TABLE OF CONTENTS. 4, Environmental Chemistry 2, Biogeochemical cycle of carbon and nitrogen

Transcription

1 Subject Paper No and Title Module No and Title Module Tag CHE_P4_M2 TABLE OF CONTENTS 1. Learning outcomes 2. Introduction 2.1. Bio-distribution of elements 2.2. Biogeochemical cycles 3. Carbon cycle 3.1. Geological carbon cycle 3.2. Marine carbon cycle 3.3. Impact of human activities on carbon cycle 4. Nitrogen cycle 4.1. Atmospheric nitrogen cycle 4.2. Marine nitrogen cycle 4.3. Impacts of human activities on nitrogen cycle 4.4. Environmental issues 5. Summary

2 1. Learning Outcomes After studying this module, you shall be able to Appreciate how the chemical elements are distributed in biological systems. Know about the nature of bio-geochemical cycles of carbon and nitrogen. Learn about the components of ecosystem and their use in various activities on earth. Identify the impact of human activities on the cycling of elements in the Biosphere. 2. Introduction 2.1 Bio-distribution of elements Bio-distribution of elements means the relative abundance of elements in biological systems. In almost all life forms on earth (weather in land or in oceans) carbon, nitrogen, oxygen and hydrogen are the chief elements making up most of the body mass of the organisms. These elements, along with sulfur and phosphorous are known as macro elements because they are required in larger quantities. These elements have a structural role i.e. they are required in the synthesis of biomolecules responsible for giving shape and strength to body of the organism. Water, a simple compound of hydrogen and oxygen, is essential for all living cells. It can dissolve a variety of ions from the atmosphere (and precipitate as rain) or in aqueous solutions. Carbon can undergo extensive covalent bonding with carbon atoms and as well as atoms of other elements, in different proportions, to form molecules which can undergo further oxidation or reduction to form ions or undergo molecular rearrangements to form complex molecules and biomolecules like carbohydrates, amino acids, proteins etc. The biomolecules are passed from organism to organism through across the food chains across the ecosystems. When the organisms die, their body decays and the elements are returned back to the environment for recycling. This forms the basis of all the biogeochemical cycles. There are also some elements, the micro elements, which are required by the living organisms in smaller quantities. These elements have a more functional role they generally act as catalysts for bio-reactions e.g. molybdenum in nitrogen fixing bacteria, boron in photosynthetic plants, copper in some enzymes and iron in hemoglobin. 2.2 Biogeochemical cycle A bio-geochemical cycle is basically a cyclic pathway through which any chemical element moves through the biotic (e.g. plants, animals etc.) and abiotic (geological e.g. water, air, soil, rocks etc.) compartments of earth. It connects the living and the non-living parts of the biosphere. In these cycles, elements in their elemental form or as compounds, in different states of matter are passed from organism to organism across the compartments of the biosphere. Recycling of elements across the biosphere is essential to maintain a state of equilibrium between different parts of the earth. Changes in climate are responsible for changing the speed, intensity, and balancing of these cycles.

3 In bio-geochemical cycles the elements are either recycled continuously (e.g. processes like respiration, photosynthesis etc.) or they may be accumulated at a place for a longer time (e.g. mineral deposits in rocks and sea beds). The area where an element is concentrated such as the atmosphere, oceans or the earth s crust is called a biogeochemical pool or reservoir. There are two types of pools in a cycle, active pool and storage pool. In active pools, the elements or their compounds are present in a form which is easily available to life processes for direct uptake. Storage pools on the other hand, contain elements in forms which need to fixed before the organisms can uptake them. Biogeo-chemical cycles can be classified into two types gaseous and sedimentary. In gaseous cycles like carbon (as carbon dioxide), nitrogen and oxygen the main pool or reservoir is in the gaseous phase while there may be pools in the atmosphere or water. In sedimentary cycles on the other hand, the main pool is the lithosphere. These cycles are generally very slow because the elements get locked up in rocks or earth s crust and are out of circulation for a long time. 3. Carbon cycle Carbon is an essential element needed by all living organisms. It forms the backbone of all organic molecules and is required in the synthesis of the building blocks like proteins, fats, carbohydrates, amino acids etc., of all life forms. Biosphere contains a complex mixture of carbon compounds which are continuously transformed from one type to another. The active carbon cycle pools are the atmosphere and the oceans, seas, rivers and lakes which contain dissolved inorganic carbon. Carbon is present as carbon dioxide (CO 2 ) in the atmosphere. The storage pools are the carbonate sediments on ocean floors and in fossil fuels as well as the carbon present in earth s crust and mantle. The soil carbon which comes from the organic non-living materials also contributes to the global carbon cycle. Fig1. The carbon cycle

