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THE CYCLING OF MATERIALS WITHIN ECOSYSTEMS

In contrast, mat­ter, the material of which living things are com­posed, moves in numerous cycles from the living world to the nonliving physical environment and back again; we call these biogeochemical cycles.

The Earth and biosphere are essentially a closed system—that is, a system from which matter cannot escape. The materials utilized by organisms cannot he "lost," although they can end up outside the organisms' reach. Usually, however, materials are reused and recycled both within and among ecosystems.

Four biogeochemical cycles of matter—carbon, nitrogen, phosphorus, and water—are representa­tive of all biogeochemical cycles and are particu­larly important to living things. Carbon, nitrogen, and water have gaseous components and so cycle over large distances with relative ease. The element phosphorus, however, is completely nongaseous and, as a result, cycles only locally with ease.

During photosynthesis, plants remove carbon dioxide from the air and fix, or in­corporate; it into complex chemical compounds such as sugar. Thus, photosynthesis incorporates carbon from the abiotic environment into the biological compounds of producers. Those compounds are usually used as fuel for cell respiration (see Chapter 3) by the pro­ducer that made them, by a consumer that eats the producer, or by a decomposer that breaks down the remains of the producer or consumer. The overall equation for cell respiration is Thus, carbon dioxide is returned to the atmosphere by the process of cell respiration. A similar carbon cycle occurs in aquatic ecosystems, between aquatic organisms that photosynthesize (aquatic plants, algae, and cyanobacteria) and dissolved carbon di­oxide in the water.

Sometimes the carbon in biological molecules limit recycled back to the abiotic environment for Mine time. For example, a lot of carbon is stored in the wood of trees, where it may stay for several hundred years.

Millions of years ago, vast coal beds formed from the bodies of ancient trees that did not decay fully before they were buried. Similarly, accumula­tions of the oils of unicellular marine organisms in the geological past probably gave rise to today's underground deposits of oil and natural gas. Coal, oil, and natural gas, called fossil fuels because they formed from the remains of ancient organisms, are vast deposits of carbon compounds, the end prod­ucts of photosynthesis that occurred millions of years ago.

The carbon in coal, oil, natural gas, and wood can be returned to the atmosphere by the process of burning, or combustion. In combustion, organic molecules are rapidly oxidized (combined with oxy­gen) and thus converted into carbon dioxide and water, with an accompanying release of heat.

Scientists think that most of the carbon chat leaves the carbon cycle for millions of years is in­corporated into the shells of marine organisms.

When these organisms die, their shells sink to the ocean floor and are covered by sediments, forming seabed deposits thousands of feet thick. The depos­its are eventually cemented together to form a sedi­mentary rock called limestone. The Earth's crust is dynamic, and over millions of years, sedimentary rock on the bottom of the sea floor may lift to form land surfaces (the summit of Mount Everest, for example, is composed of sedimentary rock). After limestone is exposed by the process of geologic up­lift, it is slowly worn away, or disintegrated, by chemical and physical weathering processes. This returns the carbon to the water and atmosphere, where it is available to participate in the carbon cycle once again.

Thus, one process (photosynthesis) removes carbon from the abiotic environment' and incorpo­rates it into biological molecules, and three pro­cesses (cell respiration, combustion, and erosion) return carbon to the water and atmosphere of the abiotic environment.

The Carbon Cycle and Global Warming Human activities have disturbed the balance of the carbon cycle. From the advent of the Industrial Revolution to the present, humans have burned increasing amounts of fossil fuels—coal, oil, and natural gas. This trend, along with a greater combustion of wood as a fuel and the burning of large sections of tropical forest, has released carbon dioxide into the ornament for the ways. Many

atmosphere at a rate greater than the natural car­bon cycle can handle.

The slow and steady rise of CO2 in the atmos­phere may be causing changes in climate called glo­bal warming. Global warming could result in a rise death of forests, extinction of animals and plants and problem for agriculture. It could face the displacement of thousands or even millions of people, particularly from coastal areas. A more thorough discussion of increasing atmospheric CO2 and glo­bal warming.

The Nitrogen Cycle

Nitrogen is crucial for all living things because it is an essential part of proteins, which are important structural components of cells and serve as enzymes and hormones, and nucleic acids, which store ge­netic information about an organism's traits.

At first glance it would appear that a shortage of nitrogen for living organisms is impossible: the Earth's atmosphere is about 80 percent nitrogen gas (N₂), a 2-atom (diatomic) molecule. But molecular nitrogen is so stable that it does not readily com­bine with other elements; therefore, living things cannot take nitrogen gas directly from the atmos­phere and combine it with other elements to manu­facture their proteins and nucleic acids. The molec­ular nitrogen must first be broken apart. The over­all reaction that breaks up molecular nitrogen and combines its atoms with such elements as oxygen and hydrogen requires a great deal of energy.

