CONCEPTS IN BIOLOGY

PART IV. EVOLUTION AND ECOLOGY

 

15. Ecosystem Dynamics. The Flow of Energy and Matter

 

15.4. The Cycling of Materials in Ecosystems—Biogeochemical Cycles

Except for small amounts of matter added to the Earth from cosmic dust and meteorites, the amount of matter that makes up the Earth is essentially constant. However, energy comes to the Earth in a continuous stream as sunlight, and even this is ultimately returned to space as heat energy. It is this flow of energy that drives all biological processes. Living systems use this energy to assemble organic matter and continue life through growth and reproduction. Because the amount of matter on Earth does not change, the existing atoms must be continually reused as organisms grow, reproduce, and die. In this recycling process, photosynthesis is involved in combining inorganic molecules to form the organic compounds of living things. The process of respiration by all organisms breaks down organic molecules to inorganic molecules. Decomposer organisms are particularly important in breaking down the organic remains of waste products and dead organisms. If there were no way of recycling this organic matter back into its inorganic forms, organic material would build up as the bodies of dead organisms.

Deposits of organic material do build up if decomposers are prevented from doing their job. This occurred millions of years ago, when the present deposits of coal, oil, and natural gas were formed. Today, new organic deposits are forming in swamps and bogs where acid conditions or lack of oxygen prevent decomposers from breaking down submerged vegetation.

One way to get an appreciation of how various kinds of organisms interact to cycle materials is to look at a specific kind of atom and follow its progress through an ecosystem. Carbon, nitrogen, oxygen, hydrogen, phosphorus, and many other atoms are found in all living things and are recycled when an organism dies.

The Carbon Cycle

All living things are composed of organic molecules that contain atoms of the element carbon. The carbon cycle includes the processes and pathways involved in capturing inorganic carbon-containing molecules, converting them into organic molecules that are used by organisms, and the ultimate release of inorganic carbon molecules back to the abiotic environment (figure 15.9).

FIGURE 15.9. The Carbon Cycle

Carbon atoms are cycled through ecosystems. Carbon dioxide (green arrows) produced by respiration is the source of the carbon that plants incorporate into organic molecules when they carry on photosynthesis. These carbon-containing organic molecules—carbohydrates, fats, and proteins—(black arrows) are passed to animals when they eat plants and other animals. Organic molecules in waste products or dead organisms are consumed by decomposers. In the process, decomposers break down organic molecules into inorganic molecules. All organisms (plants, animals, and decomposers) return carbon atoms to the atmosphere as carbon dioxide when they carry on cellular respiration. Oxygen (blue arrows) is being cycled at the same time that carbon is. The oxygen is released during photosynthesis and taken up during cellular respiration.

The same carbon atoms are used over and over again. In fact, you are not exactly the same person today that you were yesterday. Some of your carbon atoms are different. Furthermore, those carbon atoms have been involved in many other kinds of living things over the past several billion years. Some of them were temporary residents in dinosaurs, extinct trees, or insects, but at this instant, they are part of you. Other organic molecules have become part of fossil fuels.

1. The Role of Producers

Carbon and oxygen combine to form the molecule carbon dioxide (CO2), which is present in small quantities as a gas in the atmosphere and dissolved in water. During photosynthesis, carbon dioxide from the atmosphere is taken into the leaves of plants where it is combined with hydrogen from water molecules (H2O), which are absorbed from the soil by the roots and transported to the leaves. Many kinds of aquatic organisms such as algae and some bacteria also perform photosynthesis but absorb carbon dioxide and water molecules from the water in which they live. (Actually about 50% of photosynthetic activity on Earth takes place in the oceans due to the activity of algae and photosynthetic bacteria.)

The energy needed to perform photosynthesis is provided by sunlight. As a result of photosynthesis, complex organic molecules such as carbohydrates (sugars) are formed. Producer organisms use these sugars to provide themselves with energy and to make other kinds of organic molecules needed for growth and reproduction. At the same time that carbon is being incorporated into organic molecules, oxygen molecules (O2) are released into the atmosphere or water—because during the process of photosynthesis, water molecules are split to provide hydrogen atoms necessary to manufacture carbohydrate molecules.