4 Plants utilize or convert CO 2 to other organic molecules like glucose, cellulose, starch, sugars etc. through the process of photosynthesis. Animals consume the plants and utilize these organic molecules to derive energy for their body functions. The respiration of animals then returns CO 2 to the atmosphere. The decomposition of dead plants and animals also returns back CO 2 to the atmosphere and other carbon compounds to the soil. However, this does not add to the CO 2 levels in the atmosphere because it is the same amount which was fixed by the plants in their life cycle. Consumption (Photosynthesis): Production (Respiration): CO 2 + H 2 O CH 2 O + O 2 CH 2 O + O 2 CO 2 + H 2 O The terrestrial biosphere considers the organic carbon which is available in all living forms. The uptake of carbon depends upon biotic factors and seasonal cycles. During the day, in the presence of sunlight, CO 2 concentration decreases because of uptake through photosynthesis. It starts increasing at dusk when the rate of photosynthesis decreases. In seasonal cycles, the lowest levels of CO 2 are observed during spring when vegetation increases and photosynthesis is at its peak. Carbon present in the soil can flow off into rivers through erosion or be released out in the environment by soil respiration. Rise in temperature increases the rate of organic matter decomposition in soil thereby releasing the trapped carbon as CO 2. In earlier times, the carbon cycle was considered perfect among all biogeochemical cycles, because both the recovery and consumption of CO 2 were balanced. But now, large amounts of CO 2 are released into the atmosphere due to the excessive burning of coal and other fossil fuels. The rate of consumption or fixation of CO 2 by plants through photosynthesis is decreasing with forest covers thinning by the day. 3.1 Geological carbon cycle The cycling of carbon in the geosphere is slower than in other parts of the global carbon cycle. However, it plays an essential role in determining the concentration of carbon in the atmosphere and temperature of earth globally. Most of the carbon on earth is stored in inert forms in the lithosphere, of which about 80% is limestone and its derivatives. Limestone is formed from the sedimentation of shells of marine organisms which are rich in calcium carbonate. The rest 20% is present in shale rocks as kerogen which is a precursor to fossil fuels. Kerogen is a mixture of organic compounds which when subjected to high temperature and pressure conditions in the earth s crust, release high molecular weight hydrocarbons (crude oil or natural gas). These changes occur over millions of years thereby trapping carbon in the geosphere for long periods of time. Carbon can return back to the bio-geochemical cycle in different ways. Carbon dioxide which is released during the metamorphosis of carbonate rocks under heat and pressure in the earth s mantle can be released back into the atmosphere and oceans through volcanic eruptions. Humans also contribute to the depletion of carbon from the geosphere by extracting and burning fossil fuels for energy. This also releases carbon into the atmosphere. 3.2 Marine carbon cycle Oceans are the second largest active reservoirs of carbon after the lithosphere. The atmospheric CO 2 is in equilibrium with the ocean surface which is rich in dissolved inorganic carbon (DIC) as carbonate. The deep layers of the oceans are 15% richer in DIC where it is stored for longer

5 times. Thermal currents exchange carbon between the surface and deep layers. DIC can be converted into organic carbon through photosynthesis by marine organisms. Dissolved organic carbon also enters the oceans through rivers. Organic carbon either moves through the food chain or sinks into the deeper layers as shells where it eventually turns into sediments. The oceans have a limited capacity to sequester CO 2. Absorption of CO 2 makes water acidic which affects the marine life forms thereby upsetting the ocean ecosystem. An increase in the acidity of water will decrease the deposition of calcium carbonate and reduce the capacity of oceans to consume carbon dioxide. 3.3 Impacts of human activities on carbon cycle Carbon exists in the atmosphere mainly as either CO 2 or methane. Both these gases can absorb and preserve heat in the atmosphere and are therefore responsible for maintaining the temperature of earth. But, if their amounts increase, then they cause global warming. Human activities directly release large amounts of carbon to the atmosphere by burning fossil fuels and by changing the terrestrial and ocean ecosystem thus impacting the carbon cycle. Deforestation also affects the carbon cycle by releasing the soil carbon directly into the atmosphere or through erosion to rivers and then marine ecosystems. Increased levels of carbon dioxide also increase the rate of photosynthesis as it allows plants to take more carbon dioxide. Now plants need more water to fix the absorbed CO 2. This can increase the chances of drought like conditions in many regions of the earth. 4. Nitrogen cycle Fig.2 Deforestation Nitrogen, like carbon, is an important element of the ecosystem. It is an essential component of all amino acids which are the building blocks of all enzymes and proteins both structural and functional. It is also required in the formation of DNA and RNA which are the main genetic code carriers in all life forms animals, plants, bacteria and even viruses. Since nitrogen is an important constituent of all the biomolecules, it indirectly plays an important role in photosynthesis in green plants. In chlorophyll molecules, light is trapped by protein molecules which initiate the reaction. The active pool of nitrogen is the soil on the earth s crust. It contains inorganic nitrogen and organic nitrogen from the decaying organic matter. The sediments and the atmosphere are the storage pools. Nitrogen as N 2, though inert, is the largest component of the atmosphere.