There are five steps in the nitrogen cycle: (1) nitrogen fixation, (2) nitrification, (3) assimilation, (4) ammonification, and (5) denitrification. All of the steps except assimilation are performed by bacteria.

    (1) Nitrogen Fixation The first step in the nitro­gen cycle, nitrogen fixation, involves the conver­sion of gaseous nitrogen (N2) to ammonia (NH3). The process gets its name from the fact that nitro­gen is fixed into a form that living things can use. Although considerable nitrogen is also fixed by combustion, volcanic action, and lightning dis­charges and by industrial process (all of which supply enough energy to breakup molecular nitrogen).

plants is mutualistic: the bacteria receive carbohy­drates from the plant, and the plant receives nitro­gen in a form that it can use.

In aquatic habitats most of the nitrogen fixa­tion is done by cyanobacteria. Filamentous cyanobacteria have special oxygen-excluding cells called heterocysts that fix nitrogen. Some water ferns have cavities in which cyanobacteria live, somewhat as Rhizobium lives in the root nod­ules of legumes. Other cyanobacteria fix nitrogen in symbiotic association with certain plants or as the photosynthetic partners of certain lichens.

The reduction of nitrogen gas to ammonia by nitrogenase is a remarkable accomplishment of liv­ing organisms that is achieved without the tre­mendous heat, pressure, and energy required to manufacture commercial fertilizers. Even so, ni­trogen-fixing bacteria must consume the energy equivalent of 12 grams of glucose in order to biolog­ically fix a single gram of nitrogen.

(2) Nitrification The conversion of ammonia (MHO to nitrate (NO2), called nitrification, s accomplished by soil bacteria. Nitrification is in two-step process. First the soil bacteria Nitrosomonas and Nitrococcus convert ammonia to it -trite (NO₂). Then the soil bacterium Nitrobacter oxidizes nitrite to nitrate. The process of nitrifica­tion furnishes these bacteria, called nitrifying bac­teria, with energy.

(3) Assimilation In assimilation, plant roots ab­sorb nitrate (NO₅) and/or ammonia (NH3) that has been formed by nitrogen fixation and nitrifi­cation, and incorporate the nitrogen of these mole­cules into plant proteins and nucleic acids. When animals consume plant tissues, they also assimilate nitrogen by taking in plant nitrogen compounds and converting them to animal compounds.

(4) Ammonification: Living organisms produce nitrogen-containing waste products such as urea (in urine) and uric acid (in the wastes of birds). These substances, plus the nitrogen compounds that occur in dead organisms, are decomposed, releasing the

(NH3)- The conversion of biological nitrogen com­pounds into ammonia is known as ammonification,

and the bacteria that perform this process both in the soil and in aquatic environments are called ammonifying bacteria. The ammonia produced by ammonification enters the nitrogen cycle and is once again available for the processes of nitrifica­tion and assimilation.

(5) Denitrification The reduction of nitrate (NO₃) to gaseous nitrogen (N₂) is called denitrification. Denitrifying bacteria reverse the action of nitrogen-fixing and nitrifying bacteria; that is, they return nitrogen to the atmosphere as nitrogen gas. Denitrifying bacteria are anaerobic, which means they prefer to live and grow where there is little or no free oxygen. For example, they are found deep in the soil near the water table, an environment that is nearly oxygen-free.

The Nitrogen Cycle and Water Pollution

Humans affect the nitrogen cycle by producing large quanti­ties of nitrogen fertilizer (both ammonia and ni­trate) from nitrogen gas. Although this process in itself is not harmful, the overuse of commercial fertilizers on the land can cause water quality prob­lems. Rain washes nitrate fertilizer into rivers and lakes, where it stimulates the growth of algae. As these algae die, their decomposition robs the water of dissolved oxygen, which in turn causes other suffocation. Nitrates from fertilizers can also leach (filter) down through the soil and contaminate groundwater. Many people who live in rural areas drink groundwater, and groundwater contaminated by nitrates is dangerous, particularly for infants and small children. The effects of nitrate contamina­tion on the environment and on human health.

The Phosphorus Cycle

Phosphorus, which does not exist in a gaseous state and therefore does not enter the atmosphere, cycles from the land to sediments in the oceans and back to the land (Figure 5-6). As water runs over rocks containing phosphorus, it gradually wears away the surface and carries off inorganic phosphate (PO₄^!~) molecules.