2. The Role of Consumers

Herbivores can use the complex organic molecules of producers as food. When an herbivore eats plants or algae, the complex organic molecules in their food are broken down into simpler organic molecular building blocks, such as simple sugars, amino acids, glycerol, and fatty acids, which then can be reassembled into the specific organic molecules that are part of the herbivore’s chemical structure. Thus, the atoms in the herbivore’s body can be traced back to the plants it ate. Nearly all organisms also carry on the process of respiration, in which oxygen from the atmosphere is used to break down organic molecules into carbon dioxide and water. Much of the chemical-bond energy released by respiration is lost as heat, but the remainder is used by the herbivore for movement, growth, and other activities.

In similar fashion, when an herbivore is eaten by a carnivore, some of the carbon-containing molecules of the herbivore become incorporated into the body of the carnivore. The remaining organic molecules are broken down in the process of respiration to obtain energy, and carbon dioxide and water are released.

3. The Role of Decomposers

The organic molecules contained in animal waste products and dead organisms are acted upon by decomposers that use these organic materials as a source of food. The decay process of decomposers involves respiration and releases carbon dioxide and water so that organic molecules are typically recycled. (Many human-made organic compounds—plastics, industrial chemicals, and pesticides—are not readily broken down by decomposers.) How Science Works 15.1 describes how human alteration of the carbon cycle has affected climate.

The Hydrologic Cycle

Water molecules are the most common molecules in living things. Because all the metabolic reactions that occur in organisms take place in a watery environment, within cells or body parts, water is essential to life. During photosynthesis, the hydrogen atoms (H) from water molecules (H2O) are added to carbon atoms to make carbohydrates and other organic molecules. At the same time, the oxygen atoms in water molecules are released as oxygen molecules (O2). The movement of water molecules can be traced as a hydrologic cycle (figure 15.10).

FIGURE 15.10. The Hydrologic Cycle

The cycling of water through the environment follows a simple pattern. Moisture in the atmosphere condenses into droplets, which fall to the Earth as rain or snow. Organisms use some of the water and some of it evaporates from soil and organisms, but much of it flows over the Earth as surface water or through the soil as groundwater. It eventually returns to the oceans, where it evaporates back into the atmosphere to begin the cycle again.

Most of the forces that cause water to be cycled do not involve organisms but, rather, are the result of normal physical and geologic processes. Because of the kinetic energy possessed by water molecules, at normal Earth temperatures liquid water evaporates into the atmosphere as water vapor. This can occur wherever water is present; it evaporates from lakes, rivers, soil, and the surfaces of organisms. Because the oceans contain most of the world’s water, an extremely large amount of water enters the atmosphere from the oceans.

Water molecules also enter the atmosphere as a result of transpiration by plants. Transpiration is a process whereby water is lost from leaves through small openings called stomates. The water that is lost is absorbed from the soil into roots and transported from the roots to leaves, where it is used in photosynthesis or evaporates. This movement of water carries nutrients to the leaves, and the evaporation of the water from the leaves assists in the movement of water upward in the stem. Thus, transpired water can be moved from deep layers of the soil to the atmosphere.

Once the water molecules are in the atmosphere, they are moved along with other atmospheric gases by prevailing wind patterns. If warm, moist air encounters cooler temperatures, which often happens over landmasses, the water vapor condenses into droplets and falls as rain or snow. When the precipitation falls on land, some of it runs off the surface, some of it evaporates, and some penetrates into the soil. The water that runs off the surface makes its way through streams and rivers to the ocean. The water in the soil may be taken up by plants and transpired into the atmosphere, or it may become groundwater. Much of the groundwater eventually makes its way into lakes and streams and ultimately arrives at the ocean, from which it originated.

The Nitrogen Cycle

The nitrogen cycle involves the cycling of nitrogen atoms between the abiotic and biotic components and among the organisms in an ecosystem. Nitrogen is essential in the formation of amino acids, which are needed to form proteins, and in the formation of nitrogenous bases, which are a part of ATP and the nucleic acids, DNA and RNA. Nitrogen is found as molecules of nitrogen gas (N2) in the atmosphere. Although nitrogen gas (N2) makes up approximately 80% of the earth’s atmosphere, it is not readily available to most organisms because the two nitrogen atoms are bound very tightly to each other and very few organisms are able to use nitrogen in this form. Since plants and other producers are at the base of nearly all food chains, they must make new nitrogen-containing molecules, such as proteins and DNA. Plants and other producers are unable to use the nitrogen in the atmosphere and must get it in the form of nitrate (—NO3) or ammonia (NH3).

