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Name: _______________________ Date: ________________________ Flynt - _____ Period ___th Grade Science Connection: What are the 5 main gases that make up the atmosphere? Main Idea: What are 4 examples of how nitrogen is important to living things? Vocab: What is a nutrient? Connection: Can you think of any other nutrients? Review: The prefix “di-” means _______. Review: What are eukaryotes? Vocab: What is a compound? Connection: Can you list any other examples of compounds? Vocab: What is Nitrogen Fixation? Summary: As you already know, ____________________ is the most abundant gas in Earth’s atmosphere. You also know that nitrogen is essential to all living things. Nitrogen is found in nucleic acids like DNA and RNA, which are molecules that carry the genetic code and allow inheritable traits to be passed down from one generation to the next. Nucleic acids also serve as the blueprints of life, as they carry instructions for building proteins and enzymes, as well as the directions for numerous cell structures and processes. Proteins and enzymes, in turn, are also made up of nitrogen. Protein and enzymes serve as the workhorses and building blocks of many cell structures and processes. Therefore, all organisms need nitrogen in order to build cell structures, carry out cellular processes, repair worn out cell parts, and make new cells (growth and reproduction). In plants, nitrogen is also part of chlorophyll (klor-oh-fill), the green pigment of the plant that is responsible for photosynthesis! Nitrogen is a great example of a nutrient (noo-tree-int). A nutrient is a substance that an organism must have in order to survive, but cannot make on its own. So if organisms cannot make their own nitrogen, how do they get it? Every time you take a breath, you are breathing in nitrogen gas (N2). In fact, _____% of each breath that you take is composed of nitrogen gas. Nitrogen gas in the atmosphere is made of two nitrogen atoms joined together; thus, it is sometimes called di-nitrogen gas. It is also sometimes called molecular nitrogen, since molecules are substances made of two or more atoms joined together (even if they are the same type of atom). Unfortunately, our bodies cannot absorb and use the di-nitrogen gas from the atmosphere. The reason why has to do with the chemical properties of di-nitrogen gas. Molecular nitrogen is often said to be inert, or unreactive. This is because of the strong triple bond between the two nitrogen atoms; the triple bond makes N2 molecules very difficult to break apart, and the process of splitting molecular nitrogen is very “expensive” energetically. Thus, while nitrogen is all around us in the atmosphere, most organisms can’t “afford” to spend energy breaking dinitrogen molecules apart, and therefore never evolved the ability to do so. Instead, all single-celled and multi-cellular eukaryotic organisms depend on other organisms and natural processes to supply nitrogen compounds that CAN be easily broken and recombined to form nucleic acids and proteins! In order for organisms to obtain the nitrogen they need, the molecular nitrogen first must be split so that each nitrogen atom can be recombined with other atoms to form a nitrogen compound. A compound is a molecule made of two or more different kinds of atoms joined together. The act of breaking the triple bond in molecular nitrogen (N2) apart so that its atoms can combine with other atoms to form new, biologically available nitrogen compounds is called nitrogen fixation. And as you can imagine, breaking that triple bond requires the input of a lot of energy; thus nitrogen fixation is very “expensive” in terms of energy. Page 2 of 9 Main Idea: What are the three methods of nitrogen fixation? NITROGEN FIXATION: Nitrogen fixation is the building up or “synthesis” part of the nitrogen cycle! There are three ways that nitrogen can get fixed: 1. Atmospheric fixation (lightning fixation). 2. Biological fixation: a. By symbiotic bacteria living in the root nodules of legumes (Rhizobia). b. By free-living bacteria in the soil or water (Azotobacter, Cyanobacteria, and nitrifying bacteria). 