And if you picture that sediment column we just described, there is a chemical called sulfide deep down in the anoxic zone that these bacteria want to use for energy. But to get the maximum paycheck, they need oxygen. Right. Which is way up at the top. The two chemicals are spatially separated by centimeters of mud.

6:06Which is a massive distance for a microscopic organism. I mean, it's huge. It is. So what do they do? They form a literal living extension cord. That's wild. They are multicellular and they use this motility to position themselves vertically in the mud. The cells at the very bottom grab the electrons from the sulfide. Okay. And they pass those electrons cell by cell like a bucket brigade all the way up the filament to the cells at the surface that have access to the oxygen.

6:34It is a stunning adaptation. I mean, they physically bridging two completely different redox zones to game the energy system. Exactly. Now, before we move to specific elements like carbon and nitrogen, we need to clarify two crucial vocabulary terms from the chapter. Right. Because these dictate how all these nutrients move. Mineralization and immobilization. Right. Because elements don't just flow smoothly. You know, they get trapped or released. So mineralization is when organic matter is decomposed down into simple inorganic compounds.

7:07Yes. Think of a complex carbohydrate being broken down by microbes until it's just carbon dioxide or methane released into the air. Got it. And immobilization is the exact opposite. Exactly. That's when nutrients get locked up into cellular biomass. Like when a microbe uses carbon to build its own cell wall, that carbon is temporarily unavailable for the rest of the ecosystem. It is immobilized. Right. It stays locked in that tiny cellular vault until that microbe dies and saprophytic microbes.

7:34Wait, saprophytic. We're talking about the cleanup crew here, right? The microbes that specifically eat dead and decaying matter. Spot on. The cleanup crew degrades the dead microbe, mineralizing its components right back into the cycle. Okay. So now that we know the rules of electronics change and we understand how elements get locked up or released, let's look at the heavy hitter. The big one. The element that literally defines life.

7:56Carbon. Yeah. To understand the global carbon cycle, we have to track three things. Sources, sinks, and reservoirs. Okay. Sources are what release carbon dioxide into the atmosphere. This includes humans burning fossil fuels, obviously, but it also includes heterotrophic microbes. Meaning microbes that have to eat other organic matter to survive. Yeah. Just like we do. Right. And as they eat and respire, they breathe out CO2. Yes. Sinks, on the other hand, pull CO2 out of the atmosphere.

8:26We always think of forests and plants doing this. Right. Trees. But marine phytoplankton and cyanobacteria are massive sinks. In fact, microbes fix at least half the carbon on Earth. Wow. Half. That's incredible. And then there are the reservoirs, which store carbon for geological periods of time. But carbon isn't just cycling as CO2. There's also methane. And here's a detail from the text that is so specific but so important.

8:52Only one specific group of microorganisms can biologically form methane. Yes. The archaea. They are a distinct domain of single -celled organisms and only they can perform methanogenesis. And they strictly do it anaerobically, right? Without oxygen. Exactly. You find them in places where oxygen is depleted, but organic matter is abundant. Places like rice paddies, landfills, marshes, and famously the guts of ruminant animals like cows. Right. I have a question about breaking down carbon.

9:22We talk about it like it's an easy fluid process. But what about really tough carbon structures? Like what? Well, like what happens to dead wood? Wood is made of lignin, which the book says is notoriously stable. That is a great question. Lignin is a highly complex polymer. It is held together by incredibly tough carbon bonds. Filamentous fungi and some bacteria can secrete specific hydrolytic enzymes to degrade it, essentially breaking those tough bonds apart.

9:48But here is the catch. That chemical cleaving process requires oxygen as a reactant. Oh. So what happens if a tree falls in a swamp where there is no oxygen? If it falls in an anoxic environment, those enzymes just can't work. It degrades so agonizingly slowly that the woody material just accumulates layer upon layer. Wow. This is exactly how peat bogs are formed over thousands of years. But even with oxygen, microbes breaking down complex carbon can hit a sudden wall.

10:18Right. And this brings us to Liebig's law of the minimum. I love this concept so much. It's highly intuitive once you grasp it. If a microbe is munching through a huge pile of carbon, it still needs other elements to build its cells. You know, it needs nitrogen, phosphorus, iron. Yeah. Liebig's law states that growth is dictated not by total resources, but by the scarcest resource. So it's a bottleneck.

10:42Exactly. A microbe might have an endless buffet of carbon in front of it, but if it runs out of nitrogen, the whole degradation process completely stalls. Right. The carbon cycle is halted because of the bottleneck in the nitrogen cycle. Exactly. Since a lack of nitrogen can literally stop the carbon cycle in its tracks, we really need to shift our focus there. The nitrogen cycle. And looking at the diagrams in Prescott specifically, figure 28 .4, this cycle is a beast.

11:09I mean, it looks like a tangled web of arrows. It is highly complex, largely because nitrogen exists in so many different redox spates. Right. Ranging all the way from minus three to plus five. That means it can act as an electron donor in some environments and an electron acceptor in others. Wow. It is chemically very stubborn, but also incredibly versatile. Let's slow this down and conceptually map out the key processes here, just so we don't get lost in the chemistry.