1. The Role of Nitrogen-Fixing Bacteria

Because atmospheric nitrogen is not usable by plants, nitrogen-containing compounds are often in short supply, and the availability of nitrogen is often a factor that limits the growth of plants. (Most aquatic ecosystems are limited by the amount of phosphorus rather than the amount of nitrogen.) Certain kinds of soil bacteria are the primary source of the nitrogen-containing molecules plants need to make proteins and DNA.

Some bacteria, called nitrogen-fixing bacteria, are able to convert the nitrogen gas (N2) that enters the soil into ammonia (NH3) that plants can use. Certain kinds of these bacteria live freely in the soil and are called free-living nitrogen-fixing bacteria. Others, known as symbiotic nitrogen-fixing bacteria, have a cooperative relationship with certain plants and live in nodules in the roots of plants such as legumes (peas, beans, and clover) and certain trees such as alders. Some grasses and evergreen trees appear to have a similar relationship with certain root fungi that seem to improve the nitrogen-fixing capacity of the plant.

2. The Role of Producers and Consumers

Once plants and other producers have nitrogen available in a form they can use, they can construct proteins, DNA, and other important nitrogen-containing organic molecules. When herbivores eat plants, the plant protein molecules are broken down to smaller building blocks called amino acids. These amino acids are then reassembled to form proteins typical for the herbivore. Nucleic acids and other nitrogen-containing molecules are handled similarly. During the animal’s manipulation and transformation of amino acids, and some other molecules, some nitrogen is lost in the organism’s waste products as ammonia, urea, or uric acid. These same processes occur when carnivores eat herbivores.

3. The Role of Decomposers and Other Soil Bacteria Bacteria and other types of decay organisms are involved in the nitrogen cycle also. Dead organisms and their waste products contain molecules, such as proteins, urea, and uric acid, that contain nitrogen. Decomposers break down these nitrogen-containing organic molecules, releasing ammonia (NH3), which can be used directly by many kinds of plants. Still other kinds of soil bacteria, called nitrifying bacteria, are able to convert ammonia to nitrite (—NO2), which can be converted by other bacteria to nitrate (—NO3). The production of nitrate is significant because plants can use nitrate as a source of nitrogen for synthesis of nitrogen-containing organic molecules.

Finally, bacteria known as denitrifying bacteria are, under conditions where oxygen is absent, able to convert nitrite to nitrogen gas (N2), which is ultimately released into the atmosphere. Atmospheric nitrogen can reenter the cycle with the aid of nitrogen-fixing bacteria.

4. Unique Features of the Nitrogen Cycle

Although a cyclic pattern is present in both the carbon cycle and the nitrogen cycle, the nitrogen cycle shows two significant differences. First, most of the difficult chemical conversions are made by bacteria and other microorganisms. Without the activities of bacteria, little nitrogen would be available and the world would be a very different place. Second, although nitrogen is made available to organisms by way of nitrogen-fixing bacteria and returns to the atmosphere through the actions of denitrifying bacteria, there is a secondary loop in the cycle that recycles nitrogen compounds from dead organisms and wastes directly back to producers. Figure 15.11 summarizes the roles of various organisms in the nitrogen cycle.

FIGURE 15.11. The Nitrogen Cycle

Nitrogen atoms are cycled through ecosystems. Atmospheric nitrogen is converted by nitrogen-fixing bacteria to nitrogen-containing compounds, which plants can use to make proteins and other compounds. Proteins are passed to other organisms when one organism is eaten by another. Dead organisms and their waste products are acted upon by decay organisms to form ammonia, which can be reused by plants and converted to other nitrogen compounds by nitrifying bacteria. Denitrifying bacteria return nitrogen as a gas to the atmosphere.

5. Agriculture and the Nitrogen Cycle

In naturally occurring soil, nitrogen is often a limiting factor of plant growth. To increase yields, farmers provide extra sources of nitrogen in several ways. Inorganic fertilizers are a primary method of increasing the nitrogen available. These fertilizers may contain ammonia, nitrate, or both.