6 4.1 Atmospheric Nitrogen cycle Fig3. The nitrogen cycle The atmospheric nitrogen cycle is the equilibrium between the inorganic nitrogen gas (N 2 ) and the organic nitrogen in the soil and the atmosphere. Organic nitrogen in the environment is present in many forms such as ammonium (NH 4 + ), nitrite (NO 2 - ), nitrate (NO 3 - ), nitrous oxide (N 2 O), nitric oxide (NO) etc. The nitrogen cycle involves transformation of nitrogen from one form to another through processes like nitrogen fixation, assimilation, ammonification, nitrification and denitrification. These biological and physical processes are mostly carried out by microbes which either derive energy from the process or require nitrogen in a particular form for their growth Nitrogen fixation The earth s atmosphere consists of about 78% nitrogen as N 2 but it is chemically inert because of high bond energy. Thus, it is not suitable for biological use and needs to be fixed in a usable form before it can be taken up by plants and animals. There are three types of fixation processes atmospheric fixation, biological fixation and industrial fixation. Atmospheric fixation occurs when lightning strikes and converts the molecular nitrogen and oxygen to nitrogen oxides. The nitrogen compounds are then washed down by rainfall into soil. Biological fixation of nitrogen oxides is done by the microorganisms in soil which convert them to nitrites and nitrates. Nitrifying bacteria like Azotobacter and Rhizobium found in roots of leguminous plants convert the molecular nitrogen to ammonia. In aquatic systems, this fixation is done by blue-green algae. 2N 2 + 6H 2 O 4NH 3 + 3O 2 2NH 3 + 2H 2 O + 4CO 2 2CH 2 NH 2 COOH + 3O 2 Industrial fixation of nitrogen to ammonia occurs by the Haber s process. In this process, nitrogen gas and hydrogen (from petroleum or natural gas) react in 1:3 ratio, in the presence of a catalyst

7 at high temperature and high pressure. Ammonia obtained by this method is further converted into fertilizers like ammonium nitrate and urea Assimilation Plants can absorb nitrate, nitrite or ammonium ions from the soil through their root hairs. The absorbed nitrate is sequentially reduced to nitrite and ammonium ions which are incorporated into amino acids, nucleic acids, and chlorophyll. In leguminous plants, the symbiotically associated Rhizobium bacteria assimilate nitrogen directly from the nodules through a complex exchange of amino acids. Many bacteria like Azotobacter are capable of utilizing inorganic compounds like ammonium ions as their sole source of nitrogen. Animals, fungi, and other heterotrophic organisms that cannot fix their own nitrogen, obtain it by the ingestion of amino acids, nucleotides and other small organic molecules synthesized by the plants Ammonification Organic nitrogen from dead plants and animals, and animal wastes is converted back into ammonium by the action of bacteria or fungi. This process of ammonification or mineralization is the reverse of assimilation and involves many enzymatic reactions Nitrification The nitrifying bacteria (like Nitrosomonas) present in the soil oxidize ammonium to nitrite which is further oxidized to nitrate by Nitrobacter bacteria. Ammonia is toxic to plants, so its oxidation to nitrites and nitrates is essential for their survival. Due to high solubility of nitrates they can enter into groundwater and percolate into drinking water. High levels of nitrate in drinking water can lead to blue baby syndrome lowering of blood oxygen levels in infants. High concentration of nitrates can also lead to eutrophication high growth of blue-green algae in streams and lakes, sometimes covering the entire surface of the water body, blocking out sunlight, affecting the growth of fish and other organisms, and unbalancing the aquatic ecosystem Denitrification This is the last step in the nitrogen cycle in which nitrates are reduced to nitrogen (N 2 ) under anaerobic conditions such as in soils that are water-logged and released back into the atmosphere. Anaerobic bacteria like Pseudomonas and Clostridium use nitrate in respiration. In anaerobic respiration nitrate ion acts as an electron acceptor (which in aerobic respiration is done by the oxygen molecule) and release molecular nitrogen which cannot be used by plants. 4.2 Marine nitrogen cycle The marine or oceanic nitrogen cycle is also an important process. The overall cycle is similar to atmospheric nitrogen cycle involving the same processes of assimilation, ammonification, nitrification and denitrification. However, the type of organisms participating in the cycle and the mechanism of transformation of nitrogen are different. The distribution of nitrogen and its forms is greatly varied depending on factors like depth of the layer, proximity to shore and seasonal and temperature variations. Nitrogen can enter the water bodies either through precipitation (rain, snow, hail, sleet etc.) or flow in from rivers and in small amounts even by dissolution of atmospheric nitrogen. Just like plants in the atmospheric cycle, the phytoplankton in oceans also cannot use molecular nitrogen. In oceans, cyanobacteria (as compared to Azotobacter and Rhizobium on land) fix nitrogen to