3. Industrial/Anthropogenic Fixation (artificial fixation by humans). Connection/Key Concept: Which of Earth’s “spheres” interact in lightning fixation? Connection/Key Concept: How is the water cycle connected to the nitrogen cycle? Micro = Scopic = Connect: What are prokaryotic organisms? Main Idea: What is the purpose of this paragraph? (What point is being made?) Lightning fixation happens in the atmosphere and probably contributes around 5– 8% of the total fixed nitrogen cycling in the biosphere. Lightning is able to provide the enormous amounts of energy needed to break nitrogen (N2) molecules apart, enabling their atoms to combine with oxygen (O2) molecules in the air, forming nitrogen oxides. These nitrogen oxides dissolve in rain and cloud droplets in the atmosphere, forming a new nitrogen compound called nitrate (NO3-). The nitrate then gets carried by precipitation like rain and snow to the ground, where it can infiltrate into and percolate through the soil. Once it has soaked into the soil, the nitrate can be absorbed (soaked up) by the roots of plants. However, not all of the nitrate ends up in the soil. Nitrate can also be carried by surface runoff into rivers, lakes, canals, and even back to the ocean. The nitrogen can also become part of the groundwater and flow into aquifers in a process called leaching. Lightning fixation only produces a tiny bit of the fixed nitrogen that living things need. Instead, most fixed nitrogen is produced by bacteria. In fact, bacteria play a key role in the nitrogen cycle and making soil fertile. They convert molecular nitrogen gas (N2) in Earth’s atmosphere into compounds like ammonium (NH4+) or nitrate (NO3-), both of which can be absorbed by plants and algae and then converted into organic macromolecules like proteins. For a long time, microbiologists thought that bacteria were the only organisms that were able to carry out nitrogen fixation. Today, there is growing evidence that some types of Archaea can fix nitrogen as well! Still, most of the fixed nitrogen that cycles through the biosphere (and especially through land-based ecosystems) is contributed by the activities of nitrogen-fixing bacteria. So just what are bacteria? Bacteria are tiny, microscopic, prokaryotic organisms that are made up of only one cell. They are so small that hundreds of thousands of bacteria would fit on a rounded dot made by a pencil (). Summary Page 3 of 9 Key Concept: What are the two main types of “fixed” nitrogen that plants can use? Key Concept: Which types of organisms CAN and CANNOT fix nitrogen? Sym = Biotic = Key Concept: Legumes include peas, soybeans, peanuts, clover, alfalfa, and lupine. Connection: What are the other types of symbiotic relationships? Bacteria are some of the oldest organisms on Earth. The earliest bacteria probably lived about 3.5 billion years ago, long before humans or other plants and animals! Today, bacteria live all around us and within us. Bacteria live in the deepest parts of the ocean and deep within Earth. They are in the soil, in our food, and on plants and animals. Even our bodies—inside and out—are home to many different kinds of bacteria. Our lives are closely intertwined with theirs, and the health of our planet depends very much on their activities. While it is true that a few types of bacteria can be harmful to humans and other mammals, most bacteria are in no way harmful and instead are extremely important to keeping our planet healthy! Some of the bacteria that fix nitrogen live in a mutually beneficial symbiotic (sim-by-ah-tik) relationship with certain plants. Symbiotic literally means “living things living together.” Plants in the legume (ley-goom) family—which includes beans, peas, and clover—have special root structures called nodules (nod-yewls). Nodules are bumpy growths housing tens of thousands of symbiotic bacteria called Rhizobia (rhyzōh-bee-ya). The Rhizobia bacteria in the nodules “fix” nitrogen for the legume, turning nitrogen (N2) gas into ammonium (NH4+) that the legume can use. In return, the legume makes glucose (and other carbohydrates) using photosynthesis and shares its food with the bacteria in the nodules. The symbiotic relationship between legumes and Rhizobia is mutualistic (both organisms benefit). The legumes receive nitrogen in a form that can easily be converted into nucleic acids and proteins, and the Rhizobia receive carbohydrates produced by the legume during photosynthesis. However, the relationship is actually much more complex than what is described here. Legumes and Rhizobia have coevolved for hundreds of thousands of years; each type of legume hosts its own unique species of Rhizobia, and therefore each type of legume has evolved unique root nodule structures that are specific to the needs of their particular species of Rhizobia. Not all of the “fixed” nitrogen made by Rhizobia in the root nodules gets used by the legume plant. Up until the time the legume begins producing flowers, the root nodules my “leak” excess fixed nitrogen into the soil. When the legume plant dies, any extra ammonium in the nodules or other plant tissues re-enters the soil, making the soil more fertile (better for growing other plants). Farmers often grow legumes like soybeans in order to build up the levels of fixed nitrogen in their