The erosion of phosphorus rocks releases phos­phorus into the soil, where it is taken up by plant roots in the inorganic phosphates. Once in the plant's cells, phosphates are used in a variety of biological molecules including nucleic acids. Ani­mals obtain most of their required phosphate from the food they eat, although in some localities drinking water may contain a substantial amount of inorganic phosphate. Thus, like carbon and nitro­gen, phosphorus moves through the food chain as one organism consumes another. Phosphoric re­leased by decomposers becomes part of the soil's pool of inorganic phosphate that can be reused by plants.

Phosphorus cycles through aquatic communi­ties in much the same way it does through terres­trial communities. Dissolved phosphorus enters aquatic communities via absorption by algae and plants, which are then consumed by plankton and larger organisms. These, in turn, are eaten by a va­riety of fin fish and shellfish. Ultimately, decom­posers that break down wastes and dead organisms it is available to hi: used by aquatic producers sigain.

Phosphate can be lost from biological cycles. Some of it is carried from the land by streams and rivers to the ocean, where it can be deposited on the sea floor and remain for millions of years. The geologic process of uplift may someday expose these sea floor sediments as new land surfaces, from which phosphates will once again be eroded.

Some phosphate in the aquatic food chain finds its way back to the land. A small portion of the fish and aquatic invertebrates are eaten by sea birds, which may defecate where they roost on the land. Their manure, called guano, contains large amounts of phosphate and nitrate; on land these minerals may be absorbed by the roots of plants. The phosphate contained in guano may enter ter­restrial food chains in this way, although the amounts involved are quite small.

Humans and the Phosphorus Cycle Humans af­fect the natural cycling of phosphorus by accelerat­ing its long-term loss from the land. Com grown in lowa {which contains phosphate absorbed from the soil) may be used to fatten cattle in an Illinois feed-lot. Part of the phosphate absorbed by the roots of the corn plants thus ends up in the feedlot wastes, which probably eventually wash into the Missis­sippi River. Beef from the Illinois cattle may be consumed by people living far away—in New York City, for instance. Hence, more of the phosphate ends up in human wastes and is flushed down toi­lets into the New York City sewer system. Sewage treatment rarely removes phosphates, and so they cause water quality problems in rivers and lakes. To compensate for the steady loss of phosphate from their land, farmers must add phosphate fertilizer to their fields. More than likely, that fertilizer is produced in Florida from the large deposits of phosphate rock that are mined there.

In natural communities, very little phosphorus is losing from the cycle, but few communities today are in a "natural" state. Phosphorus loss from the soil is accelerated by land-denuding practices such

As the clear cutting of timber and by erosion of agri­cultural and residential land. For practical purposes, phosphorus that washes from the land into the sea is permanently lost from the terrestrial phos­phorus cycle, for it remains in the sea for millions of years.

The Hydrologic Cycle

Water continuously circulates from the oceans to the atmosphere to the land and back to the oceans, providing us with a renewable supply of purified water on land. This complex cycle, known as the hydrologic cycle, results in a balance among water in the oceans, water on the land, and water in the atmosphere (Figure 5-7). When water evaporates from the ocean's surface, it forms clouds in the at­mosphere. Water also evaporates from soil, streams, rivers, and lakes. Transpiration, the loss of water vapor from land plants, also adds water to the at­mosphere. Roughly 97 percent of the water ab­sorbed from the soil by a plant is transported to the leaves, where it is transpired back to the atmos­phere.

Water moves from the atmosphere to the land and oceans in the form of precipitation (rain, snow,

Sleet or hail). Once on land, the water can move in several ways through the hydrologic cycle:

Milking Mountains for Moisture

In many parts of the world, moisture-laden clouds (fog) pass tantali:ing!y close to ex­tremely arid regions without releasing rainfall. In the Middle Eastern Sultanate of Oman, people have "harvested" water for centuries from such clouds using the eaves of olive.Small tanks built at the foot of the trees col­lect droplets that form on the leaves. The idea has been taken one step farther in Chile's Atacama Desert. Fifty tow-cost "cap-ton" that resemble volleyball nets have been built abng a ridge of the Andes Mountains. Moist clouds from the Pacific Ocean pass through the captors, releasing 7,200 liters of fresh water each day. The water is channeled by an aqueduct to the coastal village of Caleta Chungungo, which formerly depended on weekly truck shipments for its drinking water. Gardens are now being grown with this new source of water. When more captor nets are built the village plans to develop a fish processing plant.