11:34Good idea. First, you have to actually get nitrogen into the biological system that's fixation. Yes. Taking nitrogen gas, N2, from the atmosphere and converting it into a usable form like ammonia. Right. And that N2 gas is held together by a triple bond. It is incredibly hard to break. Only specialized microbes can do it. They're either free living in the soil or acting as plant symbionts, like the rhizobia bacteria that live in the root nodules of legumes.

12:02Okay. So once it's ammonia, it gets incorporated into organic matter. But when that plant or microbe dies and decays, that ammonium is released back into the soil. Yes. And because ammonium is fully reduced, it can only act as an electron donor. And that leads to the next major process. Which is nitrification. And here's where it gets really interesting. Okay. Lay it on me. Nitrification is the process of oxidizing ammonium first to nitrite and then to nitrate.

12:29For the longest time, science taught that this required a tag team effort. It's like an assembly line. Exactly. One group of bacteria did the first step, handed off the chemical, and a totally different group did the second step. But the textbook notes that scientists recently discovered a bacteria called nitrospera that can do the entire assembly line all by itself, right? Yes. They dubbed it the process chamamix, short for complete ammonia oxidation.

12:53That's amazing. It completely changed our understanding of the nitrogen food web. It proved that a single organism could reap the energy benefits of the entire chain. Wow. But there is another shortcut in the cycle that is equally fascinating. Anamix. Oh, right. Walk us through anamix. This is an anoxic reaction performed by very unique bacteria, primarily in the plankton isotophyllum. Okay. They take ammonium and nitrate. And rather than slowly oxidizing them through the usual chain, they force them to react directly into nitrogen gas.

13:27Completely bypassing the nitrates. Exactly. Which makes them absolute superstars in wastewater treatment. I mean, if you have municipal water filled with unwanted ammonia, these anamix bacteria are exactly what you want to safely and quickly off -gas it back into the atmosphere. Right. Without needing to pump in tons of expensive oxygen. That is so smart. And finally, we have denitrification. If nitrate isn't used to build biomass, some microbes use it as that terminal electron acceptor we talked about earlier.

13:55Right. They breathe it in anaerobically. Yes. This dissimilatory reduction, meaning they aren't incorporating it, just using it to breathe, can return nitrogen gas to the atmosphere, completing the cycle. But, and this is a critical but, from the text, it can also produce nitrous oxide N2O. Yeah. Which is a very potent greenhouse gas. Yeah. We will definitely be coming back to that nitrous oxide in a minute. Definitely.

14:21But before we look at the global climate impacts, I want to make sure we don't ignore the supporting cast. We've seen how carbon and nitrogen build life, but the environment's thermostat and navigation systems are often controlled by trace elements. It's true. Phosphorus, for example, is essential for ATP and nucleic acids, and it's mostly derived from the slow weathering of rocks. Right. But then you have an element like sulfur, which is incredibly dynamic.

14:43Like nitrogen, it exists in many oxidation states. The detail about the sulfur cycle that really caught my eye is what happens out in the ocean. Oh, the DMSP. Yes. Marine phytoplankton produce this compound called DMSP, and when those plankton die, bacteria eat that DMSP and convert it into a gas called DMS, dimethyl sulfide. That DMS escapes into the atmosphere, and it actually serves as a nucleus for water droplets to form around.

15:11Yeah, and if we connect this to the bigger picture, those microscopic bacteria are literally seeding cloud formation over the ocean. Which is wild. It is. And because clouds reflect sunlight away from the Earth, this invisible sulfur reaction is actively helping to keep the planet cool. It's like nature's air conditioning. Exactly. But to build the enzymes to do any of this, microbes need specific trace metals. Which brings us to the iron cycle.

15:38The challenge with iron is that in an oxygen -rich environment with a neutral pH, it's mostly insoluble. Because it's locked up as rust, basically. Ferric iron. Yes. It is incredibly hard for a microbe to absorb. So microbes have to secrete these little molecular grappling hooks called cidophores. Right. They bind the iron in the environment and drag it back into the cell. But some microbes use iron for more than just basic nutrition.

16:03There are magnetotactic bacteria that take extra cellular iron and transform it into magnetite inside their cells. I couldn't believe this when I read it. They're building intracellular magnetic compasses. It's amazing. They literally use the Earth's magnetic field to navigate up and down in swamps. They use this iron compass to find the exact perfect microscopic layer of oxygen they need to survive. Yeah. It is like built -in GPS made of iron.

16:30It is brilliant biology. And similar redox cycling happens with other metals like manganese across anoxic interfaces. But this ability of microbes to pull heavy metals from the environment has a terrifying dark side when it meets human pollution, specifically with mercury. Okay. I'm struggling to picture this. How does inorganic mercury sitting in some anoxic mud suddenly cause a massive neurological disaster in humans miles away? It comes down to a process called methylation.