soil and make the soil more fertile. After harvesting the Is corn a legume? soybeans, the farmers plow the remnants of the soybean plants into the soil, so Can it fix it’s own that any left-over ammonium in the nodules or plant tissues will be released into nitrogen? the soil. Farmers can then plant crops like corn that DO NOT have symbiotic bacteria living in their roots. Eventually the corn will use up all of the left over fixed Connect: What does nitrogen in the soil, and the farmer will have to grow more legumes to replace the it mean when soil is nitrogen. This is called crop rotation. Small-scale organic gardeners often grow said to be fertile? legumes simultaneously in and around other crops to help add nitrogen to the soil throughout the growing season; this method is an example of companion planting. Summary Page 4 of 9 Key Concept: How do practices like crop rotation and companion planting impact soil fertility? Connection: Cyanobacteria (syan-oh-bak-teer-eeah) play a significant role in what other cycle of matter? Synthesize: How many types of bacteria have been mentioned so far? What are their roles in the Nitrogen Cycle? Key Concept: Why is nitrate leached more than ammonium? Connect: Do you think that nitrate leaching is a problem in Florida? Comprehension Check: What is nitrate enrichment? Summary While the symbiotic relationship between legumes and Rhizobia plays an important part in the nitrogen cycle, most plants do NOT have symbiotic bacteria in their roots. Luckily there are also free-living bacteria that can “fix” nitrogen and turn it into a form that plants can use. Azotobacter (As-zōh-tōh-bak-ter) is a type of nitrogen-fixing bacteria that lives freely in the soil. Like Rhizobia, Azotobacter can carry out chemical reactions that transform molecular nitrogen into ammonium, but Azotobacter adds this ammonium directly to the soil where it is available to all plants, not just legumes. Cyanobacteria (sometimes mistakenly called blue-green algae) are another type of free-living nitrogen-fixing bacteria. Like Azotobacter and Rhizobia, Cyanobacteria also convert nitrogen to ammonium. Cyanobacteria are arguably the most successful group of microorganisms on earth. They occupy a broad range of habitats across all latitudes, and are widespread in freshwater, marine and terrestrial (land-based) ecosystems. Thus, cyanobacteria play a critical role in nitrogen fixation in a variety of habitats all over the world, including in aquatic ecosystems. Technically speaking, there are two distinct processes involved in turning molecular nitrogen into a form that is biologically available: the transformation of unfixed molecular nitrogen into ammonium (nitrogen fixation) and the transformation of ammonium into nitrate (nitrification). Often times, both processes are lumped together and referred to generally as nitrogen fixation. All the bacteria discussed so far—Cyanobacteria, Azotobacter, Rhizobia—are nitrogen-fixing bacteria that convert molecular nitrogen into ammonium. Nitrifying bacteria are different; they take fixation one step further and perform nitrification, a process that converts ammonium to nitrate! While both ammonium and nitrate are biologically available and can be absorbed by plants, too much ammonium can actually be toxic to many organisms. Thus, nitrifying bacteria and the process of nitrification play a critical role in transforming toxic ammonium into the less-toxic nitrates preferred by more plants. Unfortunately, excess nitrate made by nitrifying bacteria does not last very long in the pedosphere, especially in ecosystems that experience periods of heavy rainfall. This is due to the fact that negatively charged nitrate (NO3-) compounds are much more easily leached from soils than positively charged ammonium (NH4+) compounds. Leaching occurs when runoff or groundwater percolating through soils washes away critical nutrients like fixed nitrogen. The ammonium ions are positively charged and therefore stick to (are attracted by) negatively charged clay particles and organic matter in soil. This attraction between positively and negatively charged ions—known as the Law of Attraction—prevents ammonium from being leached out of the soil by groundwater flow. In contrast, the negatively charged nitrate ions are actually repelled by negatively changed soil particles, and so nitrates can be washed out of surface soils in ecosystems where there is heavy rainfall, leading to decreased soil fertility. The leached nitrates don’t just disappear; they are carried downstream by surface runoff, stream flow and groundwater flow, leading to nitrate enrichment of ponds, lakes, aquifers and other downstream bodies of water. Algae and aquatic plants often thrive