Since the manufacture of nitrogen fertilizer requires a large amount of energy and uses natural gas as a raw material, fertilizer is expensive. Therefore, farmers use alternative methods to supply nitrogen and reduce their cost of production. Several different techniques are effective. Farmers can alternate nitrogen-yielding crops such as soybeans with nitrogen-demanding crops such as corn. Since soybeans are legumes that have symbiotic nitrogen-fixing bacteria in their roots, if soybeans are planted one year, the excess nitrogen left in the soil can be used by the corn plants grown the next year. Some farmers even plant alternating strips of soybeans and corn in the same field. A slightly different technique involves growing a nitrogen-fixing crop for a short period of time and then plowing the crop into the soil and letting the organic matter decompose. The ammonia released by decomposition serves as fertilizer to the crop that follows. This is often referred to as green manure. Farmers can also add nitrogen to the soil by spreading manure from animal production operations or dairy farms on the field and relying on the soil bacteria to decompose the organic matter and release the nitrogen for plant use.

HOW SCIENCE WORKS 15.1

Scientists Accumulate Knowledge About Climate Change

Humans have significantly altered the carbon cycle. As we burn fossil fuels, the amount of carbon dioxide in the atmosphere continually increases. Carbon dioxide allows light to enter the atmosphere but does not allow heat to exit. Because this is similar to what happens in a greenhouse, carbon dioxide and the other gases that have similar effects are called greenhouse gases. Therefore, many scientists are concerned that increased carbon dioxide levels are leading to a warming of the planet, which will cause major changes in our weather and climate.

In science, when a new discovery is made or a new issue is raised, it stimulates a large number of observations and experiments that add to the body of knowledge about the topic. Concerns about global climate change and the role that carbon dioxide plays in causing climate change have resulted in scientists studying many aspects of the problem. This has been a worldwide effort and has involved many different branches of science. This effort has resulted in critical examination of several basic assumptions about climate change, the collection of much new information, and new predictions about the consequences of global climate change.

Several significant studies include:

• Examination of gas bubbles trapped in the ice of glaciers has allowed scientists to measure the amount of carbon dioxide in the atmosphere at the time the ice formed. This provides information about carbon dioxide concentrations prior to human-caused carbon dioxide releases and allows scientists to track the rate of change.

• Long-term studies of the atmosphere at various locations throughout the world show that carbon dioxide levels are increasing.

• Measurements show that sea level is rising almost 2 millimeters per year.

• Measurements of the temperature of the Earth's atmosphere have allowed tracking of temperature. According to NASA, 10 of the warmest years on record occurred in the 12-year period between 1998 and 2009.

• Satellite images of the Arctic Ocean show reduced ice cover.

• Observations of bird migration in Europe document that birds that migrate long distances are arriving in Europe earlier in the spring.

• Many studies of the rate at which different ecosystems take up carbon dioxide have been done to determine if assumptions about the carbon dioxide trapping role of natural ecosystems are correct.

• Warming of the Arctic has resulted in less permafrost.

• Increased water temperatures have been linked to increases in the number and extent of blooms of cyanobacteria in lakes and oceans.

• Studies suggest that an increase in the level of carbon dioxide in the atmosphere could result in increased amounts of dissolved carbon dioxide in the ocean. Increased carbon dioxide will lower the pH of the ocean, which could have a negative effect on animals that make shells.

• Warming of the oceans is linked to more intense hurricanes.

• Earlier arrival of spring is linked to increased numbers and intensity of forest fires in the western United States.

The United Nations established the Intergovernmental Panel on Climate Change (IPCC)—a panel of scientists, political leaders, and economists—to analyze the large amount of information generated on the topic of climate changes. The IPCC has issued several reports about the nature, causes, and the impacts of climate change on ecosystems and culture.

The Phosphorus Cycle

Phosphorus is another atom common in the structure of living things. It is present in many important biological molecules, such as DNA, and in the membrane structure of cells. In addition, animal bones and teeth contain significant quantities of phosphorus. Most of the processes involved in the phosphorus cycle are the geologic processes of erosion and deposition. The ultimate source of phosphorus atoms is rock. In nature, new phosphorus compounds are released by the erosion of rock and are dissolved in water. Plants use the dissolved phosphorus compounds to construct the molecules they need. Animals obtain phosphorus when they consume plants or other animals. When an organism dies or excretes waste products, decomposer organisms recycle the phosphorus compounds back into the soil, where they can be reused. Phosphorus compounds that are dissolved in water are ultimately precipitated as mineral deposits. This has occurred in the geologic past and typically has involved deposits in the oceans. Geologic processes elevate these deposits and expose them to erosion, thus making phosphorus available to organisms. Animal wastes often have significant amounts of phosphorus. In places where large numbers of seabirds or bats have congregated for hundreds of years, their droppings (called guano) can be a significant source of phosphorus for fertilizer (figure 15.12).