16:59When inorganic mercury from industrial waste settles into anoxic sediments, anaerobic bacteria, specifically a genus called desulfovibrio, take it up and add a methyl group to it. And why is that dangerous? Because methylated mercury is volatile and crucially, it's lipid soluble. It absorbs into fat. Oh, I see. And this leads to biomagnification. Got it. So a plankton absorbs a little bit of this methylated mercury into its fat.

17:25Right. A small fish eats a thousand plankton, keeping all that mercury. A big fish eats a hundred small fish. And because it stores in the fat and isn't excreted, the mercury concentrates exponentially at every single step of the food chain. Exactly. Our sources specifically reference the tragic 1950s Minamata Bay poisoning in Japan. Right. Where humans ate heavily contaminated seafood from the very top of that food chain, leading to severe permanent neurological disorders.

17:55Wow. It is a stark warning of what happens when natural microbial cycles process human industrial waste. Which perfectly sets up the final overarching theme of our dive today. Yeah. For billions of years, these microbial cycles, carbon, nitrogen, sulfur, iron, they interacted in a steady state mass balance. Yes. The amount of an element entering the cycle was basically equal to the amount leaving it. But now human activity has tipped those scales.

18:19Global climate change is fundamentally a mass balance problem. We are moving carbon and nitrogen from the earth into the atmosphere way faster than the microbial sinks can pull it back down. Let's look at the sources in sinks again with that in mind. Fossil fuels are organic matter that took millions of years of microbial carbon fixation to create. Yeah. And when we burn them, we release that sequestered carbon in a fraction of a second.

18:45At the same time, deforestation is removing the very sinks needed to reabsorb it. Exactly. And as the temperature warms, it creates dangerous feedback loops. For example, the permafrost in the subarctic is melting. Okay. That allows soil microbes that have been frozen for millennia to wake up and start respiring, which releases even more CO2 and methane into the air. And just looking purely at the raw data presented in Chapter 28, without any political spin whatsoever, the measurements track a 1 .2 degrees Celsius increase in average global temperature since 1880.

19:17Right. A trend directly correlated with the industrial era. And the data on methane in the text is particularly notable. Atmospheric methane levels have increased 2 .5 times over the last 150 years. Which matters immensely because, as we noted earlier, a single methane molecule has 30 times the heat trapping potential of a molecule of CO2. Exactly. But the disruption to the carbon cycle is really only half the story.

19:42The disruption to the nitrogen cycle is arguably just as profound. Oh, absolutely. And it mostly comes down to something called the Haber -Bosch process. Right. The Haber -Bosch process is how we make synthetic fertilizer. Remember how we said the nitrogen gas triple bond is incredibly hard to break? Yeah. Well, humans figured out how to do it artificially. We use intense industrial heat and pressure to force nitrogen gas to become ammonium.

20:07Wow. It is the backbone of modern agriculture, honestly. It allows us to field billions of people. But the scale is staggering. Since 1940, fertilizer use is up 500 -fold. So what does this all mean for the microbes? Well, think about Leibig's law again. The bottleneck. Exactly. Plants can only absorb so much nitrogen before they hit a bottleneck with some other nutrient, like phosphorus. So what happens to all that extra synthetic ammonium?

20:33We dump it on a field, the plants take what they can, and the rest washes away. Right. It runs off into lakes and streams, causing eutrophication. You know, those massive suffocating algae blooms you sometimes see covering a pond. Yeah, that's the visible effect. But the real invisible danger is in the soil. All that excess ammonium kicks off a massive microbial chain reaction. Okay, walk us through it.

20:57First, chemolithotrophic microbes, the ones that eat inorganic chemicals for energy, they gorge on that ammonium. They use it for nitrification, turning it into massive amounts of nitrate. Right, which floods the next step of the system. Other microbes see all that excess nitrate and use it for denitrification. Breathing it in anaerobically. Exactly. And that denitrification process releases N2O nitrous oxide into the atmosphere. And as we mentioned, this is a greenhouse gas with 280 times the global warming potential of CO2.

21:27280 times. Yeah. By trying to feed the planet, we have inadvertently flooded the microbial nitrogen cycle, resulting in the highest N2O levels in 650 ,000 years. It really reframes the whole concept of climate change for me. It's not just about smokestacks and tailpipes. No. It's about fundamentally altering the chemical diet of the microbes beneath our feet, which leaves us with a pretty heavy thought to mull over as we wrap up.

21:54Microbes are incredibly resilient. They have adapted to extreme environmental changes over billions of years. Right. But the current rate of greenhouse gas accumulation is unprecedented in the history of life on Earth. As global temperatures rise, environments are shifting fast, lakes are shrinking, and as our notes point out, frinking lakes alter the local balance of those methanogenic archaea in the soil, changing how much methane they produce. So here is the question to leave you with.

22:24As the environment rapidly changes, will microbial evolution find a new equilibrium that helps stabilize the planet? Or will these shifts in the microbial world ultimately accelerate global warming beyond our control? It is without a doubt the defining question of modern microbial ecology. Absolutely. A warm thank you from the Last Minute Lecture team.