in aquatic environments enriched by nitrates that were leached from other ecosystems. Page 5 of 9 NITROGEN AND THE FOOD CHAIN Herbivore = Carnivore = Omnivore = Once nitrogen enters a plant, it becomes part of the food chain. The fixed nitrogen absorbed by the plant is used to make proteins, amino acids and DNA (nitrogen-containing organic macromolecules). If the plant gets eaten by an herbivore (or other primary consumer), the herbivore will digest the nitrogencontaining organic macromolecules stored in the plant and use that nitrogen to make new nitrogen-containing organic macromolecules. Through predation, herbivores pass on the nitrogen in their bodies to carnivores (secondary consumers). Omnivores can get the nitrogen they need from both the plants and the animals that they eat. Thus, the only organisms that actually absorb fixed nitrogen from the surrounding environment and allow it to enter the food chain are producers like photosynthetic plants and algae. Decomposers, the final step in the food chain, also play a vital role in the nitrogen cycle. Decomposers are often called “Nature’s recycling system” because of the role they play in the cycling of nutrients like fixed nitrogen back into the abiotic (non-living) environment. Decomposers—which include some types of fungi, mold, mushrooms, earthworms, and even our old friends bacteria—can break down the nitrogen-containing organic compounds stored in the bodies of dead organisms and release that nitrogen back into the soil. The process by which these organic compounds (like proteins, amino acids, and DNA) are chemically changed back into ammonium is called ammonification. Usually, the nitrogen released (produced) by decomposers is ammonium (NH4+). This fixed nitrogen then can be re-absorbed by plants and enter the food chain again. In other words, ammonification returns valuable fixed nitrogen back to the soil (abiotic environment) where it can re-enter to food chain (biotic environment) through uptake by plants. Organic farmers sometimes use decomposers to help create natural soil fertilizer for their crops. Autumn leaves, grass clippings, weeds, fruit and vegetable trimmings, egg shells, coffee grounds, and manure from horses, chickens and cows can be mixed with special decomposing bacteria in a compost bin or compost pile. The decomposing bacteria break down the proteins and other organic nitrogen compounds and turn them back into ammonia. After a certain amount of time, the “mature” compost can then be applied to gardens and fields to enrich the soil, thus turning “garbage” and “waste” into a valuable resource for growing crops. The inert (unreactive) nature of the N2 molecule (remember that triple bond?) means that biologically available (fixed) nitrogen is often in short supply in natural ecosystems. Thus, the availability of fixed nitrogen in an ecosystem can be an important limiting factor. A limiting factor is an environmental factor that is essential for life that is absent or depleted below the critical minimum, or that exceeds the maximum tolerable level for the species. When fixed nitrogen is in short supply, the growth of plants and other producers is restricted, and this cascades up through the entire food chain, limiting biomass growth throughout the entire ecosystem. However, population sizes of organisms in natural ecosystems have evolved in balance with the limited availability of fixed nitrogen sources in each ecosystem. Disrupting the balance of nitrogen, either through addition or removal, therefore can have significant negative consequences on the health of an ecosystem. Summary Page 6 of 9 HUMAN IMPACTS: BURNING FOSSIL FUELS Sometimes plants and animals die and their bodies do not get decomposed. Instead, their bodies may quickly become buried along with other plants and animals. Over millions of years, the remains of these organisms can be turned into fossil fuels like oil, coal, and natural gas. If humans extract (dig up) these fossil fuels and burn them, the combustion reactions produce harmful nitrogen compounds like NO (nitric oxide), N2O (nitrous oxide), and NO2 (nitrogen dioxide) that enter the atmosphere where they are considered pollutants. The nitrogen/oxygen compounds produced by the burning of fossil fuels can cause serious environmental and health-related hazards, including acid rain, photochemical smog, and high levels of tropospheric ozone (remember good up high, bad nearby). Acid rain—also known as acid precipitation— is rain or any other form of precipitation that is unusually acidic, meaning that it has a low pH. Precipitation becomes acidic when combustion products (mainly gasses) from the burning of fossil fuels enter the atmosphere and form condensation nuclei