In many soils, phosphorus is in short supply and must be provided to crop plants in fertilizer to get maximum yields. Phosphorus is also in short supply in aquatic ecosystems.

FIGURE 15.12. The Phosphorus Cycle

The primary source of phosphorus is phosphorus-containing rock. The erosion of rock and the dissolving of phosphorus compounds in water makes phosphorus available to the roots of plants. Animals obtain phosphorus in their food. Decomposers recycle phosphorus compounds back into the soil.

Nutrient Cycles and Geologic Time

The nutrient cycles we have just discussed act on a short-term basis in which elements are continually being reused among organisms and on a long-term basis in which certain elements are tied up for long time periods and are not part of the active nutrient cycle. In our discussion of the phosphorus cycle it was mentioned that the source of phosphorus is rock. While phosphorus moves rapidly through organisms in food chains, phosphorus ions are not very soluble in water and tend to precipitate in the oceans to form sediments that eventually become rock on the ocean floor. Once this has occurred, it takes the process of geologic uplift followed by erosion to make phosphorus ions available to terrestrial ecosystems. Thus, we can think of the ocean as a place where phosphorus is removed from the active nutrient cycle (this situation is known as a sink).

There are also long-term aspects to the carbon cycle. Organic matter in soil and sediments are the remains of once-living organisms. Thus, these compounds constitute a sink for carbon, particularly in ecosystems in which decomposition is slow (tundra, northern forests, grasslands, swamps, marine sediments). These materials can tie up carbon for hundreds to thousands of years. Fossil fuels (coal, petroleum, and natural gas), which were also formed from the remains or organisms, are a longer-term sink that involves hundreds of millions of years. The carbon atoms in fossil fuels at one time were part of the active carbon cycle but were removed from the active cycle when the organisms accumulated without decomposing. The organisms that formed petroleum and natural gas are thought to be the remains of marine organisms that got covered by sediments. Coal was formed from the remains of plants that were buried by sediments. Once the organisms were buried, their decomposition would be slowed, and heat from the Earth and pressure from the sediments helped to transform the remains of living things into fossil fuels. The carbon atoms in fossil fuels have been locked up for hundreds of millions of years. Thus, the formation of fossil fuels was a sink for carbon atoms.

Oceans are a major carbon sink. Carbon dioxide is highly soluble in water. Many kinds of carbonate sedimentary rock are formed from the precipitation of carbonates from solution in oceans. In addition, many marine organisms form skeletons or shells of calcium carbonate. These materials accumulate on the ocean floor as sediments that over time can be converted to limestone. Limestone typically contains large numbers of fossils. The huge amount of carbonate rock is an indication that there must have been higher amounts of carbon dioxide in the Earth’s atmosphere in the past.

Since fossil fuels are the remains of once-living things and living things have nitrogen as a part of protein, nitrogen that was once part of the active nitrogen cycle was removed when the fossil fuels were formed. In ecosystems in which large amounts of nonliving organic matter accumulates (swamps, humus in forests, and marine sediments), nitrogen can be tied up for relatively long time periods. In addition, some nitrogen may be tied up in sedimentary rock and, in some cases, is released with weathering. However, it appears that the major sink for nitrogen is as nitrogen in the atmosphere. Nitrogen compounds are very soluble in water, so when sedimentary rock is exposed to water, these materials are dissolved and reenter the active nitrogen cycle.

15.4. CONCEPT REVIEW

10. Trace the flow of carbon atoms through a community that contains plants, herbivores, decomposers, and parasites.

11. Describe four roles that bacteria play in the nitrogen cycle.

12. Describe the flow of water through the hydrologic cycle.

13. List three ways the carbon and nitrogen cycles are similar and three ways they differ.

14. Describe the major processes that make phosphorus available to plants.