8 biological forms that can be taken up for organic synthesis. Phytoplankton assimilate the organic nitrogen and release out ammonia and urea in water. Ammonia can then be converted to ammonium by bacteria in the process of ammonification. Ammonium ions undergo nitrification to nitrites and nitrates. Denitrification releases back molecular nitrogen to the atmosphere. Nitrite and ammonium ions are intermediates in the nitrogen cycle. They are generated in one step and used up in some other. For the nitrite ion, the rates of production (from the assimilation, nitrification, and denitrification steps) and consumption are comparable. The concentration of nitrates in the ocean is about 1000 times higher than the oceanic ammonium concentration. 4.3 Impacts of human activities on nitrogen cycle The conversion of nitrogen to biological forms has almost doubled due to human activities like excessive cultivation of leguminous plants (pulses, soya etc.); increasing production and use of chemical fertilizers; vehicular and industrial pollution; and unchecked burning of fossil fuels. The global nitrogen cycle has been affected by the release of nitrogenous gases into the atmosphere and water bodies, thus decreasing the quality of the air we breathe and the water we drink. Over the years, due to extensive use of fertilizers, burning of biomass and emissions from industries the concentration of N 2 O (nitrous oxide) has increased in the atmosphere. High levels of N 2 O in the atmosphere are very dangerous for earth because it not only increases the global temperatures by contributing as a greenhouse gas, but also depletes the ozone layer. The atmospheric concentration of NH 3 (ammonia) has increased to thrice its value due to human activities in the name of development. NH 3 dissolves in water to form HNO 3 (Nitric Acid) which precipitates as acid rain. Acid rain is detrimental to the health of animals, fishes and plants; can cause damage to the respiratory system in humans; and can also cause damage to property like the yellowing of the marble walls of Taj Mahal in Agra, India. Both NH 3 and NO 2 act as aerosols, which stick to the water droplets and lead to the formation of smog close to the surface of earth, especially in winters when the temperatures are low. 4.4 Environmental issues The increasing concentration of biological nitrogen has caused an imbalance in the ecosystems. Human activities like burning of fossil fuels, production of synthetic fertilizers by Haber-Bosch method and release of ammonia from waste water treatment plants is adding to the already high concentrations of nitrogen. Ammonia in high levels is fatal for fish. High concentration of inorganic nitrogen increases the acidity of water; causes eutrophication in water bodies and may be toxic for animals and humans. Eutrophication can lead to the death of aquatic animals because of decrease in dissolved oxygen levels and lesser availability of sunlight. 5. Summary The term bio-geochemical cycle means a cyclic pathway through which any chemical element moves through the biotic and abiotic (geological) compartments of earth. The element may be recycled continuously or be stored at a place for a long time. Carbon is an essential element needed by all living organisms. It forms the backbone of all organic molecules and is required in the synthesis of the building blocks like proteins, fats, carbohydrates, amino acids etc., of all life forms. Carbon is present as carbon dioxide (CO 2 ) in the atmosphere.

9 Plants convert CO 2 to organic molecules like glucose through photosynthesis. Animals consume the plants to derive energy for their body functions. The respiration of animals then returns CO 2 to the atmosphere. Human activities like burning fossil fuels releases large amounts of carbon into the atmosphere thus impacting the carbon cycle. High concentration of CO 2 in the atmosphere can cause global warming. Nitrogen is an essential component of amino acids which are the building blocks of all enzymes and proteins both structural and functional. It is also required in the formation of DNA and RNA which are the main genetic code carriers in all life forms. The atmospheric nitrogen cycle is the equilibrium between the inorganic nitrogen gas (N 2 ) and the organic nitrogen in the soil and the atmosphere. Nitrogen cycle involves transformation of nitrogen through processes like nitrogen fixation, assimilation, ammonification, nitrification and denitrification. Global nitrogen cycle has been affected by the release of nitrogenous gases into the atmosphere and water bodies. High level of N 2 O in the atmosphere increase the global temperatures by contributing as a greenhouse gas, and also depletes the ozone layer. NH 3 dissolves in water to form HNO 3 (Nitric Acid) which precipitates as acid rain which can damage the respiratory system in humans. Both NH 3 and NO 2 act as aerosols, which stick to the water droplets and lead to the formation of smog close to the surface of earth.