for cloud droplets. Acid rain can have harmful effects on plants, aquatic animals, and infrastructure. The picture shown here illustrates a spruce forest that was destroyed by acid precipitation. Ozone forms when harmful nitrogen compounds released by the combustion of fossil fuels enter the atmosphere and react with oxygen (O2) gas molecules. According to the American Lung Association, your lungs and heart can be permanently affected by ozone pollution and smog. While the young and the elderly are particularly susceptible to the effects of ozone, anyone with both short and long term exposure can suffer ill health effects. Problems include shortness of breath, coughing, wheezing, bronchitis, pneumonia, inflammation of pulmonary tissues, heart attacks, lung cancer, increased asthma-related symptoms, fatigue, heart palpitations, and even premature aging of the lungs and death. N2O is also a dangerous greenhouse gas, with 310 times the ability per molecule of gas to trap heat in the atmosphere. Thus, increased levels of N2O in the atmosphere are exacerbating global warming. Summary Page 7 of 9 HUMAN IMPACTS: ANTHROPOGENIC FIXATION Besides the burning fossil fuels, the application of nitrogen-based fertilizers is another human activity that can significantly impact the nitrogen cycle. Farmers use man-made fertilizers to increase crop yields per acre (more productive harvest on same amount of land), which lowers food prices for consumers. Thus, the use of man-made, nitrogen-based fertilizers is an accepted and common agricultural practice that is of great significance both to farmers and to people like you and me that want to be able to purchase inexpensive food for our families. The nitrogen-based fertilizers used in agriculture are man-made in a process generally referred to as industrial fixation (anthropogenic fixation). More specifically, the Haber-Bosch process produces synthetic fertilizers by causing N2 to react with H2, resulting in the production of ammonium. As second step converts the ammonium to ammonium-nitrate. Industrial fixation using the Haber-Bosch process has increased significantly over the past several decades as agricultural demands have increased. In fact, today, nearly 80% of the nitrogen-containing organic macromolecules found in human tissues originated from the Haber-Bosch process rather than natural atmospheric or biological sources of fixation! The current methods used to apply fertilizers are incredibly inefficient. Not only must fertilizer must be repeatedly added to the soil after each harvest (before the next planting), but very little of the fertilizer added to a field ever actually gets absorbed by the intended target plants. As mentioned earlier, fixed nitrogen can be easily leached from the soil, and so much of the fertilizers used in agriculture ultimately make their way into our rivers, lakes, estuaries and oceans. Nitrates can even end up in our drinking water when runoff carrying fertilizers infiltrates and percolates through the bedrock and contaminates our aquifers. The inefficient use of man-made fertilizers in agriculture is dramatically increasing the amount of biologically available nitrogen in the Earth’s natural ecosystems, even hundreds of miles away from agricultural lands. In fact, human activity has doubled the amount of global fixed nitrogen in the biosphere, and this is becoming a significant environmental issue. Since fixed nitrogen is often a limiting factor, you might think that increasing the amount of fixed nitrogen in an ecosystem would be a good thing. However, the biological communities in an ecosystem co-evolve over millions of years and develop a balance based on the available resources provided by the various cycles of matter (nitrogen cycle, carbon cycle, oxygen cycle, etc.). A change in any one component in any part of one of the cycles of matter can ripple throughout an ecosystem and cause disruptions in other cycles of matter as well. In terrestrial (land-based) ecosystems, the addition of nitrogen can lead to nutrient imbalance in trees, changes in forest health, and declines in biodiversity. As the availability of fixed nitrogen increases, tree growth may temporarily explode and strip the soil of other, non-nitrogen-based nutrients, basically starving other plants with less extensive root systems. As the plant diversity shrinks, a similar collapse occurs in populations of consumer organisms that depend on the impacted plant species. In addition, since trees absorb and store huge amounts of carbon for much longer periods of time than other forest organisms, rapid tree growth can disrupt the cycling of carbon, thus impacting more than just the nitrogen cycle. Summary Page 8 of 9 When nitrogen-enriched runoff reaches a body of water—be it river, lake, spring, estuary, or ocean—the sudden increase in biologically available nitrogen can trigger a population explosion in aquatic algae, an event known as an algal bloom. The rapidly reproducing algae quickly use up important nutrients needed by other aquatic plants and animals (resource depletion). Often the affected water becomes cloudy, typically colored a shade of green, yellow, brown, or red (depending on the species of algae). As the algal bloom grows, it can block sunlight and prevent it from reaching other photosynthetic organisms that live in deeper waters. Eventually, the algae population itself collapses due to exhaustion of resources. As the algae die, they sink to the bottom where they are decomposed. The increase in dead algae causes a spike in the decomposition process, which in turn uses precious amounts of dissolved oxygen, leading to anoxic (no oxygen) or hypoxic (low oxygen) conditions that can kill fish, shellfish (mussels, oysters, crabs, crawfish), amphibians (frogs and salamanders) and other aquatic organisms. This entire process—from algae bloom to the creation of “dead zones” due to polluted runoff—is known as eutrophication. About half of all the lakes in the United States are now eutrophic, while the number of oceanic dead zones near inhabited coastlines is also increasing. There are many different types of algae that can “bloom” suddenly and lead to eutrophication events. Algae is also known as phytoplankton (which literally means “plant-like floaters”) and includes many different kinds of tiny, plant-like, photosynthetic organisms that drift near the surface of the water. Dinoflagellates (dy-nōh-FLAJ-jell-ates) are one of the most important types of phytoplankton. They are small single-celled organisms, which swim freely in water with a forward spiraling motion propelled by two flagella: one flagellum oriented around the cell, and the other directed backwards (like a tail). These single-celled protists have plates covering their cells, and numerous internal cellular structures, including chloroplasts, mitochondria and a nucleus. Many dinoflagellates are important primary producers in aquatic food webs. When certain species of dinoflagellates reproduce in great numbers, the water may appear golden or red, producing and algae bloom known as a "red tide". Red tides sometimes occur naturally; long before the invention of anthropogenic fixation, seafarers and coastal dwellers in certain places would report occasional colorful “blooms” during the warm months of summer. However, over the last several decades, many areas of the world, including the United States and Florida, have experienced a growing trend in the frequency and size of toxic dinoflagellate blooms. There is growing evidence linking red tides to runoff polluted with anthropogenic fixed nitrogen. During a red tide event, many kinds of marine life suffer, and not just because of hypoxic/anoxic conditions and resource depletion. For in additional to all of the typical hazards associated with algal blooms, the dinoflagellate blooms carry an additional threat, since many dinoflagellates produce a neurotoxin which affects muscle function in some organisms. Because of these toxins, red tides have been associated with high mortality (death) rates in populations of fish, shellfish, birds, sea turtles, and even marine mammals like manatee and dolphin. Summary Page 9 of 9 When consumed, the dinoflagellate toxins can be quickly carried up the food chain and passed onto secondary consumers and apex predators—including humans—via fish and shellfish consumption. The human health risks associated with the consumption of fish and shellfish contaminated by red tides include gastrointestinal illness, permanent neurological (nerve/brain) damage, and in rare cases, even death. As the size and frequency of algal blooms like red tides and their associated dead zones continue to increase, socio-economic concerns grow due to the impact on commercial fisheries, human health, and coastal tourism near the affected areas. For our wildlife, the end results potentially include organism death, decreasing biodiversity, and the potential loss of certain species in the local ecosystem (local extinction). In extreme cases, such events may even lead to such drastic changes in food-web structure that an entire ecosystem fails. Because of these concerns, as of 2006, the application of nitrogen-based fertilizer is now increasingly controlled in Britain and the United States. Still, even if eutrophication can be reversed, it may take decades before the accumulated nitrates in groundwater that feeds local rivers and lakes can be broken down by natural processes. Summary